• TABLE OF CONTENTS
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 Title Page
 Dedication
 Acknowledgement
 Table of Contents
 List of Tables
 List of Figures
 Abstract
 Chapter I: Introduction
 Chapter II: Productivity aspects...
 Chapter 3: Soil fertility aspects...
 Chapter 4: Dynamics of soil fertility...
 Chapter 5: Interactions between...
 Chapter 6: Potential for farmer...
 Chapter 7: Summary and conclus...
 Literature cited
 Biographical sketch






Title: Soil fertility and productivity aspects of alley cropping leucaena leucocephala and cassia siamea under semiarid conditions at Machakos, Kenya
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Permanent Link: http://ufdc.ufl.edu/UF00056215/00001
 Material Information
Title: Soil fertility and productivity aspects of alley cropping leucaena leucocephala and cassia siamea under semiarid conditions at Machakos, Kenya
Physical Description: xvi, 267 leaves : ill. ; 29 cm.
Language: English
Creator: Adan, Bashir Jama, 1957-
Publication Date: 1993
 Subjects
Subject: Forest Resources and Conservation thesis Ph. D
Dissertations, Academic -- Forest Resources and Conservation -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Thesis: Thesis (Ph. D.)--University of Florida, 1993.
Bibliography: Includes bibliographical references (leaves 247-266).
Statement of Responsibility: by Bashir Jama Adan.
General Note: Typescript.
General Note: Vita.
Funding: Electronic resources created as part of a prototype UF Institutional Repository and Faculty Papers project by the University of Florida.
 Record Information
Bibliographic ID: UF00056215
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 001915155
oclc - 30335723
notis - AJZ0674

Table of Contents
    Title Page
        Page i
    Dedication
        Page ii
    Acknowledgement
        Page iii
        Page iv
    Table of Contents
        Page v
        Page vi
    List of Tables
        Page vii
        Page viii
        Page ix
        Page x
    List of Figures
        Page 11
        Page xii
        Page xiii
    Abstract
        Page xiv
        Page xv
        Page 16
    Chapter I: Introduction
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Chapter II: Productivity aspects of alley cropping with Leucaena Leucocephala and Cassia Siamea in semiarid tropics at Machakos, Kenya
        Page 11
        Introduction
            Page 11
            Page 12
            Page 13
            Page 14
        Materials and methods
            Page 15
            Page 16
            Page 17
            Page 18
            Page 19
            Page 20
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            Page 25
            Page 26
            Page 27
            Page 28
            Page 29
        Results
            Page 30
            Page 31
            Page 32
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            Page 56
        Discussion
            Page 57
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            Page 72
        Conclusions
            Page 73
            Page 74
    Chapter 3: Soil fertility aspects of alley cropping with Leucaena Leucocephala and Cassia Siamea in semiarid tropics at Machakos, Kenya
        Page 75
        Introduction
            Page 75
            Page 76
            Page 77
        Materials and methods
            Page 78
            Page 79
            Page 80
            Page 81
            Page 82
        Results
            Page 83
            Page 84
            Page 85
        Discussion
            Page 86
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            Page 94
        Conclusions
            Page 95
            Page 96
    Chapter 4: Dynamics of soil fertility changes : rates of decomposition, n-mineralization and uptake by maize from mulch of Leucaena Leucocephala and Cassia Siamea under semiarid conditions at Machakos, Kenya
        Page 97
        Introduction
            Page 97
            Page 98
            Page 99
        Materials and methods
            Page 100
            Page 101
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            Page 109
        Results
            Page 110
            Page 111
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        Discussion
            Page 132
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        Conclusions
            Page 145
            Page 146
    Chapter 5: Interactions between maize and hedgerows of Leucaena Leucocephala and Cassia Siamea in alley cropping system in semiarid tropics at Machakos, Kenya
        Page 147
        Introduction
            Page 147
            Page 148
        Materials and methods
            Page 149
            Page 150
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            Page 152
            Page 153
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        Results
            Page 158
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            Page 182
        Discussion
            Page 183
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            Page 189
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            Page 193
            Page 194
        Conclusions
            Page 195
            Page 196
    Chapter 6: Potential for farmer adoption of alley cropping of multipurpose trees and shrubs in semiarid areas of Machakos, Kenya
        Page 197
        Introduction
            Page 197
            Page 198
            Page 199
            Page 200
        Study area and methodology
            Page 201
            Page 202
            Page 203
        Results
            Page 204
            Page 205
            Page 206
            Page 207
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        Discussion
            Page 226
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        Conclusions
            Page 238
            Page 239
            Page 240
    Chapter 7: Summary and conclusions
        Page 241
        Page 242
        Page 243
        Page 244
        Page 245
        Page 246
    Literature cited
        Page 247
        Page 248
        Page 249
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    Biographical sketch
        Page 267
        Page 268
        Page 269
Full Text













SOIL FERTILITY AND PRODUCTIVITY ASPECTS OF ALLEY
CROPPING LEUCAENA LEUCOCEPHALA AND CASSIA
SIAMEA UNDER SEMIARID CONDITIONS
AT MACHAKOS, KENYA














BY

BASHIR JAMA ADAN


A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE
UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


1993


































To my late father, Jama Adan Derow, who passed away in

November 1992, while I was in Gainesville writing this

dissertation.














ACKNOWLEDGMENTS

The author would like to express his sincere thanks to

Dr. P.K.R. Nair for his constant advice, patience,

sincerity, and moral support during the course of this

project. This research certainly would have been more

difficult if it were not for the support and encouragement

that Dr. Nair extended on many difficult occasions. The

author also would like to thank the other members of the

supervisory committee, Drs. H.L. Gholz, C.P.P. Reid, P.E.

Hildebrand, and H.L. Popenoe. Special thanks are due to Dr.

M.R. Rao of the International Centre for Research in

Agroforestry (ICRAF), Nairobi, for his field supervision,

support and suggestions regarding the dissertation. In

addition, thanks are due to Peter Kurira Wambugu, manager of

ICRAF Research Station, Machakos, Kenya, for supervising

field activities and data collection, especially when the

author was away in 1989-1990.

Appreciation is extended to ICRAF, the Ford Foundation

and the Rockefeller Foundation, who have provided financial

support for this research; and the National Dryland Research

Station (Katumani), Kenya, which has provided laboratory

facilities; and to ICRAF once again, for granting leave-of-

absence to the author during the study period.


iii









Special thanks are given to Richard Coe of ICRAF and

Steven Linda, Department of Statistics, University of

Florida, for their suggestions and assistance in the

analysis of the experimental data. Thanks are also due to

forestry/agroforestry students Paramu Mafongoya, Todd

Johnson, Reinhold Muschler, Paul Campell, Loice Omoro,

Mutiah Govindarajan, Don Mee, Mark Follis, Chris Latt,

Antonio Brunori, and Anne Todd-Bockerie and to Christine

Kelly of agronomy department.

The secretarial services provided by Marie Kimenye of

ICRAF during the period of field research are highly

appreciated. Furthermore, appreciation is extended to Pam

White for her assistance in the preparation of this

manuscript.

Finally, the author wishes to express his deepest

appreciation for the support, understanding and sacrifice

made by his wife Fatuma (Nimoo), daughters (Aisha and

Khadija), father (who passed away in the while the author

was writing-up the dissertation--"rahimuallah"), mother,

brothers, and sisters.
















TABLE OF CONTENTS

Pace

ACKNOWLEDGMENTS. .. . . . ii

LIST OF TABLES . . ...... vii

LIST OF FIGURES . . ..... . x

ABSTRACT . . . . . . xv

CHAPTERS

1 INTRODUCTION .. . . .. 1

2 PRODUCTIVITY ASPECTS OF ALLEY CROPPING
WITH LEUCAENA LEUCOCEPHALA AND CASSIA SIAMEA
IN SEMIARID TROPICS AT MACHAKOS, KENYA 11

Introduction . . .. 11
Materials and Methods .....15
Results. . . . 30
Discussion . . . 57
Conclusions . . 73

3 SOIL FERTILITY ASPECTS OF ALLEY CROPPING
WITH LEUCAENA LEUCOCEPHALA AND CASSIA
SIAMEA IN SEMIARID TROPICS AT MACHAKOS,
KENYA .. . . . .... 75

Introduction . . . 75
Materials and Methods . .. 78
Results . ... . 83
Discussion . . . 86
Conclusions . .. . 95

4 DYNAMICS OF SOIL FERTILITY CHANGES: RATES OF
DECOMPOSITION, N-MINERALIZATION AND UPTAKE
BY MAIZE FROM MULCH OF LEUCAENA LEUCOCEPHALA
AND CASSIA SIAMEA UNDER SEMIARID CONDITIONS
AT MACHAKOS, KENYA . .. .. 97

Introduction . . . 97
Materials and Methods .. .. 100
Results . . . 110
Discussion . .. . .. 132
Conclusions . . .. 145




















5 INTERACTIONS BETWEEN MAIZE AND HEDGEROWS
OF LEUCAENA LEUCOCEPHALA AND CASSIA SIAMEA
IN ALLEY CROPPING SYSTEM IN SEMIARID TROPICS
AT MACHAKOS, KENYA . . .


Introduction . . .
Materials and Methods . .
Results . . .
Discussion . . .
Conclusions . . .

6 POTENTIAL FOR FARMER ADOPTION OF ALLEY
CROPPING OF MULTIPURPOSE TREES AND SHRUBS
IN SEMIARID AREAS OF MACHAKOS, KENYA .

Introduction .. . .
Study area and methodology . .
Results . . .
Discussion . . .
Conclusions .. . .

7 SUMMARY AND CONCLUSIONS . . .

LITERATURE CITED . . . . .

BIOGRAPHICAL SKETCH . . . .


S 147
S 149
S 158
S 183
S 195



S 197

S 197
S 201
204
S 226
S 238

S 241

S 247

S 267


Pace


147
















LIST OF TABLES


Pace


2-1. Mulch yield of Leucaena leucocephala and
Cassia siamea intercropped hedgerows or
planted in sole stands outside the
cropped area . .. ...


2-2. Concentration and amounts of nutrients in
Leucaena leucocephala and Cassia siamea
mulch from alley cropped hedgerows .

2-3. Effect of alley cropping and block planting
systems of Leucaena leucocephala and
Cassia siamea on maize grain yield
during six cropping seasons . .

2-4. Effect of alley cropping and block planting
systems on relative grain yield of maize


. 39


2-5. Land equivalent ratios (LER) of alley cropping
Leucaena leucocephala and Cassia siamea
compared with the block planting system
with same ratios of tree and crop land
occupancy . . . .. 48

2-6. Nitrogen budgets, utilization efficiencies
and yield of maize grown in alley cropping
systems of Leucaena leucocephala and Cassia
siamea at a hedgerow spacing of 6.7 m . 50

2-7. Addition of phosphorus through mulch and
removal by maize in six seasons of cropping
under Leucaena leucocephala and Cassia
siamea alley cropping . . . 52

2-8. Fine root density at two soil depths along a
meter-long transect from 6 m apart hedgerows of
Leucaena leucocephala and Cassia siamea
intercropped with maize .. . 55

3-1. Chemical and physical characteristics of
the top soil (0-20 cm) at the beginning of
the study, October 1987 . . .. 79


vii


r*




r+r


* .










Page
3-2. Chemical characteristics of the dry mulch
of alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea . .. 81

3-3. Chemical and physical characteristics of
the top soil (0-20 cm) under alley cropping
systems with hedgerows of Leucaena leucocephala
and Cassia siamea in November, 1991 . 84

3-4. Changes in soil Mehlich-l P levels after
three years of alley cropping hedgerows of
Leucaena leucocephala and Cassia siamea with
maize, November 1991 . . .. .. 85

3-5. Concentration of phosphorus in the mulch
(leaves plus twigs) of alley cropped
hegderows of Leucaena leucocephala and
Cassia siamea at the start (1987) and
end (1991) of the study .. . 87

3-6. Relative indices of KCl-extract N from
ion-exchange resins incorporated into the
soil of alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea with maize
during the "short-rains" cropping season
of 1991 . . . . 88

4-1. Chemical and physical characteristics of the
mulch of alley cropped hedgerows Leucaena
leucocephala and Cassia siamea . . 106

4-2. Intercepts, slopes, spline points and
coefficients of determination (r2) of the
regression lines for phase one and phase
two of leucaena and cassia mulch
decomposition .. . ... 111

4-3. Decomposition parameters of soil incorporated
and surface placed Leucaena leucocephala and
Cassia siamea twigs placed outside the
litter bags, second season of 1991 . 118

4-4. Effects of soil incorporated Leucaena
leucocephala and Cassia siamea mulch on
the biomass yield, N-concentration, N-yield
and apparent N recovery of maize planted in
pots, 1991 . . . . 128


viii










Pace
4-5. Relative indices of cumulative NH4- N, NO,3 N
and total N mobilized 16 weeks after soil
incorporation of 2 t ha-I equivalents of
Leucaena leucocephala and Cassia siamea
mulch in pots with maize, 1991 . .. 131

5-1. Nitrogen content of 30 day-old maize shoots
(leaves plus stalks) sampled near (0.45 m) and
away (0.45 mm) from hedgerows of Leucocephala
leucocephala and Cassia siamea, fifth season,
1991 . . . . .. 164

5-2. Relative water content of flag leaves of
maize plants sampled near (0.45 m) and
away (3.5 m) from alley cropped hedgerows
of Leucaena leucocephala and Cassia siamea,
on April 26, 1991 (sixth season) . 165

5-3. Water content of whole maize plants sampled
near (0.45 m) and away (3.5 m) from alley
cropped hedgerows of Leucocephala leucocephala
and Cassia siamea on April 26, sixth season,
1991 . . . 166

