Historic note

Group Title: AREC-H research report - Agricultural Research and Education Center-Homestead ; SB-82-3
Title: Impact of irrigation-fertilization on groundwater
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00067841/00001
 Material Information
Title: Impact of irrigation-fertilization on groundwater
Series Title: Homestead AREC research report
Physical Description: 11 leaves : ; 28 cm.
Language: English
Creator: Orth, Paul G
Agricultural Research and Education Center, Homestead
Publisher: University of Florida, Agricultural Research and Education Center
Place of Publication: Homestead Fla
Publication Date: 1982
Subject: Groundwater -- Quality -- Florida   ( lcsh )
Irrigation -- Florida   ( lcsh )
Fertilization -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
Bibliography: Includes bibliographical references (leaf 11).
Statement of Responsibility: Paul G. Orth
General Note: "July 15, 1982."
 Record Information
Bibliographic ID: UF00067841
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 72665315

Table of Contents
    Historic note
        Historic note
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
Full Text


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source

site maintained by the Florida
Cooperative Extension Service.

Copyright 2005, Board of Trustees, University
of Florida

Homers. id AREC Research Report1 SB82-3* July 15, 1982

g 'Ch Impact of Irrigation-Fertilization Gr. .un'' rr .r--.--
Paul C. Orth L
University of Florida, IFAS
Agricultural Research and Education Cen er JUL 2 1' 8
Homestead, Florida 33031
1.F..S. Ur- n of -I ,l
ABSTRACT F.--- .... __

Agriculture is intensive in southern Dade County (Miami), Florida and exists
on a shallow soil, generally 10-20 cm deep overlying porous limestone. The
major soils usually contain 30-70% rocks which fact combined with soil shal-
lowness gives a low retention of the soil for soluble plant nutrients, pri-
marily nitrogen and potassium. Management of fertilization and irrigation
can affect inputs of these nutrients into the Biscayne aquifer, usually 20 -
300 cm below ground surface during the cropping season.

A field experiment was carried out to measure the impact of crop fertilization
and carefully managed irrigation on nitrates and potassium in the groundwater.
Plots were 2100 m with main treatments of sprinkler irrigation, trickle irri-
gation, or plastic mulch with trickle irrigation. Two replicates of each
treatment were located in two different fields. One field was fertilized at
the rate of 750 kg/ha and the other received twice as much fertilizer. Butter-
nut squash was grown, and over a 130 day period 332 mm of rain fell and either
111 mm of sprinkler irrigation or 70 mm of trickle irrigation was applied.
Water in a well in the center of each plot was sampled as well as the water in
other scattered wells at the research center.

Nitrate and potassium concentrations were lower in the plot area during active
cropping than either before or after. Average concentration of NO -N during
cropping was 2.0 ppm at the surface of the aquifer and 1.4 ppm at 3 m. The
corresponding concentrations for K were 5.4 and 5.0 ppm. It was not possible
to prove that any of the management practices increased nutrients in the
groundwater, however a reasonable estimate would be a 1-2 ppm temporary in-
crease in NO -N during some part of the season. Water samples from nearby
less well-managed crops averaged as high as 5.6 ppm NO3-N as compared with 3.6
ppm for the highest average concentration for one of the experimental plots.


Much of the agriculture in Dade County (Miami), Florida exists in a unique
environmental combination of soil, groundwater aquifer, and climate. Two of
the main soils used for agriculture are Rockdale and Rockland (1). These soils
contain numerous rocks, 30-70%, and are generally shallow, 10-23 cm. The fine
material ranges between fine sand and fine sandy loam. The soil lays on a very
porous limestone, Miami Oolite, and the groundwater is part of the Biscayne
aquifer. Since land elevation ranges mostly from 1-3 m above mean sea level,

*This paper was presented at the American Society of Civil Engineers
National Specialty Conference on Environmentally Sound Water and Soil
Management in Orlando, Florida July 20-23, 1982.

the consumable portion of the aquifer is only that portion above sea level.
Excessive pumpage causes intrusion of salt water into the aquifer. The pro-
ductivity of the aquifer is among the greatest in the world. Rainfall re-
charges the aquifer, with much of the recharge occurring locally. Some of
the recharge water comes from as far away as the Lake Okeechobee system, 150
km. Since the aquifer supplies water to agriculture, industry, and homes,
maintenance of the water quality is important.

