Group Title: Bulletin ;
Title: Maintaining quality turfgrass with minimal nitrogen leaching /
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 Material Information
Title: Maintaining quality turfgrass with minimal nitrogen leaching /
Series Title: Bulletin ;
Physical Description: 11 p. : ill. ; 28 cm.
Language: English
Creator: Cisar, J. L ( John L )
Snyder, George H ( George Heft ), 1939-
Nkedi-Kizza, P
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville, Fla
Gainesville, Fla
Publication Date: 1991
Copyright Date: 1991
 Subjects
Subject: Turfgrasses -- Fertilizers -- Florida   ( lcsh )
Turf management -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Includes bibliographical references (p. 11).
Statement of Responsibility: J.L. Cisar, G.H. Snyder, and P. Nkedi-Kizza.
General Note: Cover title.
General Note: "July 1991."
 Record Information
Bibliographic ID: UF00101287
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: oclc - 25208546

Full Text


July 1991 Bulletin 273


Maintaining quality turfgrass

with minimal nitrogen leaching


J. L. Cisar, G. H. Snyder, and P. Nkedi-Kizza


















Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida
John T. Woeste, dean


Bulletin 273


July 1991










































































J. L. Cisar, assistant professor and turfgrass specialist, Fort Lauderdale Research and Education Center; G. H. Snyder, professor
and soil scientist, Everglades Research and Education Center, Belle Glade; and P. Nkedi-Kizza, associate professor and soil
scientist, Department of Soil Science, IFAS, University of Florida, Gainesville FL 32611








Abstract


Nitrogen (N) is essential for plant growth and N
fertilization is an important component of a turf-
grass maintenance program. Since in aerated soils
N can be converted to the highly-mobile nitrate
form which is an unwanted contaminant of ground-
water, turfgrass managers need to use N fertilizers
in a manner that minimizes the potential for N
leaching. Fortunately, the objective of minimizing
N leaching generally is compatible with the objec-
tive of producing quality turfgrass. Turfgrasses,
with their total coverage of the soil surface and
dense root systems, are excellent biological filters
that can be used to minimize surface run-off and
leaching of contaminants. Nitrogen fertilization
should be accomplished so as to not exceed the
capacity of this 'living filter.'
To minimize N leaching, quantities of water-
soluble N should be kept to a minimum in the
root-zone. This can be accomplished by using
controlled-release (slow-release, water-insoluble)
N sources or by limiting the amount of water-
soluble N applied at any one time. Nitrogen
fertilizer rates should be kept as low as is compat-
ible with the turf usage. Irrigation systems should
be operated in a manner that minimizes percolation
beyond the rootzone. Turfgrass diseases, insects,
and nematodes should be controlled and fertilizers
should be used so as to produce a healthy, active
turfgrass root system that can effectively absorb N
in the soil solution.

Introduction

Nitrogen fertilization: needs and concerns
Nitrogen (N) is the nutrient element applied in
largest quantities to turfgrasses. It is one of the 16
elements essential for plant growth. Nitrogen is a
constituent of plant amino acids and proteins,
including chlorophyll, the primary interceptor of
the light energy required for photosynthesis (the
food or carbohydrate-generating plant process). The
intensity of plant leaf green color is positively
correlated to chlorophyll, and N fertilization
positively affects chlorophyll production in a plant.
As a result, both turf professionals and hobbyists,
including golf course superintendents, sod produc-
ers, home owners, and garden enthusiasts, use N
fertilization as one component of a cultural pro-
gram to obtain desirable growth, vigor, color, and
overall visual appearance of turfgrasses.
Along with the many positive benefits of N
fertilization, there also are potential negative


results. One concern is the impact of mankind's
activities on N concentrations in both surface and
ground waters. For drinking water, the US Envi-
ronmental Protection Agency has set an upper limit
of 10 ppm-N for nitrate-N (one form of inorganic
nitrogen) (Alexander, 1972).
This document will review important factors that
affect nitrate-N leaching (for reasons explained
below, most N leaching occurs in the nitrate-N
form). Management strategies to stabilize N within
the turf root zone and to reduce the potential for
nitrate-N leaching from turfgrass systems will also
be discussed.