5-4. Relative water content of flag leaves of maize
plants sampled near (0.45 m) and away (3.5 m)
from alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea leucaena and
cassia on May 28, 1991 (sixth season) . 168

5-5. Relative water content of flag leaves of maize
plants sampled near (0.45 m) and away (3.5 m)
from alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea on January 6,
1992 (seventh season) . . . 169

5-6. Saturated hydraulic conductivity of soil near
(0.45 m) and away (3.5 m) from alley cropped
hedgerows of Leucaena leucocephala and Cassia
siamea, March 1992 (season seven) . 173

5-7. Effects of three nitrogen rates and two
irrigation levels on the biomass yield of
maize after two months growth in pots, 1991 184

6-1. Farm and household characteristics of the 60
farmers interviewed at Machakos, Kenya,
August 1991 . . .. . 205










Pace


6-2. Interests of farmers in alley cropping, farm
size and number of members in a household
at Machakos, Kenya, August 1991 . .. 207

6-3. Factors which influenced significantly
interests of farmers in alley cropping for
fodder or for mulch and the frequency of
farmers under each factor, Machakos, Kenya,
August 1991 . . . . 216

6-4. The p-values of dependency of resource
possession, their use or purchase on the presence
of coffee on the farm, Machakos, Kenya,
August 1991 . . . . 217

6-5. Estimates of maize yield (with or without
inputs) of farmers with interests in fodder
or mulch aspects of alley cropping, Machakos,
Kenya, August 1991 . .. . 219

6-6. Functions of tree nurseries possessed by
30% of the 60 farmers, 1991 interviewed,
Machakos, Kenya, August 1991 .. . 224

6-7. Reasons given by farmers without nurseries
on their farms, Machakos, Kenya, August 1991 225















LIST OF FIGURES


Page
2-1. Seasonal rainfall and the local names
(long-rains and short-rains) of the
cropping seasons during the study . 16

2-2. Mulch yield of Leucaena leucocephala and
Cassia siamea in alley cropping and
block/sole planting systems . .. .. 33

2-3. Effect of three hedgerow spacings on maize
grain yield, averaged over the yields
under the two species, Leucaena leucocephala
and Cassia siamea . . . .. 37

2-4. Effect of sole stands of Leucaena leucocephala
and Cassia siamea planted at three land
occupancy on maize yield during six seasons 38

2-5. Relationship between rainfall and maize mean
yield of all treatments in each of the six
cropping season . . . 42

2-6. Maize grain yield under alley cropping and
block planting systems of Leucaena leucocephala
and Cassia siamea expressed as percent of the
sole maize control yield . . ... 44

2-7. Maize productivity (Kg m"2) under alley cropping
Leucaena leucocephala and Cassia siamea hedgerows
for six seasons . . . . 45

2-8. Productivity of maize (kg mm"1 of rainfall)
under alley cropping of Leucaena leucocephala
and Cassia siamea hedgerows for six cropping
seasons .. . . . 46

2-9. Fine root density in the top 40 cm soil profile
of alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea . . 54

2-10. Effect of three mulch rates of Leucaena
leucoc--yhala and Cassia siamea on biomass
yield of maize grown in pots, 1991 . 56










Pace


4-1. Monthly rainfall and mean temperature during
the study, 1991 . . . 101

4-2. Decomposition patterns of the leaves of
Leucaena leucocephala and Cassia siamea 113

4-3. Decomposition patterns of the twigs of Leucaena
leucocephala and Cassia siamea . 117

4-4. Decomposition patterns of the mulch (leaves
plus twigs) of Leucaena leucocephala and
Cassia siamea . . . . 120

4-5. Cumulative percent of N mineralized from soil
incorporated mulch of Leucaena leucocephala and
Cassia siamea after 16 weeks . .. 123

4-6. Relationship between the patterns of N release
from the mulch of Leucaena leucocephala and
Cassia siamea and biomass yield of maise
from four consecutive crops of maize sown
in pots and harvested at an interval of
four weeks . . .. . ...... 126

5-1. Maize yield response to distance from alley
cropped hedgerows of Leucaena leucocephala
and Cassia siamea, 1990 short-rains . 159

5-2. Maize yield response to distance from alley
cropped hedgerows of Leucaena leucocephala
and Cassia siamea, 1991 long-rains . 160

5-3. Maize yield response to distance from alley
cropped hedgerows of Leucaena leucocephala
and Cassia siamea, 1991 short-rains . 161

5-4. Bulk density of top 5 cm soil profile
near (0.45 m) alley cropped hedgerows of
Leucaena leucocephala and Cassia siamea,
1991 short-rains . . . 170

5-5. Infiltration rates of water into the soil
near (0.45 m) and away (3.5 m) from alley
cropped hedgerows of Leucaena leucocephala
and Cassia siamea, 1991 short-rains . 172

5-6. Soil water content near (0.45 m) alley cropped
hedgerows of Leucaena leucocephala and Cassia
siamea determined at the end of 1991
long-rains .. . .. . 174


xii










Page


5-7. Soil water content near (0.45 m) alley cropped
hedgerows of Leucaena leucocephala and Cassia
siamea determined at the end of 1991
short-rains . . . 175

5-8. Difference in gravimetric water content
(%, w/w) between soil sampled at 0-20 cm
depth near (0.45 m) and away (3.5 m) from
alley cropped hedgerows of alley cropped
hedgerows of Leucaena leucocephala and
Cassia siamea in 1991 . . . 177

5-9. Monthly average readings of tensiometers placed
in the soil at 15 cm depths near (0.45 m) and
away (3.5 m) from alley cropped hedgerows
of Leucaena leucocephala and Cassia
siamea in the long- and short-rain
seasons of 1991 . . . 178

5-10. Difference in mean monthly temperature of
top soil (10 cm) measured near (0.45 m) and
away (3.5 m) from alley cropped hedgerows of
Leucaena leucocephala and in the long- and
short-rain seasons of 1991 . . 179

5-11. Count of fauna in 25 x 25 x 25 cm top soil
monoliths taken from near (0.45 m) and away
(3.5 m) from alley cropped hedgerows of Leucaena
leucocephala and Cassia siamea at the end of
1991 short-rain season .. . 181

5-12. Effects of irrigation and fertilization, in
a combination or alone, on maize grain yield
in the field (no alley cropping with hedgerows
of Leucaena leucocephala and Cassia siamea),
1991 .. . . . 182


xiii














Abstract of Dissertation Presented to the Graduate School of
the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy


SOIL FERTILITY AND PRODUCTIVITY ASPECTS OF ALLEY
CROPPING LEUCAENA LEUCOCEPHALA AND CASSIA
SIAMEA UNDER SEMIARID CONDITIONS AT
MACHAKOS, KENYA

By

Bashir Jama Adan

May 1993

Chairman: Dr. P.K.R. Nair
Major Department: Forest Resources and Conservation

Soil productivity aspects of alley cropping under

semiarid conditions were investigated in a three-year study

at Machakos, Kenya. The objectives were to: (1) determine

if alley cropping with leguminous trees and shrubs improves

soil fertility and productivity, (2) separate the effects of

mulch per se from those of in situ presence of the hedges,

(3) assess the extent of tree-crop interaction patterns, and

(4) determine the major factors that influence farmers'

adoption of alley cropping. The study consisted of Leucaena

leucocephala and Cassia siamea alley cropped with maize (Zea

mays); the woody species were also grown in sole-stand

blocks outside the cropped field, their prunings applied as

mulch to the cropped area in equivalent land areas.


xiv







Difference in maize yield between alley-cropped plots

and the plots receiving mulch from the block plantings

showed that growing the hedges in situ had a positive effect

on maize yield. Yield advantage of alley cropping over the

separate crop and hedge planting systems, expressed as land

equivalent ratios, were 36% and 12% for cassia and leucaena,

respectively. Leucaena produced more biomass (4 t ha' yr-')

than cassia (2 ha-' yr-'); also leucaena had higher fine-root

density than cassia in the top soil (0-40 cm). Therefore,

leucaena was more competitive than cassia, and over the

years, maize yield declined by 12% under leucaena whereas it

increased by 8% under cassia.

For both species, maize nearer the hedgerows yielded

more than those away from them. Soil water content was also

higher near the hedgerows. Chemical and physical properties

of the top soil (0-20 cm) did not differ significantly

between treatments (with or without mulch or alley cropping)

or over time for a given treatment. The quantities of mulch

produced by the hedgerows were generally low, and they

decomposed quickly (half life of four weeks); these could be

the main reasons for the lack of significant changes in soil

properties.

Farmers were generally more interested in the fodder

than the mulch aspects of alley cropping. Because of the

initial high investments of inputs such as labor and

seedlings that required to establish the hedgerows, alley

cropping is more likely to be adopted by relatively









resourceful farmers such as coffee growers than by poorer

farmers.


xvi















CHAPTER 1
INTRODUCTION


In many parts of the tropics, semiarid lands that have

traditionally not been under arable farming are increasingly

being brought under rainfed agriculture. For example, in

Kenya where arid and semiarid lands comprise four-fifths of

the country's land area, arable farming has been extended to

vast areas of dry or marginal lands during recent years.

However, there are some formidable obstacles to food

production in many of these areas. In addition to rainfall

constraints, the soil's productive capacity is inherently

low in many areas. In areas where low level subsistence

farming has been followed, fertilizer use and other means of

nutrient input to soil are seldom practiced. Consequently,

low and declining yields of crops are common. For instance,

maize yields less than 1 t ha'1, even in seasons of normal

rainfall are typical of farms in semiarid areas of Machakos,

Kenya (Nadar and Faught, 1984; Mbogo, 1991).

Under traditional shifting cultivation, which has been

the predominant agricultural system in many areas of the

tropics (both humid and semiarid), long rest or fallow

periods in between short cropping periods have traditionally

been used to improve fertility of soils exhausted by

cultivation (Nye and Greenland, 1960). However, land

1











fallowing results in satisfactory regeneration of soil

fertility only when the length of the fallow is adequately

long. The length of the fallow-period or rest-period

requirement of a soil depends on a number of factors, such

as soil type, climatic conditions and land-use practices

(Young, 1989). For example, on Alfisols in semiarid zones,

it is estimated that fallow periods of one to two years out

of every three years are required even with intermediate

levels of inputs. With the increasing population, fallow

periods have either been reduced drastically or totally

abandoned in many places. Thus, keeping the land in fallow

as a means of sustaining soil fertility is no longer a

feasible management option.

Use of fertilizers is an option considered by many to

increase soil fertility and productivity. Chemical

fertilizers are, however, seldom used by many resource-poor,

tropical, subsistence farmers under conditions as in

Machakos (Ockwell et al., 1991). Fertilizers are expensive

and/or not readily available (Mbogo, 1991). Application of

farmyard manure is another widely adopted practice which, in

fact, is suggested to be the principal source of nutrients

for crops on most farms in Machakos (Probert et al., 1990).

However, the quantity of manure available or applied is

often too low to sustain continuous crop production at

meaningful yield levels (Ikombo, 1984; Ockwell et al.,

1991).











Thus, expansion of arable lands, extended fallows

and/or increased fertilizer use are not viable solutions to

the problem of declining soil productivity and poor crop

yields in most semiarid parts. There is a need, therefore,

for alternative low-input technologies that can intensify

and/or sustain production from existing croplands. One

technology that has been tried with some success in the

humid lowland tropics is alley cropping (also called

hedgerow intercropping). It consists of growing food crops

in alleys between planted hedgerows of multipurpose trees

and shrubs (MPTS), usually nitrogen-fixing ones. The

hedgerows are periodically cut back at the beginning of, as

well as during, cropping. This prevents shading of the

associated crop and the hedge prunings can be a source of

nutrients to the crop (Kang et al., 1990). By retaining

woody perennials in crop production fields on a continuing

basis, alley cropping simulates the role of fallow in soil

fertility regeneration in shifting cultivation. It also

involves, to some extent, the principle of intercropping of

legumes and cereals (e.g., maize and beans), which is a

traditional farming practice in Kenya as well as many other

African countries.

The most distinctive element of agroforestry

practices, and in particular, alley cropping, is the MPTS.

The notion of trees as multipurpose plants is based on their

productive attributes (for example, food, fodder, fuelwood











and poles) and their protective or service functions in

relation, for example, to soil conservation and soil

fertility improvement. Leucaena leucocephala Lam. De Wit

(Leguminosae, Mimosoideae) and Cassia siamea Lam.

(Leguminosae, Caesalpinoideae) are two MPTS widely used in

alley cropping studies in both the humid lowlands and the

semiarid tropics (Nair, 1987; Kang et al., 1989; Singh et

al., 1989). This is probably so because both species are

legumes and it is assumed that soil fertility improvement is

among their potential benefits. (In subsequent references

to the two species, the generic names leucaena and cassia

will be used as common names.)

Leucaena had long been used for shade and soil

improvement in tree and coffee plantations in Java,

Indonesia, as early as 1900 (Dijkman, 1950). Properties

that make leucaena suitable for soil fertility improvement

are summarized in Young (1989). They include: high biomass

production (10-25 t ha" yr-, DM), high nitrogen fixation

(100-500 kg N ha-' yr'1), high levels of nitrogen in leaves

(2.5%-4.0%), and, thus, a high rate of nitrogen return in

litter and prunings. The main soil limitation is a

reduction in its growth on acid soils (Brewbaker, 1987).

Also, attack by the psyllid, Heteropsylla cubana, is

currently serious in some regions. Like leucaena, cassia

has the capacity to grow on poor soils and has been used in

alley cropping trials (Yamoah et al., 1986a), although the









5

extent of its soil-improving potential is little known

compared to that of leucaena. There is even doubt as to

whether it is nitrogen fixing, and established opinion is

that it is not.