Normally, agriculture is a land use that is compatible with water recharge (2).
Much of the rain falling on industrial and urban areas picks up enough con-
taminates to render it significantly poorer in quality than water entering the
aquifer in an agricultural area. Thus Dade County agriculture exists in an
environment where groundwater quality is very responsive to activities occurring
less than 3 m above it. This paper describes some research to measure the im-
pact of irrigation and fertilization practices on water quality.


The experiment was carried out on the 65 ha site of the University of Florida
Agricultural Research and Education Center near Homestead. Two fields sepa-
rated by 400 m were used, figure 1. The research center is divided into 16
blocks or fields numbered as shown in the figure, and fields 5 and 16 were
used for the experiment. Each of these two fields contained six square plots
containing 2100 square meters, 0.2 ha. Plots were separated by a minimum of
2 m. A water monitoring well 7 m deep was drilled in the center of each plot.
Other wells existing throughout the Research Center was sampled from time to
time to assess groundwater quality in the surrounding area.

Treatments are summarized in table 1. In each field there were two repli-
cations of three treatments: (1) sprinkler irrigation, (2) trickle irrigation,
and (3) trickle irrigation plus crop beds covered with plastic mulch. Ferti-
lizer was applied at 750 kg/ha in field 16 and 1500 kg/ha in field 5. The
complete analysis is given in table 1. Butternut squash was planted in field
16 on May 4-5, 1981 and in field 5 on May 5-6. One third of the fertilizer
was mixed into the soil, another third placed in a band 2-4 cm deep and 10 cm
to one side of the plant row, and the last third similarly banded 20 cm from
the plant row on the opposite side. The incorporated fertilizer was applied
during plant bed preparation, and the two fertilizer bands were applied after-
ward. Where trickle irrigation tubing was used it was placed about half way
between the plant row and the fertilizer band offset by 20 cm. Soil prepara-
tion included forming the soil into beds about 1.2 m wide with bed centers
1.8 m apart.

Groundwater was sampled periodically using a hand operated vacuum pump. One
sample was taken from the surface of the water table and another from 3 m
lower. During land preparation the water table was about 0.8 m MSL, at
planting it was 0.44, on May 27 was a low of 0.3, and at harvest it was slight-
ly higher than 0.8 m. Changes with time showed variation because rain causes
increases in the water table or slows decreases. Land surface at the research
center is 2.9 3.2 in MSL. A float-mounted U-tube was used to collect a sample


(14-4) 16 12 II 10
(4-2) 6 1
2 3
S4 5 *(2-4) (-)
(2-4) E (2).(2-2) __ _
15 9 8 7

(1-I, ).______________ _______
14 6 (4-3 5 4
4 1
13 3 2
i3 3 2

100 m
g. 1 Map of plot locat s and o a s.OOm
Fig. 1. Map of plot locations and covecal wells.

Table 1. Treatments in irrigation-fertilizer management test.

Treatment Location Fertilizer Irrigation Plastic
number field rate* method mulch
1 16 750 sprinkler no
2 16 750 trickle no
3 16 750 trickle yes
4 5 1500 sprinkler no
5 5 1500 trickle no
6 5 1500 trickle yes

*Fertilizer Analysis N 7.1% Nitrate 5.3 Ammoniacal 1.8
P 6.1%
K 11.6% Fe 0.45%
Mg 1.2% B 0.97%
Mn 0.18% Mo 0.005%
Cu 0.07% Cl 2.07

from the top 1-2 cm of groundwater. Care was taken to minimize disturbing
of the water, especially the upper sample. Samples brought from the field
were filtered through Whatman 42 paper and stored in a refrigerator. Nitrate-
nitrogen was determined colorimetrically by the brucine method and potassium
and calcium by flame photometry. A few samples were analyzed for orthophos-
phate but the concentrations found were generally less than 0.2 ppm P.