Fate of nitrate-N
Nitrate-N is found in soils either because of
direct nitrate-N application as a N source in fertil-
izer (e.g., ammonium nitrate, calcium nitrate) or
because of conversions of other N forms to nitrate-
N by soil microorganisms (Figure 1). In a well-
aerated soil suitable for turfgrass growth, microor-
ganisms tend to convert all N sources to the nitrate
form. These convertible sources of N include
organic N (e.g., urea, sewage sludge), ammoniacal
N (e.g., ammonium sulfate, diammonium phos-
phate), decomposing plant or animal residues (e.g.,
returned leaf blade clippings, manure), soil organic
matter, and dissolved N from rainfall. Regardless
of the original source of N, once it is in the ammo-
nium form, it can be rapidly converted to nitrate-N
by soil micro-organisms. Some ammonium-N,
however, can be lost to the atmosphere through
volatilization, i.e., conversion to the ammonium gas
form (Figure 1).
Nitrate-N is water soluble and is rapidly taken
up by plants along with water required for plant
transpiration and growth. Additionally, nitrate-N
may be lost from the soil in a gaseous form
following denitrification, and in water that perco-
lates through the turf root zone to the underlying
water table.
Denitrification is a biological process favored by
anaerobic (oxygen deficient) soil conditions. These
conditions are most probable in Florida when soils
are saturated or are nearly water saturated due to
poor drainage, intense rainfall, excess irrigation or
a combination of these factors. These conditions
lead to poor soil aeration, which does not favor
turfgrass quality, and therefore should be avoided.
During the process of denitrification, nitrate-N is
reduced by anaerobic microorganisms to various
gaseous nitrous oxide compounds (e.g., NO, N20).



































-Ground Water
SGround Water >s


Figure 1. Nitrogen transformations in turfgrass soils.


Nitrate-N leaching below plant root-zones can be
a serious problem under certain conditions. Soil
colloids, including soil organic matter, have many
negatively charged surfaces that attract positively
charged elements cationss) such as potassium or
calcium, thereby acting to retain these elements
against leaching. But nitrate-N is an anion (nega-
tively charged), and soils have little or no ability to
retain anions. As a consequence, nitrate-N that is
not retrieved by plants or denitrified is subject to
downward movement in percolating water. Once
beyond the root zone, nitrate-N is no longer avail-
able for plant uptake, and may ultimately reach the
water table.
Many of the soils in Florida used for either
agriculture or turfgrass are well-drained coarse-
textured sands that have little ability to retain
either water or nutrients (even positively charged
cations). As a consequence, applied N is quite
mobile in Florida soils. The problem of N leaching
is exacerbated by the fact that rainfall often is
intense, and irrigation and fertilizers are applied
liberally to produce high-yielding crops and quality
turfgrass.


Leaching losses of nitrate-N can be minimized by
using sound agronomic practices. It is to the
turfgrass manager's benefit to utilize practices that
reduce N leaching so that more of the applied N is
utilized by the turfgrass. Fortunately, a variety of
university and industry-tested methods are avail-
able to limit the potential for nitrate-N losses.


Sand soils do not retain either water or soluble nutrients.








Knowledge of these methods, and an understanding
of how they interact, are the keys to sound fertilizer
N management. Properly utilized, these methods
can help turfgrass managers retain N in turf root-
zones and make it available for plant uptake. The
methods for achieving this goal involve a consider-
ation of 1) N sources, 2) N application rates, 3)
frequency of N application, 4) irrigation, and 5)
cultural practices that encourage deep, active root
systems.

Nitrogen sources
Fertilizer N is commonly sold either as a water
soluble material, or as a controlled-release (slow-
release, water-insoluble) material (Table 1), al-
though commercial fertilizer mixes often contain a
combination of both types. Prudent use of these
sources will help achieve a balanced N nutrition
program that can result in maintenance of quality
turfgrass with minimal nitrate-N losses. For
example, turfgrass quality, measured visually as
intensity of green color, has been shown to improve
markedly shortly after application of a water-
soluble material, but then to decrease to an unac-
ceptable level with time thereafter (Figure 2)
(Snyder et al., 1980). Application of N in a water-
insoluble form, on the other hand, resulted in a
somewhat lesser, but nevertheless acceptable,
improvement in quality that was sustained for a
longer period of time (Figure 2). The degree of
improvement in quality and the extent of the time
during which acceptable quality is maintained
following application of a controlled-release mate-
rial has been shown to depend on the interac-
tion between the environment and the release