In the humid lowlands of Nigeria, many trials have

shown that crop yields could be improved under alley

cropping with MPTS. For instance, an eight-year alley

cropping trial conducted by Kang et al. (1990) showed that

using Leucaena leucocephala prunings only (in the range of

8-10 t ha'1 yr'1), maize yield could be maintained at a

reasonable level of 2 t ha-1, compared to 0.66 t ha-' without

leucaena. Additions of leucaena prunings also maintained

higher soil organic matter than when no prunings were added.

Several other studies in the humid lowlands have also given

favorable results from alley cropping (e.g., Yamoah et al.,

1986a). As a result, there has been considerable interest

in alley cropping, and the technology has been tried in a

number of other places including the semiarid tropics

(SATs). However, the results of alley cropping have not

been positive in all places.

Fundamental to the concept of alley cropping is the

proposition that trees improve soils. There is evidence to

support that soil fertility and, to some extent, crop yields

in semiarid areas could be improved by the presence of trees

on farmlands e.g., Faidherbia albida (synonym: Acacia

albida) in West Africa (Bon Kongou, 1992) and Prosopis











cineraria in India (Mann and Saxena, 1980). The various

mechanisms, demonstrated or hypothesized but not yet fully

proven, are reviewed by Sanchez (1987), Young (1989), and

Nair (1990). Some of the processes demonstrated or

hypothesized by which trees improve soil fertility include

additions of litter, N2-fixation, nutrient uptake from

subsoil and protection from erosion. While trees may

improve fertility of the soil beneath them and the yield of

crops near them, they will also compete with crops for

nutrients and moisture if their root distribution is similar

to and in the same soil zone as that of annual crops (Huxley

et al., 1977; Ewel et al., 1982; Jonsson et al., 1988). For

example, most of the fine, feeder roots of many common trees

are found within the 20 cm top soil (Comerford et al.,

1984). Besides, roots of the tree may be well established

by the time the annual crop is sown. It is this potential

for effective competition, particularly for soil resources,

that has been suggested as the reason for decreased crop

yields under alley cropping in some humid lowlands of the

tropics (Evenssen, 1989; Szott et al., 1990) and semiarid

tropics (Singh et al., 1986; Nair, 1987; Mittal and Singh,

1989).

To minimize the potential for crop yield losses under

alley cropping, there should be benefits of soil fertility

and crop performance that can only be realized if the hedges

and crops are planted together, i.e., benefits that would be












lost if they were grown separately. These include mostly

gradual changes in soil organic matter, nutrient content and

physical properties such as bulk density, water infiltration

and porosity. The presence of the hedges in situ may,

through their root turnover and reduction of soil erosion,

sustain soil organic matter (Young, 1989). This may be of

particular importance in low-input agricultural systems

where all above-ground biomass is removed from the field.

If the contribution of the in situ presence of the

hedgerows to soil fertility relative to that of the mulch is

negligible, then planting the hedges separately from crops

and then transferring their mulch to crop fields would be a

more logical approach than growing the two together. Even

then, an understanding of the relative roles of the quantity

and quality (i.e., decomposition rates) of the mulch

produced would be relevant as this may enhance the

efficiency of its use.

In the humid tropics, mulch yields from the hedgerows

are high (in the range of 8-10 t ha-1 yr1) Also, the rates

of decomposition are rapid. This makes nutrients in the

mulch rapidly available, for instance, in about 30 days in

the case of leucaena (Budelman, 1988; Gichuru and Kang,

1989). So, the mulch, in essence, resembles inorganic

fertilizer that provides a readily available supply of

nutrients to the crop. Crop response when mulch is removed

is therefore observable within a growing season. In the












SATs, unlike in the humid tropics, mulch yields from alley

cropped species are lower than in the humid tropics. For

instance, mulch yields of 2 t ha'1 yr"1 from leucaena is

typical in SATs (Nair, 1987) as opposed to 8-10 t ha-1 yr-1

in lowland humid tropics. As Huxley (1986) postulated, this

low mulch yield in SATs may limit the potential of alley

cropping to improve fertility and productivity of soils in

these regions.

Information on the yields as well as the rates of

decomposition of mulch from alley cropped MPTS (which in

turn affects the ability of the mulch to provide nutrients

to the crops and to contribute to soil organic matter) are

relatively scarce in the SATs. Also, the benefits of the in

situ presence of hedgerows to soil fertility and

productivity are not well understood in either the humid or

the semiarid tropics.

In addition to these technical questions, there is

also the issue of social acceptability of the new technology

by the target farmers. It is necessary to determine how

farmers will adapt and use the technology under conditions

unique to their systems of farming. For instance, in the

semiarid areas where prunings from hedgerows as well as crop

residues are used as fodder (Singh et al., 1986), the in

situ presence of hedgerows may be the main avenue for

improving soil fertility. Studies of this nature will

provide insights into determining the technical feasibility












as well as the management regime of alley cropping to

improve the fertility and productivity of soils in the

semiarid tropics. These technical and social questions were

the prime motivations in undertaking this study. This study

had with four main objectives:

1. To determine if alley cropping with leguminous

MPTS substantially improves soil fertility and

productivity in the semiarid conditions of

Machakos, Kenya.

2. To determine or separate effects due to mulch per

se from those due to the in situ presence of the

hedges within the crop field.

3. To assess the patterns of tree-crop interaction

and to determine factors that may be responsible

for the observed interactions.

4. To determine factors that may significantly

influence adoption of the technology by farmers.

Plant productivity aspects of the crop and hedgerows

of leucaena and cassia are presented in Chapter 2. Chapter

3 addresses the effect of the alley cropping and mulch

quantity and quality on soil chemical, physical and

biological properties; trends over time as well as within a

cropping season are discussed. Patterns of N availability

determined by means of frequent soil sampling and/or in situ

mineralization using ion-exchange resins are also discussed

in Chapter 3.










10

In Chapter 4, studies on decomposition, mineralization

and uptake patterns of leucaena and cassia mulches are

presented. Uptake patterns of N and P from the mulch was

assessed by means of monthly harvests and determination of

nutrient content of potted maize plants. Chapter 5

addresses crop yield profile at the tree-crop interface.

For this, the last three of the seven croppings seasons of

the study were used. Also presented in Chapter 5 are the

results of field and pot studies of water and nutrient

status of soil at the tree-crop interface. These studies

were conducted to explain the observed interaction between

the hedgerows and the crop. Chapter 6 presents results of a

survey of 60 farms and discusses factors that may

significantly influence adoption of alley cropping in the

semiarid tropics.

Finally, Chapter 7 provides a summary of Chapters

1-6. It highlights the major findings of the study and

suggests aspects of alley cropping in the semiarid tropics

with potential for further research.













CHAPTER 2
PRODUCTIVITY ASPECTS OF ALLEY CROPPING WITH LEUCAENA
LEUCOCEPHALA AND CASSIA SIAMEA IN SEMIARID TROPICS AT
MACHAKOS, KENYA


Introduction

Alley cropping, also called hedgerow cropping, is an

agroforestry technology where annual crops are grown in the

alleys between rows of trees/shrubs that are regularly

pruned and maintained as low hedges. The prunings from the

hedgerows, when applied to the crops in the alleys, are

reported to improve or sustain crop yields. This is

suggested by many studies on cropping from the humid lowland

tropics. For instance, Kang et al. (1985) reported from

southern Nigeria that, on an Alfisol, using leucaena

prunings only, maize yield could be maintained at a

"reasonable" level of 2 t ha-' as against 0.66 t ha"'

without leucaena prunings and fertilizer. An extensive

review by Kang et al. (1990) of other such studies from the

humid tropics showed similar results. Several other studies

also show that alley cropping can sustain soil nutrient

status and minimize organic matter decline in humid to

subhumid regions (Lal, 1989; Yamoah et al., 1986b).

Typically, alley cropping studies involve use of

leguminous trees/shrubs. This is because nitrogen

availability is commonly (though by no means exclusively) a

limiting factor in tropical agriculture. -It is also the

11













CHAPTER 2
PRODUCTIVITY ASPECTS OF ALLEY CROPPING WITH LEUCAENA
LEUCOCEPHALA AND CASSIA SIAMEA IN SEMIARID TROPICS AT
MACHAKOS, KENYA


Introduction

Alley cropping, also called hedgerow cropping, is an

agroforestry technology where annual crops are grown in the

alleys between rows of trees/shrubs that are regularly

pruned and maintained as low hedges. The prunings from the

hedgerows, when applied to the crops in the alleys, are

reported to improve or sustain crop yields. This is

suggested by many studies on cropping from the humid lowland

tropics. For instance, Kang et al. (1985) reported from

southern Nigeria that, on an Alfisol, using leucaena

prunings only, maize yield could be maintained at a

"reasonable" level of 2 t ha-' as against 0.66 t ha"'

without leucaena prunings and fertilizer. An extensive

review by Kang et al. (1990) of other such studies from the

humid tropics showed similar results. Several other studies

also show that alley cropping can sustain soil nutrient

status and minimize organic matter decline in humid to

subhumid regions (Lal, 1989; Yamoah et al., 1986b).

Typically, alley cropping studies involve use of

leguminous trees/shrubs. This is because nitrogen

availability is commonly (though by no means exclusively) a

limiting factor in tropical agriculture. -It is also the

11










element which more than any other is under biological

regulation in natural ecosystems (Swift and Sanchez, 1984).

Nitrogen available to crops in alley cropping may be

enhanced through processes of biological fixation of

atmospheric N (Roskoski, 1986; Dommerques, 1987) and

subsequent release through processes of nodule sloughing

(Sanginga et al., 1986) or excretion of nitrogen from the

root nodules (Brophy and Heichel, 1989; Wacquant et al.,

1989). However, the significance of.the latter process

remains doubtful (Vallis, 1978).

Prunings from the hedgerows are the main source of

nutrients to the crops. This is evident from the

significant declines in crop yield that occur when the

application of mulch is withdrawn (Kang et al., 1981). The

extent of supply of N from the mulch to the crop would

depend, among other factors, on the rainfall conditions of

the site, the soil, the hedgerow species, and management

practices such as spacing and the pruning regime. In the

humid lowlands of West Africa, Kang et al. (1981) and Duguma

et al. (1988) reported a wide range of N yields of about

150-560 kg N ha-1 yr-1 for alley cropped leucaena. Similar

information from the semiarid tropics is scarce. A major

exception is the work of Lulandala and Hall (1990) in

Tanzania that noted nitrogen returns through frequent

pruning of leucaena hedges could be as high as 88 kg ha-'

yr-.









13

One of the impacts of the above-mentioned studies has

been to stimulate interest and studies on alley cropping in

the SATs. In many areas of SATs, soil fertility is low

(Steiner et al., 1988) and chemical fertilization alone

(i.e., no irrigation) could improve yields (Ludwig, 1987).

Fertilizers are, however, expensive for many resource-poor

farmers and/or are not readily available.

In many regions, and in particular the SATs, the

prunings are more likely to be used for fodder than as mulch

(Singh et al., 1986; Ong et al., 1991). Even if the

prunings are used as mulch for the crops, the effects on

crop yields may not be significant (Singh et al., 1989).

Under these conditions, the N contribution by the hedgerows

to the soil may primarily be through below-ground processes,

particularly where N fixing species are used. Whether a

significant amount of N can accrue through these processes

and whether this N is available to the companion crop have

not been convincingly established (Ta et al., 1986; Danso et

al., 1987; Rao et al., 1987).

If growing the trees within the crop fields is not

beneficial for the crops but the trees can provide other

valuable benefits (e.g., fodder), separate planting of trees

might be preferable. This would be particularly appropriate

in the SATs where many studies report significantly reduced

crop yields under alley cropping (Singh, et al., 1986;

Mittal and Singh, 1989; Nair, 1987). Under these











conditions, the trees could be grown separately from the

crops, and their biomass harvested for use as fodder or

mulch in a "cut and carry system."

The magnitude of the effects of in situ planting of

the hedgerows would be influenced significantly by

differences in the rooting patterns and densities of the

hedgerow species. Trees with high root densities (expressed

as cm cm'3 of soil volume) in the rooting zone of the crops

are likely to result in less positive in situ benefits to

crops than trees with low root densities. Also, for a given

species, between-row spacing of the hedges, or put in other

words, the ratio of tree:crop land occupancy, could modify

the effects of in situ planting. Because of the increased

chances of competition, hedgerows with narrow between-row

spacings would be more likely to have less positive in-situ

effects on crops than widely spaced hedgerows. Narrowly

spaced hedgerows may, on the other hand, produce higher

quantities of biomass and provide greater scope for nutrient

recycling than widely spaced hedgerows.

From the foregoing, it is apparent that an

understanding of the in situ versus mulch benefits of

growing hedgerows in crops is necessary. Additionally, it

is important to have an understanding of the rooting

densities of species involved. The objective of this study

was to separate the effects of mulch from those of in situ

planting of the hedgerows on productivity (biomass and crop











yields) in alley cropping systems of Leucaena leucocephala

and Cassia siamea in semiarid Machakos, Kenya.



Materials and Methods

Site Description

The study was carried out under rainfed conditions at

the Field Station of the International Centre for Research

in Agroforestry (ICRAF) located at Machakos, Kenya (latitude

1 N 33' S, longitude of 370 14' E and 1596 m elevation).

Mean annual temperature is 19" C and annual rainfall ranges

from 400-1100 mm with a mean of 750 mm. This rain falls in

two seasons in almost equal amounts. There are two rainfed

cropping seasons for the principal agricultural crops. The

first rains of the year during March-June are locally

referred to as the long-rains; the second rains of the year

during October-January are referred to as the short-rains.