Initial irrigation included bringing the soil to near field capacity 2-3
days before planting except for beds covered with plastic mulch where adequate
moisture was supplied before the soil was covered with mulch. Irrigation
after seeding was 3-5 mm. Further irrigation was managed in accord with past
experience since no specific quantitative guidelines have been developed.
Squash leaf appearance was a useful indicator of irrigation need along with
observations of soil appearance and daily evaporation measurements from a
standard weather service pan. A strict water budget approach is not reliable
until the parameters related to water stress are more completely evaluated.
Plants grown on these soils receive some moisture from capillary movement in
the underlying limestone which brings water to the soil-rock interface. Fre-
quent high nighttime humidity and dew help moderate potential moisture stress.

It was not possible to maintain an identical soil moisture status in both
trickle irrigation treatments because theywere irrigated by a single system,
and the unmulched plots had more water loss by evaporation than the mulched
plots. The goal of the experiment was to manage irrigation as carefully as
possible to prevent nutrient leaching into the groundwater.


To prevent leaching of fertilizer nitrate and potassium into the groundwater,
the capacity of the soil to retain moisture should not be exceeded. The
retention capability of the test plot soil was a function of soil depth, the
amount of fine material compared with gravel and rocks, and the water holding
capacity of the fine fraction. The soil depth averaged 17 cm, and the bulk
soil held approximately 12% water by weight. A summary of dry sieving 12
soil samples is given in table 2. Such an analysis of these soils does not


Table 2. Sieve analysis of soil in irri-
gation-fert1 ization management test.

Percent by weight
Particle size Moan S.D.

(2.5 mm 31.0 1.5
2,5 6.4 mm 12.0 0.8
6.4 12.7 mm 16.2 2.2
12.7 25.4 mm 29.4 2.0
25.4 50.8 n:m 11.4 3.6

give a complete separation into different sizes because some fine material
adheres to the various sized particles of limestone. Also, much of the
fine soil exists in aggregates. However, the analysis demonstrates that a
large portion of the soil consists of rocks which, naturally, have little
capacity to hold water. The smallest sieved fraction includes everything
from clay through sand particle size. Further separation techniques were
not used because of the difficulty in applying them to this soil. This
soil contains many fragile limestone particles which are broken by most
techniques used to disperse soil aggregates. Such breakage introduces bias
into the analyses. The technique used is adequate when studying water re-
tention since the two finer fractions hold considerable moisture. The dry
weight of cultivated soil averaged 8.80 kg for a 1 m area 1 cm deep. For 17
cm of soil at 12% moisture this equals 18 1 of water or a depth of 18 mm over
the meter square area.

Information on rainfall and sprinkler irrigation are summarized in table 3.
Rainfall information is arranged in dry and wet periods to save space and to
isolate times when rainfall in excess of the retention capacity of the soil
could have leached fertilizer. Irrigation by sprinkler are listed individually
and irrigation by the trickle system is listed in groupings. Irrigation is
joined to rainfall periods by a vertical bar. Obviously, leaching potential
was greater from rainfall than from controlled irrigation. TLickle irrigation
was applied more frequently, but only in sprinkler equivalents of 0.5 0.8
mm of water per irrigation. There were 2 3 irrlgations/day when plants
reached maximum size. Total irrigation applied through the trickle system was
70 mmn as compared with l11 mm by sprinker. Trickle irrigation wets only a nar-
row strip of soil, 15 25 cm, before water penetrates to the underlying lime-
stone. Additional irrigation will cause additional lateral water movement,
but most of the additional water passes into the limestone carrying any dis-
solved plant nutrients with it.

Results from the water samplings are summarized in tables 4-8. Data have
been separated into three groups, before planting, during crop management,
and harvest into post-harvest. The first period (1) includes three sampling
dates: Feb. 25, Mar. 23, and Apr. 20, 1981. The second (2) includes four
dates: May 8 and 22 and June 8 and 22. The third interval (3) consists of
two dates: July 21 and 30. Table 4 is a summary of the nitrate-N and K
analyses from all the wells within plots. Wells from the two fields and the

Table 3. Rainfall and irrigation received by irrigation-

fertilizer management plots.