mechanism of the N source (Snyder et al., 1976,
Snyder et al., 1980). For example, N in
ureaformaldehyde (UF) is released by microbial
action. Cool weather, therefore, slows the release
of N and reduces turfgrass response to an applica-
tion of UF. Nitrogen release from isobutylidene
diurea (IBDU), on the other hand, is primarily by
dissolution, so temperature has much less effect on
N release than do particle size and moisture.
During extended periods of heavy rainfall or
irrigation, N release from IBDU can be faster than
is desired.
Commensurate with the observation that a
single application of a controlled-release N source
resulted in a longer period of acceptable turfgrass
quality than was obtained by a single application of
a water-soluble material was the observation that a
greater proportion of the N applied in the water-
soluble form leached beyond the root-zone. For
example, in the experiment illustrated in Figure 2,
13% of the N applied as calcium nitrate was found
to leach beyond the root-zone, whereas virtually
none of the N applied as UF leached (Snyder et al.,
1980). Clearly, water-insoluble N sources can be
used both to produce quality turfgrass and to
minimize N leaching. This benefit is especially
important during periods of excessive rainfall.
The availability of various sources of N provides
flexibility for the management of turfgrass. A turf
manager may use controlled-release N as part of a
routine fertilizer program and then use small
amounts of rapid release products as required to
gain quick green-up after winter dormancy; rapid
growth during turf establishment; and recuperation


Table 1. Nitrogen fertilizer sources commonly used for turfgrass.

Nitrogen Nutrient
Nitrogen source Content Release Comments
(%)
Ammonium nitrate 33 Rapid High N content
Ammonium sulfate 21 Rapid Acidifying, supplies sulfur
Calcium nitrate 16 Rapid Deliquescent, supplies calcium
Diammonium phosphate 20 Rapid Supplies phosphorus
Monoammonium phosphate 11 Rapid Supplies phosphorus
Potassium nitrate 13 Rapid Supplies potassium
Sodium nitrate 16 Rapid Naturally occurring inorganic (guano)
Urea 45 Rapid Very high N content, water-soluble organic
Isobutylidene 31 Slow Release controlled by hydrolysis, largely
diurea (IBDU) temperature independent
Resin-coated Variable Slow Release by diffusion through the resin
(e.g., Osmocote) coat
Sewage sludge 6 Slow Natural organic, release by microbial
(e.g. Milorganite) action, very low N content
Sulfur-coated urea 32 Slow Release by weathering
(SCU) of sulfur coating
Ureaformaldehyde (UF) 38 Slow Release by microbial action,
temperature dependent







Color rating


0 1 2 3 4 5 6 7 8
Time since N application (weeks)
Source: Snyder et al., 1980

Figure 2. Effect of two N sources on turfgrass color ratings.


from damage caused by insects, nematodes, dis-
eases, cultivation practices, and heavy play or use.
Another example of choosing a N source to affect
turfgrass management is the use of acid-forming
inorganic N sources to correct soil chemical related
deficiencies. For example, manganese deficiency
due to high soil pH has been corrected by using


ammonium sulfate as the N-fertilizer source
(Snyder et al., 1979).
Although turfgrass managers can purchase
single source fertilizers, fertilizer companies often
blend N sources in order to provide to consumers
the benefit of both type of sources. The rapid-
release material provides quick plant response,

I


Fertilizers are often applied as granular materials. In photo A (left), by a homeowner. In photo B (right), by commercial application.


'11






































Figure 3. Sample fertilizer label.


while the slow-release N provides a stable level of
nutrition for an extended period of time.
When purchasing fertilizer, consumers should
make informed choices based on the contents of the
bag, and not just on price. Soluble forms of fertil-
izer generally are less expensive than slow-release
forms. Florida law protects consumers by requiring
fertilizer companies to state on the bag or tag the
precise nutrient contents including a wealth of
information regarding N quantity, and N source
(Sartain, 1989). At the top of a fertilizer tag are
three numbers which represent the guaranteed
analysis of the three most frequently supplied
nutrients: N, phosphorus (P), and potassium (K)
(Figure 3). By convention, N is represented as the
element (N), while P and K are represented in
oxide form as P205 and O20, respectively. Refer-
ring to Figure 3, the analysis "16-4-8" indicates
that this bag contains by weight 16% N, 4% P205,
and 8% KIO. The various forms of N are also listed
on the label. In this example (Figure 3), there is no
nitrate-N, and the readily available ammoniacal