Of the two, the short-rains are more reliable than the long-

rains at Machakos. The seasons and the monthly rainfall

during the study period are presented in Figure 2-1.

Generally, monthly rainfall equals or exceeds

evapotranspiration only in April, May and November.

The soil at the study site is texturally well-drained,

dark, reddish-brown sandy clay, derived from the basement

complex gneiss. According to the Kenya/FAO (Food and

Agriculture Organization of the United Nations), the soils

are Luvisols, including the ferric and eutric forms (or























300

LR = Long-rains
250 SR = Short-rains


-200
E

S150
'0-
'(5
S100


50


SI -------I I ----------- I----



1988 1989 1989 1990 1990 1991 1991
SR LR SR LR SR LR SR
Month and cropping season


Figure 2-1. Seasonal rainfall and the local names
(long-rains and short-rains) of the cropping seasons during
the study.









17

Kandic Rhodustalfs according to the United States Department

of Agriculture Soil Taxonomy). The soils of the research

station are in general weakly to moderately leached, with a

weakly acid reaction (pH [water] 6.0-6.5), a medium base

saturation (50-80%), low to moderate levels of organic

matter (top soil organic carbon: 1%-1.5%), nitrogen (N)

(0.08%) and phosphorus (P) (<16 ppm). Other nutrients are

reportedly present in adequate levels to support crop

production (Kibe et al., 1981).

Clay and sand content in the top 15 cm averaged 28%

and 67%, respectively, in 1987. Kaolinite is the

predominant mineral in the clay fraction. The top soil had

an effective cation exchange capacity of 9.0 meq per 100 g

of soil at the initiation of the study in October 1987.



Methods


Field study

Experimental layout.--Hedgerows of Leucaena leuco-

cephala and Cassia siamea were established in October 1987.

Alley widths of the hedgerows were 4, 5 and 6.7 m. In-row

spacing of trees within the hedgerows was 0.5 m. Alley

widths used were computed based on field observations (Nair,

1987) that 3-4 year old hedgerows, spaced 4 m apart and

pruned frequently to low heights, occupied 25% of total

(aboveground) land area. Similarly, the land occupancy

ratio by the hedgerows spaced 5 and 6.7 m would be 20% and











15%, respectively. Sole stands of leucaena and cassia

occupying 15%, 20% and 25% of crop land were also

established for comparison to the hedgerow intercropped

plots. In-row spacing of the trees within the block/sole

stands of hedgerows was 1.0 m by 1.0 m. Control plots

included in the experiment were (a) maize crop which

received mulch only in amounts equivalent to what was

produced by the hedgerow intercropped plots and, (b) maize

crop which received no mulch but fertilizer only at the

rates of 40 kg and 17 kg of N and P ha' season-'.

The experimental design was split-plot, replicated

three times. Main plot treatments, randomized within the

blocks, were the hedgerow species (i.e., leucaena or

cassia). Subplot treatments were the factorial combination

of the three ratios of crop to tree land occupancy and the

control plots with mulch-only. The fertilizer-only control

plot was included in each main plot (i.e., 2 replications

per block). This gave 6 replications for the fertilizer-

only control. Main plot size was 20 x 50 m, with two meter

borders separating sub-plots of 5 x 20 m. Between blocks, a

4-meter wide path was provided. Sole stands of leucaena and

cassia hedges which provided mulch for the mulch-only

control plots were also included in each block.

Initially, a control plot with no inputs was

inadvertently left out. The control plot of an adjacent

experiment, established about the same time as this study









19

was used instead. Nevertheless, since this control plot was

outside the setting of the present study, it was not

included in the statistical analyses. Rather, it was used

only for relative comparisons.

Root studies.--Root studies were initiated in the last

cropping season of the study with the objective of

identifying the cause of differences in the effects of the

two hedgerows species on maize yield. These studies were

conducted on hedgerows outside the main experiment

consisting of two blocks of cassia and leucaena hedgerows,

6 m wide, established in 1983. Until 1988, maize and beans

were sown in rotation in the alleys between the hedgerows in

these plots. The maize crop was fertilized with N and P at

the rate of 40 and 18 kg ha' season-'; the beans were not.

In addition, prunings from the hedgerows were applied to the

cropped alleys. In 1988, fertilizer application was stopped

and maize was sown in both seasons.

For each species, blocks of soil 20 x 20 x 20 cm were

removed from six transects 3 meters long from the base of

the hedgerows during the 1992 short-rains cropping season.

The soil blocks were taken from 0-20 and 20-40 cm soil

depths. The samples were brought back to washing sheds in

plastic bags. The soil blocks were then washed with a spray

of water aided by hand manipulation to separate medium-sized

and fine roots from the soil in sieves with mesh sizes of 5

mm and 0.5 mm, respectively. Roots less than 2 mm in











diameter were classified as fine roots, those of 2-5 mm as

medium-sized or small roots (Bohm,.1979).

The fine roots of leucaena and cassia were

distinguishable easily from maize roots based on color and

texture. Maize had whitish and succulent roots, cassia had

black roots, and leucaena had fine, cream-colored roots,

which were strong relative to cassia. The majority of the

fine roots of the two species (which were planted

separately) and the maize roots were sorted in the same way.

Some were hard to differentiate and were separated under a

magnifier. Dead roots with brittle steel and degenerated

epidermal and cortical tissues were not included.

Root length was determined by the line-intercept

method of Tenant (1975) using a tray with grid units of 1.3

cm (half-inch). With these grid units, the total number of

intercepts equals the total length of root in centimeters.

Crop and tree management.--The hedgerows occupied 15%,

20% and 25% of the gross land area. The remainder of the

land, viz, 85%, 80% and 75%, was sown to maize at

approximately 37,000 plants ha-' with a spacing of 90 cm

between rows and 30 cm within rows. Katumani Composite, a

drought-tolerant maize variety was used. The crops and

trees were only rain-fed.

Fertilizer was applied at the rate of 40, 18 and 0 kg

ha-' season-' of N, P and K, respectively, to maize in the

fertilizer-only control, alley cropped maize and maize next












to the block/sole planted hedgerows. The mulch-only plots

received no fertilizer.

The hedgerows were pruned 2-3 times in a cropping

season (depending on the rainfall conditions of the season)

to 50 cm height aboveground. First pruning was done in

November 1988, this being the first of the seven cropping

seasons with the treatments applied. At each pruning of the

hedgerows, which typically took place 1-2 weeks before the

onset of rains, the biomass harvested (hereafter referred to

as mulch) was determined. It was then chopped into small

pieces, samples taken for dry matter determination and the

rest spread on the field. Typically, mulch harvested before

the onset of rains was incorporated into the soil during

field preparation and planting of the crops. However, mulch

from later pruning during the life of the crop was either

partially incorporated (in the process of weeding the crops)

or generally left on the soil surface. In all crops, the

plots were hand tilled and weeded.

At each harvest of maize, which took place

approximately 120 days after sowing, the yields of the

grain, cobs and stalks were determined. Samples of each

component were oven-dried at 1050C for 48 hours and dry

weight determined. Maize stalks and cobs were not returned

to the plots, in line with the farmers' practice in the

study region where maize stalks are a valuable livestock

feed.









22
In cropping seasons 1-4, yields of all maize rows in a

plot were composite at harvest. However, in seasons 5, 6

and 7, maize yields of each row in a plot was separately

harvested and recorded. Also, in the last three cropping

seasons, sub-samples of the grain, cobs and stover of some

selected treatments, as explained later, were analyzed for

nitrogen and phosphorus concentrations. Nitrogen was

determined by the standard micro-kjeldahl method and P by UV

visible spectrophotometry. In all seasons, two rows of

maize on all four sides of the plot were left as guard rows.



Pot Study

A pot study was initiated in January 1992 to determine

the amounts of mulch that would produce significant effects

on maize yields under a given rainfall condition. The

treatments included a factorial combination of 4 rates of

leucaena and cassia mulch and 2 levels of irrigation. The

four mulch rates were applications equivalent to 0, 2, 4 and

8 t ha'I season-', dry matter. The two irrigation levels

were the equivalents of 200 and 400 mm received in the last

two cropping seasons of the study. The pots were arranged

in a randomized block design with 3 replications in a

transparent polythene-roof house. In each block, pots with

one level of fertilizer at a rate equivalent to 40 N and 17

P kg ha-1 season' were also included.











Soil put into the pots (approximately 20 kg) was

collected from three randomly selected positions near (0.45

m) and away (3.5 m) from the alley cropped plot of the 6.7 m

wide hedgerows. The amounts of irrigation water applied to

the pots were based on the difference between soil water

storage at field capacity (15%) and wilting point (7.8%)

determined at the initiation of the study. The plots were

irrigated twice weekly.

Three seeds of maize were sown in each pot. Three

days after germination, the seedlings were thinned to one

per pot. Total plant harvest (i.e., shoot plus roots) was

done after four weeks. After oven drying at 650C for 48

hours, dry mass was determined. On the same day as the

harvest, the pots were resown for another four weeks. This

repeated sowing and harvesting from the pots was performed

for a total period of 16 weeks.


Yield Computation--Crop and Hedge
Biomass from Field Study

Biomass and maize yields of the hedgerow planting

systems (expressed in t ha'-, dry weight) were computed to

include area occupied by both the hedgerows and the crop.

Hence, the yields are expressed on gross as opposed to net

area basis or area occupied by crop only. In situ effects

of the hedgerows were calculated as the difference in mulch

and crop yields of the systems where hedgerows were












intercropped and where they were planted in sole/block

stands outside the crop area.

To estimate in situ effects of the intercropped

hedges, the block-planted hedgerows should not affect the

adjacent crop. In an on-farm situation, the block-planted

trees could be on land not fit for cropping that is at some

distance from the crop land. In on-station situations,

distant planting may not be feasible, especially under

conditions where treatments are randomly located in small

plots. Two approaches, polythene sheets vertically inserted

into the soil to 50 cm depths (Ong et al., 1991) and

trenching (Verinumbe and Okali, 1985) have been used to

minimize effects of adjacent rows or blocks of trees on

crops. The effectiveness of these approaches depends on the

depths of insertion of the polythene sheets into the soil or

of trenching. Tree roots can go deep below the polythene

sheet or the trench and resurface (Corlett, 1989; Ong et

al., 1992).

In the present study, row-by-row harvest of the crop

of the widest alley (6.7 m) was used to determine the

distance of influence of the block-planted hedgerows on the

yield of the adjacent crop. A graphical plot of the yield

of the rows against the distance of the rows from the

hedgerows was then made. From this plot, the rows of maize

influenced by the block-planted hedges were determined and

discarded. The yields of the remaining rows were used to










25
calculate the per hectare yield of maize planted next to the

block planting system of hedges outside the crops.

In the section on results below, only yields of the

second season to the sixth season crops are discussed. The

yield of the first season was excluded because the hedgerows

were pruned only once (since they were not yet well-

established). In the other seasons the hedgerows were

pruned 2-3 times. Also, the crop yield of the first season

crop was at least twice as much as the yield of the other

seasons, ascribed more to the initial site conditions than

to the effects of treatments. Detailed presentation and

discussion of the biomass and crop yields are limited to the

6.7 m wide alley in the block planting system. This spacing

of the alleys and ratio of tree:crop land occupancy offers

the most land for cropping. This is of practical

significance to farmers.


Trends in Maize Yield over Time

Trends in maize yield over time were analyzed by

expressing the yield of each treatment in a season as a

percent of the average yield of all treatments of the second

crop. The patterns of maize yield over time were also

examined by expressing the yields of the two maize planting

systems as a percent of the yields of the fertilizer-only

control treatment.












The productivity of alley cropping compared to the

sole planting system of the hedgerows and the crop was

assessed by calculating land equivalent ratios (Willey,

1979) based on grain and total biomass yield of grain, stalk

and the hedgerows. The block planting and alley cropping

systems with the same ratio of land apportioned to tree and

crop (15:85) was used.



Nutrient Budget of Nitrogen and Phosphorus

A simple budget for N and P was computed for selected

treatments during the last three cropping seasons of the

study. It was only in these seasons that nutrient content

of the crop was regularly determined. The N budget was

calculated by considering N applied through the mulch or as

fertilizer, N removed in crop (grain, cob and stover), and N

still on the plot in mulch not decomposed. The P budget was

similarly calculated except that P remaining in mulch not

decomposed was not computed because sufficient amounts of

mulch did not remain for determination of both N and P.

Nitrogen and P removed from the plots were calculated

by summing the N and P removed by the crop (i.e., grain,

cobs and stover) for the last three seasons of the study.

Since N and P data for the crop were available for only

three out of the seven crops, regression equations relating

N and P removed in crops 5-7 to yields in these seasons were

determined. These equations were then used to calculate N











and P removed in crops of seasons 2-4. The regression

equations were obtained first by multiplying the N and P

concentrations of the crop (grain, cobs, stover) by their

respective yields to obtain N and P yields of the crop. The

N and P values were then regressed against the crop yields

which resulted in the following equations:

N(kg) grain yield = 0.015 grain yield 0.0006; r =0.98

N(kg) cob yield = 0.0028 cob yield + 0.0002; r2=0.95

N(kg) stover yield = 0.0056 stover yield + 0.0008; r2=0.91

Similarly, regression equations for P yield in grain, stover

and cob yields are as follows:

P(kg) grain yield = 0.0018 grain yield + 0.0002; r2=0.93

P(kg) stover yield = 0.0021 stover yield -0.001; r2=0.92

P(kg) cob yield = 0.0005 cob yield 0.00002; r2=0.96

Nitrogen remaining in mulch not decomposed was calculated

using the N concentration of the soil-incorporated mulch at

the end of the study in March 1992. Typically, N remaining

in mulch is calculated using the decomposition constant of

the mulch (Palm, 1988). However, where the pattern of

decomposition of the mulch has more than one phase as was

the case in the present study (Chapter 4), use of one

decomposition constant for both phases may not be

appropriate. Nitrogen remaining was calculated for each

season's mulch input into the soil and then summed over all

the seasons.