Days Rain Irrig evaporation
Date(s) elapsed (mm) (mm) (mm)

Mar 27 Apr 15 20 111
Apr 16-30 15 6 76
May 1-17 17 13 102

May 18 5
May 21 10
May 24 15
May 18-24 7 52

May 25-29 5 57 27
May 30 June 3 5 32
June 4-11 8 56 37

June 15 17
June 18 20
June 12-18 7 46

June 19-22 4 53 14

June 25 10
June 23-30 11 8 37

July 1-3 3 54 13

July 6 1.0
July 4-8 5 30
July 13 12
July 16 12
July 9-16 8 11 45

July 17-18 2 39 11

July 19-31 13 35 69

Total 130 332 111 702


Table 4. Average concentrations of nitrate-nitrogen and potassium, in two
plot areas at two depths during three intervals of time.

Depth Nutrient Location Time Interval*
1 2 3

Surface NO -N 5 3.00.9 2.30.8 5.61.4
16 2.611.0 1.81.2 1.80.5
K 5 7.31.2 5.81.0 12.25.1
16 6.41.3 5.01.4 6.50.9

3 Meters NO -N 5 2.80.8 2.10.4 3.2t0.4
16 2.20.8 0.60.3 0.80.2
K 5 7.11.2 6.0+0.9 8.00.7
S16 5.81.2 4.00.3 4.9+0.9

*Time intervals represent preplanting, planting up to harvest and harvest,
post harvest, respectively.

Table 5. Average nitrate-nitrogen concentrations at
two depths during three intervals from selected

Well Surface Sample 3 Meter Sample
Designation Time Interval Time Interval
1 2 3 1 2 3



2.2 1.8 (5.1)* 2.1 3.1 (2.6)*
3.1 2.3 (3.0) 2.8 1.6 (3.8)
1.5 2.0 (2.6) 1.3 1.6 (1.4)
2.9 3.3 (3.2) 3.1 2.1 (1.6)
5.0 3.4 0.1 5.2 2.8 2.7
5.9 5.6 7.1 6.6 5.4 2.5
1.3 3.0 0.1 1.3 1.5 <0.1
2.1 0.4 1.7 1.9 0.2 0.1
0.4 0.5 1.1 0.4 0.2 0.6

0.9 1.5

0.9 1. 1

indicate only one sampling date during this

interval .

1-1-----1-----`---- ~1~1- I----~

Table 6. Average concentration of nitrate nitrogen and
potassium in water at the aquifer surface in twelve
plots during three intervals of time.

Field 5 Field 16
Plot Time Interval Time Interval
Number Treatment 1 2 3 1 2 3

PPM Nitrate Nitrogen
I Sprinkler 2.1 2.3 1.1 2.7 1.0 1.0
2 3.o 1.6 2.1 1.3 2.4 2.8
3 Trickle 2.7 2.7 7.6 2.2 1.0 2.9
4 1.9 1.8 6.8 2.3 2.1 0.9
5 Tr. + Plastic 4.5 3.6 8.9 4.3 2.8 1.4
6 3.2 1.9 6.9 2.8 1.4 1.9
S.D.* .86 .83 1.4 1.0 1.2 .48

PPM Potassium
1 Sprinkler 5.8 5.4 14.0 6.1 3.8 6.4
2 8.1 4.5 6.0 4.7 6.2 5.8
3 Trickle 7.2 6.0 17.7 5.4 3.4 8.6
4 5.0 5.4 14.6 6.5 5.3 6.2
5 Tr. + Plastic 9.5 7.5 11.4 9.0 8.0 9.3
6 8.2 5.6 9.4 6.5 3.2 2.9

S.D.* 1.2 1.0 5.1 1.3 1.4 .91

*Standard deviation of the mean, 5% level.