nitrogen content is 12% of the total fertilizer
weight. Therefore, in this example, 75% (12%/16%)
of the total N in the bag is readily available as the
ammonium form. The label also indicates that the
ammonium-N is derived from sulfate of ammonia
and diammonium phosphate. Since sulfate of am-
monia (ammonium sulfate) is listed first, it is the
predominant source of ammoniacal N. According to
the label, there is 0.55% water-soluble organic and
3.45% water-insoluble N. IBDU is the source of
these N forms. Note that the quantity of urea
(none in this example) must be specified on the tag.
Therefore, fertilizers may claim to be "100% or-
ganic," but contain only water-soluble N. From the
information listed on the tag, we can calculate that
12.55/16.0 or 76.4% of the N in this bag is readily-
available water-soluble N. The product may be
advertised to contain slow-release N, as it does, but
by looking at the label the buyer can observe that
most of the N is water soluble. Since this is a 50 lb
bag, 6.28 lbs of N (0.1255 x 50) out of a total 8 lbs of
N (0.16 x 50) in this bag is readily available.


~~-48







The mechanism of N release from slow-release
products varies with the product. Nitrogen release
from sulfur-coated urea (SCU) is related to the
thickness and integrity of the sulfur coating and
sealant barrier surrounding the urea granule.
Breakdown of the sulfur barrier is controlled by a
combination of mechanical, physical, chemical, and
biological processes. Nitrogen release from resin-
coated materials occurs as diffusion through the
resin coating.
The slow-release property of other N products is
a function of the molecular structure of the N
containing compound (Turgeon, 1985). For ex-
ample, N from IBDU is released as a function of
hydrolysis of two ureas which are attached onto the
molecule isobutyraldehyde. Therefore, N availabil-
ity is governed by water and IBDU granule size.
Smaller granules release N more rapidly than large
granules. Nitrogen is slowly made available from
various sized methylene ureas in UF via microbial
activity. Release of UF-N is mainly controlled by
temperature, as microbial activity is temperature
dependent, and by the mixing of various molecular
weight methylene ureas in the product formulation.
Since larger molecules must be broken down into
lower-weight molecular units, the greater the
proportion of large molecular weight methylene
ureas the slower release of N. Another source of
slowly-available organic N is composted sewage
sludge, such as Milorganite. Milorganite is a
virtually completely water-insoluble material that
depends on microbial decomposition for N release.
From this discussion it is clear that different
slow-release N materials will be more effective at
certain times. IBDU might be a better slow-release
choice during Florida's cool, dry season while long-
chain UF or Milorganite might be a more appropri-
ate choice during the warm, rainy season. Turf-
grass managers should not rule out the use of
readily-available forms of N. These sources have
an important role in turfgrass management, but
good judgement is necessary when using readily
available sources to avoid over-exposure of both the
turfgrass and the environment to excess water-
soluble N.

Nitrogen rates
Turfgrasses often respond dramatically to N
fertilization. Growth generally increases, the grass
competes better with certain weeds, and it develops
a dark-green color that many desire and assume to
be a sign of healthy turf. Nitrogen fertilization may
be used to mask a variety of problems that probably


should be corrected. However, in recent years
turfgrass authorities have become more aware that
unusually dark-green color is not always a sign of
healthy turf, but rather, may be associated with
poor rooting, higher incidence of disease and insect
problems, and reduced tolerance to environmental
stress (Beard, 1973; Turgeon, 1985). Excessive N
fertilization leads to a reduction in the quantity of
carbohydrates needed for protein synthesis, which
can result in the death of the turfgrass root system,
and a restriction in rhizome and stolon growth.
Excessive N fertilization also stimulates unwanted
thatch production. For these reasons, turf manag-
ers should use N judiciously, and limit application
rates to the minimum consistent with turfgrass
use.
Necessary N application rates vary with the type
of turfgrass grown, its growth stage, where in
Florida it is grown, how the grass is utilized, and
the level of culture desired or required. For ex-
ample, more N is needed for sod production than
for home lawn maintenance. Turfgrass in south
Florida may need more N than that in north
Florida, because of the longer growing season in the
southern part of the state.


Beautiful turfgrass areas such as this golf course are enjoyed
by residents and visitors alike.