Apparent nitrogen utilization efficiencies were

calculated for crops seasons 2-6 and includes grain, cobs

and stover. The efficiencies were calculated by determining

the nitrogen yield of the treatments where mulch was applied

to the crop (dry matter x nitrogen concentration) less

nitrogen yield of the no fertilizer control treatment and

expressing this as a percentage of the amount of nitrogen

via the mulch or the fertilizer. This method has also been

used by others to estimate utilization efficiency of mulch N

by crops (Kang et al., 1981; Yamoah et al., 1986b; Palm,

1988).


Statistical Analyses

The effects of the two species and systems of hedge

planting on crop and biomass yields were determined using

the General Linear Models (GLM) procedure in the Statistical

Analysis System (SAS, 1992). Because of the unbalanced

design caused by the fertilizer-only treatment (which was

applied at only one level as opposed to other treatments

which were applied at three levels), the analysis was

performed in two stages. First, the fertilizer-only

treatment was omitted and the analysis of split-plot design

was performed. Second, depending on what factors were

significant in the split-plot analysis of variance (ANOVA)

test, comparisons with controls were performed in a

subsequent ANOVA that included all treatments. In both











analyses, differences between means of treatments were

determined by pairwise contrasts of the Least Squares Means

(LSM). A statistically significant difference was declared

to exist between means of any pair of treatments when the p-

value was less than or equal to 0.05.

Yield response of the systems over seasons was also

analyzed with the GLM/SAS procedure. The option for

repeated measures ANOVA was included in the GLM procedure to

account for the systematic effects of harvests from the same

plots over time. The difference in mean yield of treatments

between seasons was determined by pairwise contrasts of the

LSM.

The GLM/SAS procedure was also used to analyze the

root density data. The option for repeated measures

analysis was included to account for the effects of "fixed"

distances of root observations from the hedgerows. The

significance of differences between means of root densities

at various distances from the stumps of the hedgerows was

determined by pairwise contrasts of the least squares means.

In the pot studies also, the GLM/SAS procedure was

used to analyze the data. As in the field study, the option

for repeated measures was included in the ANOVA to account

for the effects of repeated harvests from the same pots. In

addition, orthogonal polynomial contrasts for linear,

quadratic and cubic effects were included in the GLM

procedure to test significance of the relationships between










30

irrigation levels, mulch species and mulch rates. Pairwise

comparison of LSM was used to test the significance of

differences between treatment means. Dunnet's test, which

controls the Type 1 experiment error for comparisons of all

treatments against a control, was used to determine

significance of differences between the mulch rates and the

control pots that had only one level of fertilizer

application.

Results


Field Study


Mulch yield

Leucaena, intercropped in hedgerows, produced

significantly higher mulch (+25%) than when the species was

planted in sole/block stands (Table 2-1). For cassia,

however, the +14% observed difference was not significant.

Mean mulch yield of leucaena, whether it was

intercropped or planted in sole/block stands, was

significantly higher (+46%) than that of cassia (Table 2-1).

Between the three hedgerow spacings of the intercropped

system, mulch yield of the 4 m hedgerows was significantly

(p = 0.04) higher (on average, 21%) than that of the other

two hedgerow spacings. The mulch yields of 5 and 6.7

m-wide hedgerows were not significantly different

(p = 0.23).









31


Table 2-1. Mulch yield of Leucaena leucocephala and Cassia
siamea intercropped hedgerows or planted in sole
stands outside the cropped area.


t ha-I vyr-
Hedgerow Alley Block
Species Cropping Planting* Mean


Leucaena 4.4" 3.3b 3.9a

Cassia 2.2a 1.9' 2.1b

Mean 3.3 2.6 3.0


*Block planting in an area equal to that estimated to be
occupied by the hedgerows in an alley cropping system.

abFor a given species, means followed by the same letter are
not significantly different (p = 0.05).












In the first year, there was no significant difference

between mulch yields from the two species, but after the

second year, leucaena yields were significantly higher than

those of cassia (Fig. 2-2). Also, a significant difference

was observed in the response of the two species to rainfall.

While changes in rainfall had a significant effect on

the mulch yield of leucaena, no effects were observed for

cassia. Mulch yield of leucaena peaked in 1990 when the

annual rainfall was highest (964 mm) during the study

period. In 1991 when rainfall dropped to only 22% of the

1990 total (Fig. 2-1), mulch yield of leucaena dropped by

half. For cassia, there was no significant response (Fig.

2-2). Cassia mulch yield tended to decline over time,

although the differences in yield between years were not

significant. Although the effect of rainfall on mulch yield

was more pronounced than the effect of age of the hedgerows,

given the same rainfall, mulch yield of leucaena did

increase with age. For instance, with similar seasonal

rainfalls total of 330 mm, mulch yields of leucaena

hedgerows were 40% more in the fifth season than in the

second season. Similar trends of increase in cassia mulch

with age were not detectable.

Where hedgerows were planted as sole stands/blocks

outside the crop fields, differences in mulch yields per

hectare among the three tree:crop ratios were not
























3.5 Leucaena
Intercropping
13.0 Leucaena
3 / Sole
D Cassia
2.5 b Intercropping
Cassia
Sole
2.0 -
Yea

1.5 b


S1.0
C
C

0.5



1989 1990 1991
Year





Figure 2-2. Mulch yield of Leucaena leucocephala and
Cassia siamea in alley cropping and block/sole planting
systems.










34

significant. However, leucaena always yielded significantly

more mulch than cassia.

A count of all plants in the three replicates in the

last season of cropping showed that cassia had a higher

percentage of dead plants than leucaena (using the number of

seedlings planted in 1987 as a base). In cassia, up to 5%

( 7% standard error) of the intercropped hedges and 11.6%

( 15% standard error) of the block planted plants were

dead. In contrast, no plants were recorded as dead in

either leucaena systems. Cassia's mortality was largely

attributed to a commonly observed stem borer, Xylentes

capensis (Cassidae) and a wood boring beetle, Xyloposus sp.

(Bostrychidae).



Nutrient concentrations and contents of the mulches

In addition to a supply of N and P, the mulch provided

substantial quantities of other nutrients to the soil (Table

2-2). While concentrations of the other nutrients remained

stable over time in both species, the concentration of P

increased considerably. For instance, the levels of P in

cassia mulch increased 25% from 1987 to 1991 (compare 0.20

to 0.25%).



Crop yields

The combined analyses of the data over seasons

indicated that there were significant interactions between













Table 2-2.


Concentration and amounts of nutrients in Leucaena
leucocephala and Cassia siamea mulch from alley
cropped hedgerows.


Nutrient concentration (%) and amounts (kg
ha" yr", in parenthesis)


Species N P K Ca Mg S


Leucaena 3.5" 0.2" 1.97 0.7" 0.3' 0.1"

(136.0) (7.5) (71.7) (26.4) (11.3) (3.8)

Cassia 2.9" 0.3" 1.8" 0.7' 0.2' 0.1a

(54.7) (5.7) (33.7) (13.2) (3.8) (1.9)


"Within a column, values of nutrient concentration with
different letters are significantly different (p = 0.05).
The amounts of all nutrients in leucaena mulch were
significantly higher than those in cassia (p = 0.05).












seasons, systems, and hedgerow species on crop yield.

Therefore, ANOVA by season was necessary in order to

understand the true main effects of species and systems.

The ANOVA by season showed that in four out of the six

(i.e., seasons 2, 4, 5, and 6), alley cropped maize yielded

higher with cassia than with leucaena (Table 2-3). A

significant difference between the yield of leucaena alley

cropped maize and that next to the block planted system of

leucaena was observed in only two out of the six seasons

(the third and the fourth). For cassia, the difference was

also significant in two out of the six seasons (the second

and the third). The difference in maize yield under the two

systems represents the in situ effects of the hedgerows on

crop yield. Averaged over the seasons, the in situ effects

of cassia on maize yields were 22% while those of leucaena

were 9%.

Except in the second season, differences between the

effects of the three hedgerow spacings (i.e., 4, 5, and 6.7

m) on maize yield were not significantly different (Fig.

2-3). In the second season, maize yields under the 6.7 m

hedgerows were significantly higher than those of the other

two spacings. In the block planting systems, significant

differences in maize yields were observed in seasons 2 and 4

only and 4 only (Fig. 2-4).

A comparison of the mean yields of all the treatments

(Table 2-3, last column) shows that the fertilized maize




























r--





,.
N
Co


2.5




2.0




1.5


1.0




0.5


RI

-.


3
Cropping


4m


5m


6.7 m
sea&@*


4
Season


Figure 2-3. Effect of three hedgerow spacings on
maize grain yield, averaged over yields under the two
species, Leucaena leucocephala and Cassia siamea. Vertical
bars indicate standard error of difference of means.


1 I 1 I 1









Percent land under tree and crop

I 25:75


20:80



15:85

. 111s.


1 2 3 4 5 6
Cropping Season


Figure 2-4. Effect of sole stands of Leucaena
leucocephala and Cassia siamea planted at three land
occupancy on maize yield during six seasons. Vertical bars
indicate standard error of difference of means.


4.0



3.5



4 3.0







S2.0
'(5 2.0


1.5



1.0











Table 2-3. Effect of alley cropping and
during six cropping seasons.
row.


block planting systems of maize grain yield
Rainfall for each season is included in the last


Maize yield (t ha-) for each season


System 2 3 4 5 6 7 Mean


Alley cropping (leucaena) 2.5a'b 3.0ab 2.3ab 2.3"ab 0.6a,bc 2.3" 2.2ab

Alley cropping (cassia) 2.4"a' 3.9d 2.4ab 3.4a 1.4a 2.8c 2.7c

Block planting (leucaena) 2.5" 2.7b', 2.0c 2.6b 0.5c 1.7b 2.0"

Block planting (cassia) 2.3ab 2.9b', 2.0 2.9ab 0.9a,b, 1.5b 2.1ab

Control (leucaena mulch) 2.0ab 2.6b,c 2. 2a,bc 1. 6d 0.8a'b,c 1.7b 1 .b

Control (cassia mulch) 1.9ab 2.3" 2.1a,b,c 2.1cd 0.7ab,c 1.8b 1.8"

Control (fertilizer) 1.6b 3. l,b 2.9a,b 3.1ab 1.3a,b 2.7a 2.5c

Control (absolute)* 2.0 2.9 1.8 2.1 1.1 2.4 2.1

Mean 2.1 2.8 2.1 2.4 0.9 2.0 2.1

Rainfall (mm) 330 441 631 333 214 373 336


"'bcdWithin a season, values followed by different letters are
(p = 0.05).

*From an adjacent experiment.


significantly different











(control) yielded significantly higher than all the other

treatments except the cassia alley cropping system. Lack of

major differences between maize yields of the various

treatments across the seasons of the study is evident from

Table 2-3. In general, maize yield was positively related

to the rainfall patterns. However, in the third season, in

spite of the relatively high rainfall (631 mm), the average

of all treatments was only 2.2 t ha'1 which was similar to

the yield obtained in some other seasons with lower rainfall

(e.g., seasons 1 and 6 with rainfall of 330 mm and 373 mm,

respectively). The low yield with 631 mm rainfall could be

ascribed to excessive soil loss and runoff that also caused

physical damage to the crop; about half the rainfall was

estimated to have been lost in the runoff (data not

presented).


Maintenance of maize yield over time

Trends of maize yield over time were analyzed by

expressing the yield of each treatment in a season as a

percent of the average yield of all treatments of the second

crop (Table 2-4). The yield of all treatments generally

remained stable over seasons, including that of the control

with no inputs. Yield increased or declined mainly in

direct response to the season's rainfall, with a linear

relationship between rainfall and yield (Fig. 2-5). This














Table 2-4. Effect of alley cropping block planting systems on
relative grain yield of maize.


System/Treatment Season

2 3 4 5 6 7 Mean


Alley cropping
Leucaena 100 124 95 95 25 90 88

Cassia 95 157 95 138 57 114 109

Block planting
Leucaena 119 129 95 124 24 81 95

Cassia 110 138 95 138 43 71 99

Mulch-only
Leucaena 96 124 105 76 38 81 87

Cassia 90 110 100 100 33 86 87

Fertilizer-only 76 148 138 149 62 129 117

Absolute control 96 138 86 100 52 114 98

Mean 98 134 101 115 42 96 98


NOTE: Relative yield was calculated as a ratio of actual to
average yield of all treatments in the second season,
expressed as percent.











3.0





2.5




2-
2.0



C

1.5
N




1.0





0.5
200


250 300 350 400 450


Rainfall (mm)





Figure 2-5. Relationship between rainfall and maize
mean yield of all treatments in each of the six cropping
seasons. Numbers on the regression line refer to the
cropping seasons.


500










43
suggests that at this site, soil moisture was more limiting

than differences between treatments.

The patterns of maize yield over time were also

examined by expressing the yields of the two maize planting

systems as a percent of the yields of the fertilizer-only

control treatment (Fig. 2-6). Averaged over seasons, maize

yield declined by 12% per season under leucaena alley

cropping, with the largest decline in the third year (season

6). In contrast, maize yield under cassia alley cropping

increased by 8% per season. In the second year (season 6),

when yield under leucaena declined by 45% compared to the

control, yield under cassia was 27% higher than the control.