two sampling depths were averaged separately. Standard deviation of the
mean is also tabulated to show the amount of variability between samples.
This table gives an overall view of the data collected. It shows generally
lower concentrations during crop production than either before or after
that time. Also, concentrations were lower in field 16 than in field 5.
Field 16 is in the NW corner of the research center and underground water
flow is generally from the NW (Fig. 1). Field 16 received less fertilizer
than field 5. Also, the groundwater may pick up nutrients as it flows
under various plots at the research center, and cause the concentration
to increase toward the SE. Therefore, rate of fertilization as well as
leaching from other experiments could be responsible for the high nutrient
concentration found in the groundwater under field 5 plots. Concentrations
were generally higher at the top of the aquifer than 3 meters below the
surface. In field 5 this difference was 0.9 ppm nitrate-N or 46% greater
and 1.2 ppm K or 21% greater in the surface samples. Recent, local leaching
contributes primarily to nutrient concentration changes found at the top of
the aquifer. Concentrations found at 3 m are related to more remote leaching
both in time and location.

Table 5 summarizes nitrate data from several wells at various locations at
the research center. These water samples were taken at the same times as
those summarized in table 4. The first number of the well designation

Table 7. Average concentration of nitrate nitrogen and
potassium in water three meters below the water
table in twelve plots during three intervals of time.
Field 5 Field 16
Plot Time Interval Time Interval
Number Treatment 1 2 3 1 2 3

PPM Nitrate Nitrogen
1/ 2/
1 Sprinkler 2.3 1.9 2.6 2.4 .52- .39-
2 2.7 1.8 3.1 1.3 .34 .19
3 Trickle 2.8 2.5 4.0 2.0 .56 .43
4 1.9 1.9 3.7 2.1 .40 1.00
5 Tr. + Plastic 4.6 2.1 3.3 3.3 1 .20 1.92
6 2.7 2.6 2.7 2.2 .61 .50
S.D.- .80 .43 .45 .75 .32 .24
PPM Potassium
1/ 2/
1 Sprinkler 6.0 5.6 7.0 6.1 3.7 3.2-
2 7.0 5.6 7.6 4.3 3.6 3.4
3 Trickle 6.9 6.1 9.6 5.7 3.9 3.6
4 4.9 5.4 8.4 5.9 3.8 6.5
5 Tr. + Plastic 10.1 6.0 8.0 7.1 4.7 6.6
6 7.6 7.2 7.4 5.6 3.9 4.6
S.D. 1.2 .86 .70 1.2 .28 .90

/ Average of two sampling dates.
/ Only one sampling date.
Standard deviation of the mean, 5% level.

indicates the field location of the well. All wells except 10 and 11 are
fairly close to the test fields (Fig, 1). Well 16(4-4) is in the NW corner
of the research center and should indicate the status of water moving into
the area. These data generally show higher nutrient concentrations in the
surface aquifer water than at 3 m. This possibly indicates that water moving
into the area contains lower concentrations of nitrate and K and/or as the
water moves horizontally in the aquifer, nutrients concentrated at the surface
are diffusing and mixing into deeper portions of the aquifer. Well 15(1-1)
showed consistently higher nitrate, 5.6 ppm N during interval 2, the cropping
season for the experimental plots. It was SE of a highly fertilized and fre-
quently irrigated papaya planting. Thus there is strong indication that
fertilization-irrigation management practices on tihe -.i:ya planting were
affecting groundwater nitrates. Nitrate nitrogen concentrations around 10 ppm
have been found in some wells at the research center from time to time.

Tables 6 and 7 are summaries of the individual plot wells with water analyses
averaged over the same three intervals as in tables 4 and 5. Plots 1 and 2
were sprinkler irrigated, plots 3 and 4 were trickle irrigated, and plots 5
and 6 were covered with plastic mulch in addition to being irrigated by the


Table 8. Effect of sampling date on average nutrient concentration at
two depths in six wells in each of two fields.

(PPM) Field 2/25 3/23 4/20 5/8 /22 6/8 6/22 7/21 7/30

Aquifer Surface Sampling

NO -N 5 3.6 3.1 2.3 1.8 2.4 1.7 3.3 4.5 6.6
"3 (1 4.4 2.2 1.2 1.6 2.6 1.2 1.7 1.3 2.4
K 5 8.2 9.6 6.8 5.8 5.7 5.2 6.3 11.2 13.2
16 7.6 6.7 4.8 5.2 7.0 3.3 4.5 5.8 7.3

3 Meter Sampling

NO -N 5 3.4 2.9 2.2 2.0 2.5 1.9 2.2 3.3 3.2
16 3.9 1.7 1.0 .88 .82 .45 .33 .74 .87
K 5 7.3 7.8 6.0 5.7 5.4 6.2 6.6 7.9 8.1
16 6.6 6.2 4.6 4.2 4.1 4.0 3.6 5.0 4.8

trickle irrigation system. These two tables are an expansion of the summary
in table 4. The data show no measurable differences between treatments dur-
ing the major crop growth period, interval 2. Note that the highest NO -N
during the interval was 3.6 ppm, plot 5. Replicates show large differences
which would obscure small differences in treatment impact. The differences
between replicates are probably the result of background nutrient concentra-
tions in the water as it moves under the plot. The data previously presented,
table 5, indicated the lack of homogeneity in nutrient concentration spatially,
especially at the surface of the water table. Thus, even though it is logical
to expect fertilizer leaching to occur under some circumstances, the amount of
leaching may be impossible to measure because oi normal background variations
in nutrient concentrations in the gronidwater. Al so, it would appear that
the amount of leaching that did occur is not only unmensurable but also had
an insignificant impact on groundwater quality.

The next step in studying the data is to look for positive indications of
fertilizer leaching. There seems to be some evidence that more leaching
occurred around harvest time or shortly thereafter, and that more leaching
had occurred on sprinkler irrigated plots than on other treatments. The
evidence of the latter is in the low nitrates under plots I and 2 in field 5,
during the third time interval, compared with the other four plots. It can
be reasoned that more leaching occurred from the sprinkler irrigated plots,
1 and 2, during the cropping season, and therefore there were less nutrients
available for leaching late in the season when leaching took place from the
other four plots. However, an alternate explanation is that plot 1 was in
the NE corner of the test area, and plot 2 in the SW corner, and thus were on
the edges relative to the normal underground water flow from NW toward the SE.
Therefore, nutrients leaching from the edge plots were subject to greater
diluting influences that nutrients leaching from the four more central plots.
The other feature of these data is the relatively high nutrient concentrations
for plots 3-6 in field 5 which seems to indicate late season leaching.

-1 1-

During cropping, the eastern three plots (1, 3, and 5) in field 5 had higher

concentrations of nitrate and K in the aquifer surface water than was found
under the western three plots. Again, the fact that water moving under the
area can be expected to come from a westerly direction could explain the
detected distribution. In field 16 the SE well (plot 5) averaged the highest
nitrate and K concentrations during the cropping period at both depths, the
highest K in sampling period 3 at both depths, and the highest nitrate at 3
m in sampling period 3. Again, these data indicate a nutrient increase due
to leaching, but the amount is small an increase of about I ppm.

Table 8 displays the summarized data in a form that emphasizes general changes
with time and the differences between the two fields. The data show the higher
concentrations of N and K before planting, low but fluctuating concentrations
during cropping, and an increasing tendency during July. The greater concen-
trations in field 5 are apparent as well as the low nitrate concentrations
usually found at 3 m in field L6.


It was not possible to prove that any of the management practices increased
nutrients in the groundwater. The data do :,,'.. t, however, that a reasonable
estimate of temporary N increase would bel-2 ppm in NO -N during part of the
season. Some wells at the research center showed much greater increases in
nutrients showing that such increases can be measured when crop management does
not conform to strict guidelines. The higher concentrations of nutrients found
before cropping and at the end of the season should be of concern from the point
of view of proper management between active cropping periods.


1. Leighty, R. G., M. H. Gallatin, J. L. Malcolm, and F. B. Smith. 1965.
Soil associations of Dade County, Florida. University of Florida,
Agricultural Experiment Station. Circular S-77A.

2. Orth, P. G. 1976. Nutrient fluctuations in groundwater under an agri-
cultural area, Dade County, Florida. Soil and Crop Science Society of
Florida 35:117-121.

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