The increase in maize yield under cassia, in spite of the

hedgerows occupying land, suggested that productivity of

maize (kg m-2) under cassia alley cropping was higher per

unit net (cropped) area than of the sole crop (Fig. 2-7).

Most of the reduction in maize yield and in productivity

under leucaena, compared to the yield of the sole crop,

occurred in the last three seasons of the study. The

patterns of productivity per unit of land or rainfall were

similar. On average, mean maize productivity per unit of

rainfall under cassia alley cropping was +9% while that

under leucaena was -15% (Fig. 2-8). The increased maize

productivity under cassia alley cropping offsets the 15%

loss in maize yield when hedgerows occupied 15% of the

croppable land.



























Leucaena Cassia Leucaena Cassia
(intercropped) (intercropped) (sole) (sole)
.. = 0 A0


ft-t tftt t-f f-f


1990


1991


Year











Figure 2-6. Maize grain yield under alley cropping
and block planting systems of Leucaena leucocephala and
Cassia siamea expressed as percent of the sole maize control
yield.


140



120



100



80


60


1989

























Leucaena

Cassia
-Conro
Control
--,,I-, .


0.5


N
E 0.4


o 0.3



2 0.2
a,
N
- 0.1



0


Figure 2-7. Maize productivity (kg m-2) under alley
cropping Leucaena leucocephala and Cassia siamea hedgerows
for six seasons. Vertical bars indicate standard error of
difference of means.


1 2 3 4 5 6
Cropping season





























TI


Leucaena
(intercropping)
-a-- *
Cassia
(intercropping)
--A-z -
Maize (sole)
-A-


- -.`j- -"


7/


1 2 3 4 5 6
Cropping season











Figure 2-8. Productivity of maize (kg mm-' of
rainfall) under alley cropping of Leucaena leucocephala and
Cassia siamea hedgerows for six cropping seasons. Vertical
bars indicate standard error of difference of means.


0


E 8
E

6


4
N











Land equivalent ratios (LER)

Alley cropping leucaena and cassia had LER greater

than 1.0 of maize grain yield or the combined yield of the

crop and hedgerow biomass. Averaged across species and

seasons, LER of maize grain yield under alley cropping was

1.24 (or 24% advantage over the maize grain of the sole

planting system of the hedges and crop). No particular

trend in LER of maize yield across seasons was discernible

under both species (Table 2-5). Between the two species,

LER of maize grain yield under cassia (1.36) was

significantly higher than under leucaena (1.12). This

difference was due to the lower crop yield under leucaena

compared to cassia.

When averaged across species, the combined biomass of

the hedgerows and the crop yield (grain plus stalks) had LER

of 1.10 (i.e., an advantage of 10% by alley cropping over

the sole system). The LER of maize grain yield under alley

cropping was greater than 1.0 because of lower yield of

maize yield under the sole system compared to alley cropping

in nearly all seasons. The biomass yield of the hedges

under alley cropping and sole planting systems were similar

when the highest yielding sole planting system was used in

LER computation.












Table 2-5.


Land equivalent ratios (LER) of alley cropping
Leucaena leucocephala and Cassia siamea compared with
the block planting system with same ratios of tree and
crop land occupancy.


Year and Seasons Within a Year
1989 1990 1991

Species 1 2 1 2 1 2 Mean


Leucaena 1.0" 1.09 1.15 .0.87 1.23 1.35 1.12

(1.10) (0.97) (1.15) (1.02) (1.29) (1.26) (1.13)

Cassia 1.04 1.33 1.21 1.17 1.56 1.87 1.36

(0.99) (1.24) (1.00) (0.96) (1.10) (1.15) (1.07)


LER = (Biomass yield of the tree species in alley cropping plus
yield of crop in alley cropping) divided by (biomass yield of the
tree species in sole system plus yield of crop in sole system).
In both the alley cropping and sole planting systems, the percent
of land occupied by the hedges and the crop was 15 and 85,
respectively. In the sole planting system, the biomass yield of
the hedgerows in the 25:75 tree:crop percent land occupancy ratio
was used because this was the most appropriate (i.e., highest
yielding) sole planting system.

aLER of maize grain yield only. Number in bracket is the total
LER, i.e., the grain and the stalk yield of the maize and the tree
biomass which was used as mulch.











Nutrient budgets

Nitrogen.--Generally, leucaena had significantly

higher mulch yield (Table 2-1) and N concentration (Table

2-2) than cassia. As a consequence, the N yield of leucaena

was significantly higher (136 kg N ha" yr-') than that of

cassia (59 kg N ha" yr').

Nitrogen uptake was high and essentially of similar

pattern in all treatments, including the absolute control

(Table 2-6). It was only the fertilizer-control and the

alley cropping system of cassia that had N-yields

considerably higher than the rest of the treatments. This

is because of the relatively high maize yields of these two

treatments (i.e., the fertilizer-only control and the

hedgerow intercropped plots of cassia).

Apparent recovery/utilization efficiency of N applied

through the mulch or fertilizer was low in all treatments

(Table 2-6, columns 7 and 8). The N-recovery values were

low primarily because maize and N-yield of the absolute

control were almost equal to those of the other treatments

with inputs. This suggests that the N needs of the crop

were adequately met by native soil N. Averaged across

treatments, an apparent N-recovery of 9% summed over the N

recoveries through the grain, stover and cobs was observed.

Most of the N (67%) was recovered in the grain component of

the crop and the rest (33%) in the stover and cobs.










Table 2-6.


Nitrogen budgets, utilization efficiencies and yield of maize grown in alley
cropping of Leucaena leucocephala and Cassia siamea, at a hedgerow spacing of 6.7
m for 6 seasons.


N from
mulch Unac- Maize
and/or N-utilization N-remaining count- yield
fertil- N removed during 6 seasons efficiency in mulch ed for (t/ha/
izer kg ha'- (%) N sea-
Treatment (kg) Grain cob Stover Total Grain Total kg ha (%) (%) son)

Absolute
control* 0 179 9 89 277 2.1

Alley
cropping
(Leucaena) 648 186 9 83 278 1.0 0.2 103 16 41 2.2

Alley
cropping
(Cassia) 417 225 9 93 327 11 12 39 9 13 2.7
Mulch-only
(Leucaena) 408 192 10 87 289 3 3 87 21 8 1.8
Mulch-only
(Cassia) 177 190 9 91 290 6 7 33 19 0 1.8
Fertilizer
only 240 222 9 97 328 18 22 0 2.5

"Fertilizer was applied to the alley cropped plots at the rate of 40 and 17 kg ha-' season-' N and
P, respectively.
"Sample calculation using maize grain yield of 6 seasons from alley cropping of cassia: N
utilization efficiency (11%) = N removed in grain yield (225 kg) less N removed in grain yield
of the absolute control (179 kg) divided by total N applied (417 kg) times 100.
cBased on N concentration of the mulch at the end of the litter bag decomposition studies of
mulch incorporated in October 1991 to March 1992.
d100 -(percent of applied N in mulch or fertilizer removed in crop plus N remaining in
undecomposed mulch).


*From an adjacent experiment.











Trends in N uptake were similar to those of the crop

yields, i.e., the higher the yield of the crop, the higher

the N-yield. Because of the lower yields under leucaena

alley cropping compared to those under cassia (Table 2-3),

apparent N utilization efficiency under leucaena was low.

However, in the sole croppings with only mulch application,

apparent N utilization efficiency values from the mulch of

leucaena and cassia were similar.

N uptake and apparent N utilization efficiency were

highest in the fertilized control plot, in both grain as

well as stover and cobs. N uptake by the maize in the

fertilizer-control treatment was 42% more than the amount of

N applied. Similar high N uptake levels exceeding the

amounts applied were also observed with cassia alley

cropping. For these two treatments, all of the N applied

through the fertilizer or mulch could be accounted for.

Phosphorus.--In six seasons, P removed by the crop

(i.e., grain, stover and cobs) significantly exceeded the

amount of P added through mulch or fertilizer (Table 2-7).

Averaged across species, P removed through the crop was 164%

more than P applied through the mulch of leucaena and

cassia. As a consequence of the lower mulch yield of cassia

compared to leucaena, P applied through cassia mulch was

significantly lower than P applied through leucaena mulch.

Hence, P removed through the crop in plots with cassia was

significantly higher (215%) than that from plots with












Table 2-7. Addition of phosphorus through mulch and removal
by maize in six seasons of cropping under
Leucaena leucocephala and Cassia siamea alley
cropping.


Phosphorus (kg ha1)


Mulch Source Addition Removal Difference


Leucaena 22.8a 52.1' -29.3a

Cassia 15.8b 49.8a -34.0"

Mean 19.3 52.0 31.7


'Values in a column with different letters are not
significantly different (p = 0.05).












leucaena mulch (127%). The interaction between N and P

uptake/removal by the crop was not examined in this study.


Root Density

Root density differed substantially between species

and also between soil depths. In both species, the

densities of both fine- and medium-sized roots were

significantly higher near than away from the hedgerows (Fig.

2-9). Fine root densities of leucaena at both 0-20 cm and

20-40 cm soil depths were significantly higher at nearly all

distances from the hedgerows than that of cassia (Table

2-8). Interaction effects between species and soil depths

on root density were not significant. In both species, root

density at 20-40 cm depth was significantly higher than at

0-20 cm.


Pot Study

During four harvests, the species effects and the

interaction effects of species and mulch rate on the biomass

yield of maize were not significant (Fig. 2-10). However,

effects of the mulch rates on maize yield were highly

significant, with the yields increasing direct proportion to

mulch rates (Fig. 2-10). Averaged across species and

irrigation levels (i.e., the equivalents of 100 and 200 mm

of rainfall in two months period), biomass yield of maize

from the 0 and 2 t ha1 mulch rates were not significantly



















0.12


0.1


0.08
E

t 0.06
Co

. 0.04
0
0
0.02


0


SLeucaena (0-20) U Cassia (0-20)

[ Leucaena (20-40) 2 Cassia (20-40)


..LLr L-fI-g. 1l~ lin u I IMa li- hii---iiini --inIEIE-----


20 40 60 80 100120140160180200220240260280300


Distance (cm) from tree hedges


Figure 2-9. Fine root density in the top 40 cm soil
profile of alley cropped hedgerows of Leucaena leucocephala
and Cassia siamea.


iiMi


~''~''~'' ~^ '~'~'~'' "'' "''''












Table 2-8. Fine root density at two soil depths along a
meter-long transect from 6 m apart hedgerows of
Leucaena leucocephala and Cassia siamea
intercropped with maize.


Soil depth (cm)

Hedgerow Species 0-20 20-40 Mean


Leucaena 0.41 0.64 0.53"

Cassia 0.23 0.47 0.35b

Mean 0.32' 0.56b 0.44


NOTE: Fine root density = cm root per cm3 of soil.
"abFor a given row and column, means followed by the same
letters are not significantly different (p = 0.05).

























Leucaena: Y = 0.15X + 16.
Cassia: Y = 0.24X 14.2;2
Cassia: Y = 0.24X + 14.2; r


7,r = 0.8

= 0.84
OP~


Leucaena Cassia
leucocephala siamea
-,


I I *
10 20 30 40 50
Mulch rates (g pot-1,dry weight)


Figure 2-10. Effects of three mulch rates of
Leucaena leucocephala and Cassia siamea on the biomass yield
of maize grown in pots, 1991.












different. The effects of these lower rates were also not

significantly different from the effects of the fertilizer

only.

The effects of irrigation depended on the harvest. In

harvests 1 and 4 (i.e., 4 and 16 weeks after mulch

application to the pots, respectively), differences in yield

between the two levels of irrigation were not significant.

However, in harvests 2 and 3, the differences in maize

biomass yield were significant.



Discussion


Mulch Yield of the Hedgerows

On average, leucaena produced 46% more mulch than

cassia. The higher yield of leucaena was observable in both

systems of hedge planting. Similar differences in the mulch

yields of the species had been noted earlier by Nair (1987)

working at the same site. The differences in biomass

production between the two species could be ascribed to

inherent growth differences as well as their site

utilization efficiencies. In the lowland humid tropics,

differences in the biomass yields of leucaena and cassia

appear to be even more pronounced than in the semiarid

tropics (Kang et al., 1981; Kang et al., 1985). Averaged

over the three years of study, mulch yield of leucaena was

approximately 4 t ha" yr1 (dry matter). Similar yields of

leucaena have also been reported by Rao et al. (1990) from












alley cropping studies spanning over 4 years from semiarid

India.

Between the two systems of planting (i.e.,

intercropped vs block/sole stands), intercropped hedgerows

of leucaena yielded significantly higher (+33%) mulch than

the block planting on an equal area basis in all but in the

first year. In cassia, differences in the mulch yield of

the two systems were not significantly different. The

higher biomass of the alley cropped plots could be

attributed to (a) availability of more space and hence

reduced competition for soil water and nutrients (especially

when the annual crop was not in the field) and, (b) indirect

benefits from husbandry practices of the maize crop, e.g.,

fertilization, weeding and tillage. The higher mulch yield

of the intercropped hedgerows compared to the yields of the

block/sole stands indicates the benefits of growing the

hedgerows within the crops.

In the block planting system, although the hedges

occupied the same land area as the intercropped hedges, they

were confined to limited space and not spread out. Hence,

plant-to-plant competition was relatively severe.

Typically, the hedges at the borders of the block planting

system always grew taller and yielded more biomass than the

rows inside the blocks. Enhanced tree-to-tree competition

may have contributed to the slightly higher mortality,











especially of cassia, observed in the hedges planted in

block/sole stands compared to those intercropped.

The high pruning frequency (4-6 times a year) may also

have stressed the cassia hedges more than leucaena, and

subsequently made cassia less resistant to pest attack.

Another experiment on the site of the present study (P.

Kiepe, personal communication) showed that with only two

prunings per year, mulch yields similar to those obtained in

the present study could be obtained from intercropped

hedgerows of cassia. Nevertheless, reduced frequency of

pruning may exaggerate crop loss under alley cropping due to

enhanced shading.

It is also possible that the frequent pruning and

"export" of mulch from the area of the block planted hedges

impoverished the site in terms of nutrient storage, which in

turn contributed to the low yields of the block planted

hedges.



Crop Yield



Effects of mulch on yield

Because yields of maize in the mulched and unmulched

control plots were comparable, it would appear that low

levels of mulch obtained from the hedgerows limited maize

yields. Significant response in biomass yield of maize to

increasing rates of mulch in the pot studies also supports











this observation. This response could be ascribed to

enhanced availability of nutrients; positive impact of mulch

on soil physical properties (e.g., improved aeration and

moisture conservation) could be another reason, but this

aspect was not examined in this study.

It has been argued that if mulch applied in a season

could be increased to 3-4 t ha', maize yield may be

increased significantly even in sites similar to the one of

the present study with relatively fertile soils. For

instance, Mugendi (1990), working at a site adjacent to the

present study, reported from a study spanning 14 seasons

that application of leucaena and cassia mulch at rates

equivalent to 5 t ha"1 consistently gave significantly

higher maize yields than applications of 1 t ha-1 or those

of the control plot with no mulch additions. Moreover,

more pronounced effects would be expected in nutrient-poor

than in nutrient-rich sites. For instance, Palm (1988)

observed that in more nutrient-poor and/or in acid soils,

increased rates of mulch application resulted in crop

responses similar to those of inorganic nitrogen

fertilizers.

Because a species like leucaena has a higher fodder

value compared to cassia which has no fodder value (NAS,

1984), in semiarid areas, the prunings from the leucaena

hedges are more likely to be removed from the field for use

as fodder (Singh et al., 1989). The use of the prunings for











fodder becomes even more appropriate when mulch may not

result in significant gains in crop yields. Similar

observation was made by Singh et al. (1989) from semiarid

India. Furthermore, in nutrient-poor and/or acidic soils,

crop yields may not be sustained by mulch even if all the

crop residues and hedge mulch remained in the crop fields

(Szott et al., 1991).

Although the small quantities of mulch from alley

cropped hedgerows may not improve crop yields, it could be

argued that their application may still be necessary to

minimize long-term declines in soil fertility and

productivity. It is hypothesized that such declines in

fertility and productivity could result from loss of

nutrients removed in continuous harvests of mulch and tree

products (Kimmins, 1977; Hook et al., 1982; Lulandala and

Hall, 1988). With or without the removal of hedgerow mulch,

initial soil conditions, crop type and their yields would

likely influence the onset of yield declines. Use of manure

from livestock fed with the prunings may reduce yield- and

soil-fertility declines. In addition, in the semiarid

tropics where the crop yields are generally low compared to

those of the humid tropics, removal of mulch in the range of

1-2 t ha'1 season-' may not show any significant impact on

yields and fertility conditions of the soil, at least in the

short term. This view is supported by the absence of

significant differences in maize yield between plots with










62
mulch and those without for the seasons of this study. From

the foregoing, therefore, it is evident that improvements in

crop yields through the effects of mulch produced by alley

cropped hedgerows are probably limited in the semiarid

tropics.



Supply of N and P by the mulch
and removal by the crop

Leucaena mulch, when returned to the soil, provided

more N than what the crop removed in 6 seasons. On the

other hand, cassia supplied only half of the crop N. Most

of the N removed was harvested in the grain (67%), an

observation in agreement with that of others (Allison,

1973). Based on the N budget analysis, it does appear,

therefore, that if all of the mulch N is available to the

crop, leucaena could meet the crop's seasonal N

requirements.

Besides N, the supply of the other nutrients is

important for the maintenance of high maize yield over time.

Removal of the mulch which supplies substantial quantities

of nutrients other than N could therefore have significant

effects on yield, even when N is provided via fertilizer.

This view is supported by observations of Gichuru and Kang

(1989) who showed that plots receiving no prunings had, in

general, lower maize grain yield regardless of N application

compared to those receiving prunings.










63
Soil P appears to be a nutrient that could rapidly be

depleted through continuous harvest of mulch and crops from

the systems studied. This is particularly so in soils such

as the Alfisols of the present study where the native

exchangeable soil P levels are low (less than 16 ppm),

inputs through the mulch are low, and, yet, outputs through

crop harvest exceed inputs significantly. The 7 kg ha-1 yr-I

P supplied through leucaena mulch is similar to P levels in

leucaena mulch noted by Lulandala and Hall (1990) from

semiarid Tanzania. Most of the P in the crop was removed in

the grain and stover, in almost equal percentages. Only

about 3% of the total P in the crop was in the cobs.

Averaged over species, only 37% of the P needs of the crop

was balanced by leucaena and cassia mulch. The shortfall

was more pronounced in cassia alley cropping because of

cassia's low mulch yield compared to leucaena's.

Based on the P budgets (notwithstanding the

possibility that not all mulch P is available to crop), it

does appear that the recycling potential of cassia mulch

from the alley cropping system is inadequate to meet the P

requirements of the maize crop. The balance between P

supply and crop need may be even less favorable under

conditions of high crop yield such as in seasons of good

rainfall. Kang and Wilson (1987) observed that the P

content of leucaena prunings was inadequate for one crop of

maize at the humid lowlands of Nigeria. Also, while












mulch-P, particularly that of leucaena, could meet the P

requirements of maize grain yield of about 2 t ha" as

suggested by Torres (1983), it is evident that it does not

meet the total needs of the crop (i.e., grain, stover and

cobs).

The considerable increase (16%) in the P concentration

of cassia mulch in 1991 over the levels of 1987 and the

absence of similar increase in leucaena mulch suggest

differences in the patterns of P uptake by the two species.

The cause of this difference is unknown. Some postulations

are that cassia is more mycorrhizal than leucaena or

supporting more phosphate solubilizing bacteria than

leucaena in its rhizosphere.



Apparent utilization efficiency of mulch-N

The apparent low use efficiency of mulch-N can best be

explained by the equally high crop and N yield of the sole

maize control compared to the yields of mulch treatments.

This is another indication of the presence of sufficient

native soil N at the study site for the maize crop without

additional inputs. Low utilization efficiencies of mulch-N

have also been reported by several other workers. For

example, Palm (1988) observed mulch-N utilization efficiency

of 16% by upland rice from mulches of leguminous shrubs.

Yamoah et al. (1986b) noted N-utilization efficiencies of

near 20% from the mulch of leguminous shrubs with the











addition of fertilizer. Sanginga et al. (1988) also

observed low N-utilization efficiencies from leucaena mulch

but they emphasized that the absence or presence of N-fixing

bacteria inoculation made a difference. Other workers (Kang

et al., 1981; Guevarra, 1976) have also reported low

apparent N-utilization efficiencies from mulch of leguminous

shrubs.

Factors such as time lags between mulch-N

mineralization and crop N demands (Anderson and Ingram,

1989), immobilization of mulch-N and soil-N by microbes in

the process of mulch decomposition, at least in the initial

stages (Palm and Sanchez, 1991) and volatilization losses

from surface applied mulch (Terman, 1979; Nelson, 1982)

could also have contributed to the apparent low mulch-N

utilization efficiencies observed. Also, about 22% of the

undecomposed mulch-N was still tied up in mulch of both

species. This cannot be described as inefficiency, because

this N is still in the system and might be utilized by

subsequent crops. Admittedly, without more precise

determinations such as the use of "N techniques, it is

difficult to assess the fate of mulch-N in soil (Sanginga et

al. 1988). Management practices that synchronize periods of

peak N release from mulch with periods of peak N demands by

the crop may enhance effects of the small quantities of

mulch available from the hedgerows.











In situ effects of hedgerows on crop yield

The in situ benefits of growing the hedgerows was

reflected in the higher maize yields between the alley

cropping system and the block planting system and LER of

alley cropping > 1.0. The in situ effects of the hedgerows

may result not only from their root and rhizosphere effects

but also from their ability to protect the soil against

erosion and to serve as windbreaks (Wiesrum, 1984). From

semiarid India, Rao et al. (1991) provide evidence for

reduction in runoff and soil loss under alley cropping with

leucaena.

The in situ benefits of the hedgerows will be in

perspective when viewed against the yields of the

appropriate sole-crop control. This will determine if,

indeed, there has been a positive gain over the no-

intercropping situation. Such a comparison can be made from

the last column of Table 2-3, which shows, for instance,

that the maize yield of the sole-maize control (fertilizer-

only) was -8% compared to that of cassia alley cropping and

+12% compared to the leucaena alley cropping. Because of

reduced maize yields under leucaena alley cropping, LER or

the yield advantage of leucaena alley cropping was

significantly lower than that under cassia. These

differences in yield reductions under alley cropping of the

two species suggest that leucaena is more competitive with

maize than cassia. The reductions in maize yield under











leucaena alley cropping could be more severe than observed

in the present study in sites that are both nutrient-poor

and dry (Chirwa, 1991).

Leucaena became more competitive than cassia starting

in the second year. Similar observations on the onset of

leucaena's competition were made also by Rao et al. (1990)

working with alley cropping in semiarid India. The

significantly higher mulch production as well as the

presence of high fine-root densities of leucaena in the top

soil compared to that of cassia provides additional clues to

leucaena's higher competitiveness. It is possible that a

higher pruning frequency than the 4-6 times a year of

leucaena in the present study may have reduced its

competition with the intercrcropped maize.

The higher maize yield under cassia alley cropping

compared with that under leucaena or under the sole crop

could be ascribed to significantly more positive crop-

hedgerow interactions observed under cassia. These positive

interactions could have mitigated high reductions in maize

yields from hedgerows with narrow alleys compared to those

with wide alleys. This could explain the lack of

significant differences in the three alley widths spacingss)

used in the present study. Because more land was under

hedges in the narrow alleys than in the wide alleys, maize

yield from the narrow alleys was expected to be less than

from those of the wide alleys.










68
Maize-yield reduction observed under leucaena was not

likely due to competition for light, because the hedges were

pruned frequently during the rainy season. Also, it was

probably not due to competition for nutrients because the

plots were fertilized and leucaena (which was inoculated at

the start of the study) may have fixed considerable levels

for its own N needs. Instead, the reduction in yield was

probably most likely due to competition for water, a

suggestion also made by others regarding leucaena alley

cropping in semiarid India (Rao et al., 1991). In the sixth

season when rainfall was 39% below normal, the significantly

lower maize yields under leucaena alley cropping compared to

the control provide support to the explanation of water

limitation. The argument that competition was more for

water and less for nutrients was supported by the results of

additional field studies (Chapter 5) involving factorial

combination of fertilizer (N and P) and irrigation levels

where a positive response in maize yield to N application

was not significantly different from that due to the effects

of irrigation only. Prior to the initiation of the present

study, the site was a savanna woodland cropped to pigeon pea

for one-and-one-half years, so that soil N may still have

been relatively high. Another evidence for water being more

limiting than nutrients is provided by the linear

relationship between the maize yield and rainfall over the

six seasons.











Trends in maize yield over time

All treatments had similar levels of relative yield

maintenance as long as rainfall received in a season was

normal, a situation more pronounced under leucaena than

under cassia. Much longer studies than three years are

required because the effects of the hedgerows are likely to

be more pronounced with age. Also, long-term studies would

more clearly reflect the high variability in seasonal

rainfall. From six-year studies in the humid lowlands of

Nigeria, Lal (1991) observed that total maize yields from

alley cropping of leucaena and Gliricidia sepium were about

10% lower than those of the control. In the present study,

total maize yield after 6 seasons under alley cropping with

leucaena was reduced by 12% while that under cassia was

increased by 8%. Therefore, under management conditions

similar to the ones of the present study, it does appear

that cassia has more potential to sustain maize grain yield

than leucaena.

Because of the low measured concentrations of

extractable N and P in the soils of the study station (Kibe

et al., 1981), fertilizer N and P was applied to the crop

each season at the minimum of the recommended rates for

farms in the area. However, the general lack of significant

differences in maize yields between treatments suggests that

N and P were not limiting, again supported by the results of

the pot study.











Root Density

The significantly higher yield of leucaena mulch and

presence of significantly higher density of its fine roots

in the top soil could be the causes of the lower maize yield

under alley cropping with leucaena compared with cassia. In

a competitive situation, the sharing of limited soil water

and nutrient resources by competing plants is proportional

to their effective rooting length/density (Bowen, 1985).

Generally, most of the fine roots of trees are found in the

upper 50 cm of the soil (Fayle, 1975; Deans and Ford, 1983).

High root density is an ecological adaptation of plants,

particularly those growing on soils with chemical and/or

physical limitation in the lower horizons (Bowen, 1973).

This is also true of plants in nutrient and water stressed

environments (Chapin, 1980). The studies of several workers

in both the humid lowland and the semiarid tropics (Kang et

al., 1981; Corlett, 1989; Singh et al., 1989; Rao et al.,

1991, 1992; Ruhigwa et al., 1992; Toky et al., 1992)

corroborate the high root densities of leucaena and cassia

found in the present study.

Unfortunately, most of the roots of annual crops and,

in particular those of maize, are found in the same soil

horizon as those of trees (Jonsson et al., 1988).

Therefore, competition for limited resources (e.g., soil

water and nutrients) between trees with well established

root systems and the seasonal crop with less









71
well-established roots is unavoidable. More often than not,

the agronomic crop is the loser in the competitive situation

as indicated, in this case, by the low maize yields under

leucaena. Studies of other workers from the semiarid

tropics also show low maize yields under leucaena alley

cropping (Nair, 1987; Singh et al., 1989; Corlett et al.,

1989; Rao et al., 1990, 1991; Ong et al., 1992). The

present study also shows that all species are not as

competitive as leucaena with crops, and that an

understanding of the rooting system of species to be used

for intercropping is necessary.

Typically, fine root density of trees declines with

distance from the stem (Roberts, 1976; Ford and Deans, 1977;

Persson, 1980; Dhyani et al., 1990) which is consistent with

the observations of the present study. Along the entire 3 m

long trenches from the hedgerows studied, root density of

leucaena was significantly higher than that of cassia. The

extended high root-density of alley cropped leucaena into

the cropped area and the resultant low yields of intercrops

would suggest that potentials of intercropping leucaena

without physical separation from crops are limited.

Approaches to minimize below-ground competition between

hedgerows and crops, at least experimentally, include root

pruning and/or placement of physical barriers such as

galvanized iron or polythene sheets (Singh et al., 1989; Rao











et al., Ong et al., 1992), trenching (Verinumbe, 1985) and

root pruning (Fernandez, 1990).

With root pruning/barrier techniques, improvements in

crop yield compared to a control have been noted, with

significantly increased yields at some times (e.g., Corlett,

1989) and not at others (Ong et al., 1992). Root pruning

is, however, labor demanding and unless there are

significant gains to be foregone by not having the hedgerows

in situ, farmers would be most unlikely to adopt it.

Besides, roots of the hedgerows can arch back to the surface

soil even when physical barriers are placed to depths of one

meter below the soil surface and hence render barrier or

trenching efforts ineffective (Ong et al., 1992).

Separate planting of hedges in blocks or sole stands

on land outside the crops that is not currently cropped, or

on land not suitable for cropping or on the periphery of the

farm may be options to minimize competition of leucaena with

crops. This could be an option particularly for leucaena

which has a desirable product (fodder) but which cannot be

intercropped because it is competitive with crops. However,

expected losses in crop yield are proportional to the

percentage of land occupied by the hedges. This lost land

occurred because the amounts of mulch obtained from the

hedges were low (on average 1-2 t ha-1 season") and had no

compensatory effects on crop yield. However, the results of

the pot studies suggest that if the levels of mulch from











hedgerows could be increased to application rates of 3-4 t

ha' season-', gains in crop yields could be increased even

from the relatively fertile site of the present study.



Conclusions

Alley cropping of Leucaena leucocephala was

detrimental to maize yield over the six cropping seasons.

On average, maize yield under leucaena alley cropping

dropped by 12% of the sole crop while that of cassia

increased by 8%. Leucaena, on the other hand, produced

significantly higher mulch biomass (50% more) than cassia.

The higher mulch biomass production and higher density of

fine roots in the top 40 cm soil are factors that contribute

to leucaena's significantly higher competitiveness with

maize than cassia's.

The in situ benefits of growing the hedges on maize

yield were positive for both species but significantly more

under cassia compared to those under leucaena. Planting the

hedges outside the crops may be a management approach to

minimize competition between the crop and the tree

hedgerows, especially for a species like leucaena with

desirable fodder value. However, this approach produced

yield reductions higher than alley cropping with both

leucaena and cassia, with the loss nearly equal to the

percentage of land apportioned to the hedges.











The magnitude of the mulch produced by leucaena and

cassia, 1 and 2 t ha"1 season", respectively, was not

sufficient enough to cause significant gains in maize yield

at the study site. There are indications that gains in

maize yield could be significant if the amounts of mulch

applied could be increased to 4 t ha' season and beyond.

It is unlikely, however, that such increases in mulch yield

from the hedges could be made without compromising further

crop yields.

At this point, the results of the present study do not

indicate sufficient short-term benefits to recommend alley

cropping with leucaena under conditions similar to the ones

of the present study. However, cassia does have potential

if (a) the 8% increase in maize yield outweighs the costs of

production and, (b) mortality of cassia after 3-4 years is

low enough not to necessitate replanting of the hedgerows.

Follow-up studies on factors responsible for cassia's

positive in situ effects may enhance its potentials for

alley cropping under semiarid conditions.














CHAPTER 3
SOIL FERTILITY ASPECTS OF ALLEY CROPPING
LEUCAENA LEUCOCEPHALA AND CASSIA SIAMEA IN
SEMIARID TROPICS AT MACHAKOS, KENYA


Introduction

Agroforestry practices are suggested as low-input

systems with potentials to meet the critical needs to improve

or sustain food production in the tropics (Swift and Sanchez,

1984). One practice of agroforestry which has been the

subject of much interest and study is alley cropping, also

called hedgerow intercropping (Kang et al., 1990). It

consists of growing food crops in alleys between planted

hedgerows of multipurpose trees and shrubs. The hedgerows are

periodically cut back at the beginning of as well as during

cropping to prevent shading and to provide mulch to the

associated crop.

One of the most important premises of alley cropping is

that the addition of organic mulch, especially if it is rich

in nutrients, has favorable effects on soil physical and

chemical properties, and hence on productivity of crops.

However, the ability of the mulch to provide nutrients and to

maintain or improve soil properties would depend, in addition

to its quality and time of application, on the quantity. For

instance, Kang et al. (1990) observed from six-year-long

studies at the humid lowland of West Africa that with

continuous addition of leucaena pruning (8-10 t ha-1 yr"1, dry

75














CHAPTER 3
SOIL FERTILITY ASPECTS OF ALLEY CROPPING
LEUCAENA LEUCOCEPHALA AND CASSIA SIAMEA IN
SEMIARID TROPICS AT MACHAKOS, KENYA


Introduction

Agroforestry practices are suggested as low-input

systems with potentials to meet the critical needs to improve

or sustain food production in the tropics (Swift and Sanchez,

1984). One practice of agroforestry which has been the

subject of much interest and study is alley cropping, also

called hedgerow intercropping (Kang et al., 1990). It

consists of growing food crops in alleys between planted

hedgerows of multipurpose trees and shrubs. The hedgerows are

periodically cut back at the beginning of as well as during

cropping to prevent shading and to provide mulch to the

associated crop.

One of the most important premises of alley cropping is

that the addition of organic mulch, especially if it is rich

in nutrients, has favorable effects on soil physical and

chemical properties, and hence on productivity of crops.

However, the ability of the mulch to provide nutrients and to

maintain or improve soil properties would depend, in addition

to its quality and time of application, on the quantity. For

instance, Kang et al. (1990) observed from six-year-long

studies at the humid lowland of West Africa that with

continuous addition of leucaena pruning (8-10 t ha-1 yr"1, dry

75








76
matter), soil organic matter (SOM) and nutrients were

maintained at higher levels than where the prunings were

removed. Similar observations are made by other workers also

from the lowland humid tropics (Atta-Krah et al. 1985; Yamoah

et al. 1986c). However, in the SATs, ecological conditions do

not allow the high rate of mulch production found in the humid

tropics. Hence, significant gains in SOM from the addition of

mulch may be limited. Based on the effects of mulch per se

there might be no conceivable advantage of alley cropping

systems on SOM in the SATs. However, turnover rates of high

root mass often associated with agroecosystems with trees

(Ewel et al., 1982), such as alley cropping, may contribute

significantly to organic matter and nutrient contents of the

soil. Moreover, where leaching losses of soil- and applied-

nutrients may be a problem, the roots could minimize these

through a combination of processes including sorption and

uptake from deeper layers (Stark and Jordan, 1978; Jaiyebo and

Moore, 1964). These processes may result in the concentration

of nutrients in the mulch of the hedgerows which could, in

turn, improve crop nutrition (Fernandez, 1990). In some

soils, there are indications that the roots may even recycle

nutrients from the subsoil which could in turn result in top

soil improvement of pH and exchangeable bases (Lal, 1989). In

addition, due to their longevity and to their protective

benefits, intercropped hedgerows may minimize runoff and soil

erosion loss (Lal, 1989; Rao et al., 1991).










77
Notwithstanding the positive attributes and potentials

of alley cropping discussed above, competition for soil

resources (nutrients and water) between the hedgerows and the

crops has to be considered since this could result in

significant losses of crop yield under alley cropping (Rao et

al., 1991; Nair, 1987; Singh et al., 1989). The losses in

yield may, indeed, obscure any gains in soil fertility due to

the presence of the hedgerows. To justify mixing of multi-

purpose tree hedgerows with crops, particularly by resource-

poor farmers, it is important, therefore, to understand the

benefits to soil fertility that would otherwise be lost if the

hedgerows were planted separate from the crops. This

knowledge is of particular significance in the semiarid

tropics where soil moisture is a major factor limiting crop

production and where soil fertility improvement aspects of

alley cropping have not yet been well understood.

This study was undertaken in this scenario with the

following objectives: (a) to evaluate the changes in some

selected soil chemical and physical properties under alley

cropping of maize with Leucaena leucocephala and Cassia siamea

over three years and, (b) to separate the effects of mulch per

se from the in situ presence of the hedgerow on the selected

soil chemical and physical properties.











Materials and Methods


Site Description

The location, rainfall and soil characteristics of the

site are described in Chapter 2. Table 3-1 shows the chemical

and physical characteristics of the top soil (0-20 cm) at the

beginning of the study (October, 1987).



Treatments and Experimental Layout

The experimental treatments consisted of two systems of

Leucaena leucocephala and Cassia siamea: (a) intercropped

hedgerows and (b) planted as blocks/sole stands outside the

crop. In the intercropped system, the hedgerows were 6.67 m

wide. The hedgerows in both the intercropped and the

block/sole planting systems occupied 15% of the total area,

and the crop occupied the remainder of the land (85%). Plots

with fertilizer-only and mulch of leucaena or cassia only were

included as controls. As an absolute control, no-mulch or

fertilizer plots from an adjacent experiment was used. The

experimental design was randomized block design with three

replications. The field experiment was conducted for more

than 3 years (October 1987 to March 1992).



Crop and Tree Management

In all systems, the soil was hand tilled and maize

(Katumani Composite var.) planted twice a year at a stand of












Table 3-1. Chemical and physical
soil (0-20 cm) at the
October 1987.


characteristics of the top
beginning of the study,


Treatments

Leucaena Cassia Control
Soil Property plots plots plots Mean


pH (1:2.5 soil:H,0) 6.2 6.2 6.2 6.2

Org. C (%) 0.9 1.0 0.8 0.9

Total N (%) 0.1 0.1 0.1 0.1

P (ppm, Mehlich-1) 12.7 12.0 12.7 12.5

CEC (meq/100 g soil) 9.6 9.5 9.0 9.4

Clay (%) 28.7 30.7 26.7 28.7

Bulk density (g/cm3) 1.3 1.3 1.3 1.3


experimental


At the time of soil sampling in 1987, the
treatments had not yet been introduced.









80

37,000 plants/ha. Moderate levels of fertilizer (40 and 18

kg of N and P/ha) were added to the maize in the hedgerow

intercropped and the block planted systems each season.

Weeds were hand removed at least twice in each season. At

harvest, maize grain, stalks and cobs were removed from the

field as was the practice of the farmers in the area.

The tree hedgerows were pruned back to 50 cm above

ground with 2-3 prunings a season. The prunings obtained

were chopped into small pieces and applied as mulch to the

plots. The chemical characteristics of the mulch are shown

in Table 3-2. If tillage and/or weeding schedules shortly

preceded periods of mulch application, the mulch generally

became incorporated into the soil; otherwise, it remained on

the soil surface.

Soil sampling was always carried to 20 cm depths. A

composite sample of 10 auger cores was taken for each

treatment. The soil was air-dried, crushed to pass a 40-

mesh sieve, and mixed. Samples were analyzed at the

Analytical Laboratory of the Katumani Research Station,

Machakos, Kenya by the following methods: total N by the

micro-Kjeldahl method (Jackson, 1958), organic carbon by the

Walkley-Black procedure (Jackson, 1958), P by the Mehlich-1

double acid procedure and cation exchange capacity (CEC) by

Atomic Absorption Spectrophotometry methods. Soil pH was

determined using a glass electrode pH meter (model Bibby),

at soil to water ratio of 1:2.5.









81


Table 3-2. Chemical characteristics of the dry mulch of
alley cropped hedgerows of Leucaena leucocephala
and Cassia siamea.


Poly-
N P K Ca Mg C:N Lignin phenol
(%) (%) (%) (%) (%) ratio (%) (%)


Leucaena 3.5 0.2 1.9 0.7 0.3 14 8.1 4.7

Cassia 2.9 0.2 1.8 0.7 0.2 17 8.4 3.7










Relative availability of N03" N and NH4, N was

monitored in the last season. Ion exchange resin bags

(Binkley and Matson, 1983; Binkley and Hart, 1989) were

used. In each plot/replicate, one bag for anions and one

for cations weighing 5 g were placed in the top 10 cm soil

profile. After the harvest of the crop, the resins were

retrieved and NO,- N and NH4 N extracted with 2 N KC1.

Levels of NO,' N and NH4/ N from the KCl-extracts were

determined using Standard Autoanalyzer methods at the Soil

Testing Laboratory of the University of Florida,

Gainesville.



Soil physical properties

Bulk density (Blake, 1965) is the only physical

property of the soil discussed in this chapter. In the last

cropping season, soil was sampled with a semi-wide auger

and 5 cm long to a total depth of 20 cm (i.e., 0-5, 5-10,

10-15 and 15-20 cm). Bulk density of each plot/replicate

was computed as the average values of samples taken at two

positions near the hedgerows (0.45 m) and two in the middle

of the alleys (3.5 m away from the hedgerows).



Statistical Analysis

ANOVA procedures were used to determine statistically

significant (p = 0.05) differences between the levels of




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