Four major warm-season turfgrasses are grown
in Florida. Each is recognized to have an optimal
utility and also a range of cultural requirements.
Generally, Florida grasses are separated by fertility
level which is either low, moderate, or high, and N
is a major component of this classification.
Bahiagrass and centipedegrass are low mainte-
nance grasses that typically receive 1 to 4 lbs N/
1000 sq. ft. each year. St. Augustinegrass, a
moderate fertility grass, receives 3 to 6 lbs N/1000









NOVEMBER-DECEMBER 1989


70


60-


50-


40


30


20-


10


0
0.5 b/1000 s
c 0.5 lb/1000 sq ft


10 20 2 12 22
TIME SINCE N APPLICATION (days)
+ 1.0 IbN/1000sqft o 2.0 Ib N/10(


)0 sq ft


Figure 4. Effect of N rate on soil-water nitrate for two application dates.


sq. ft. per year. Bermudagrass has the greatest
variety of turf uses in Florida and therefore the
widest range of fertility. Bermudagrass on high-
way roadsides typically receives 1 to 3 lbs N/1000
sq. ft. per year. However bermudagrass usually is
grown for its beauty or for its ability to endure
traffic, and it therefore generally receives rates of N
that are high compared to the other Florida
grasses. For home lawns, athletic fields, and golf
course roughs and fairways, the general practice is
to apply between 0.5 and 1 lbs N/1000 sq. ft. per
growing month. On intensively used golf greens
and tees, N is applied at 1 to 2 lbs N/1000 sq. ft./
growing month. By choosing the turfgrass type
that requires the least N, while still serving the
needed purpose, the chances of N leaching will be
diminished.
The quantity of nitrate-N leaching is positively
related to the amount of readily-available N
applied. For example, a recent study on N applica-
tion rates to bermudagrass turf cultured on sand
soils in Florida demonstrated that significantly


more nitrate-N leached beyond the root zone as the
N rate increased from 0.5 to 2.0 lbs N/1000 sq. ft.
(Figure 4). In an earlier study utilizing calcium ni-
trate, doubling the amount of N in a single applica-
tion (0.8 vs. 1.6 lb N/1000 sq. ft.) during a rainy
July-August period resulted in nearly eight times
as much N leaching (Snyder et al., 1980). Clearly,
nitrate-N losses can be substantially reduced if
managers use the least amount of soluble N com-
mensurate with acceptable turfgrass quality.
When using readily available N sources such as
ammonium sulfate, ammonium nitrate, and urea,
turf managers can closely control the N nutrition of
the turfgrass. In doing so, they may be able to
reduce N rates to the minimum required to achieve
acceptable turfgrass, and thereby decrease the
quantity of soluble nitrogen exposed to leaching in
the soil. This will be accomplished by making light,
but frequent, applications of N, a subject that will
be discussed in the next section.
Turf managers should also recognize that vari-
ous plant and soil amendments applied to turf also







may be sources of N. Many so called "weed and
feed" products provide N, so application rates need
to be accurately followed to avoid foliar burn or
ground water contamination from herbicides or N.
Turfgrass clippings are a source of slow-release
N (Starr and DeRoo, 1981), so it is reasonable to
assume that N rates can be reduced when clippings
are returned during mowing rather than being
removed and bagged. This practice is incompatible
with some turf uses, such as golf greens, but it
often can be used in other turf situations. It may
be possible to reduce N fertilizer rates by one-third
or more when clippings are returned.

Frequency of nitrogen application
Nitrogen application frequency varies with the
annual N rate and with the N source. For
turfgrass, IFAS generally recommends that no
more than 1 lb N/1000 sq. ft. be used per applica-
tion if the N fertilizer contains more than 50%
readily available N (Sartain and McCarty, 1990).
This applies to most commercially available turf
fertilizers, since many contain both rapid and slow-
release materials, with the former generally being
supplied in greater quantity than the latter. If an
upper limit of 1 lb N/1000 sq ft per application is
adhered to, it is obvious that application frequency
will increase as the required annual rate of N
increases. Therefore, N will be applied more
frequently to intensively-used bermudagrass than
to roadside bahiagrass.
There are several reasons for limiting the
amount of readily-available N applied at one time.
Certainly the goal of reducing the opportunity for N
leaching is one. By minimizing the amount of
available N in the soil at any one time, greater
opportunity is afforded the turfgrass to absorb most
of the N, leaving less N susceptible to leaching.
But there are horticultural reasons for this recom-
mendation as well. Because turfgrass growth
generally increases markedly after a heavy applica-
tion of N, turf managers may have trouble mowing
frequently enough to keep up with this growth.
Furthermore, excessively lush turfgrasses are more
susceptible to certain insect and disease problems.
So the practice of limiting the amount of readily-
available N applied at one time makes sense both
from an environmental and a horticultural view-
point. The frequency of application can be de-
creased when controlled-release N sources are
used, since N in these materials is protected from
entry into the soil solution and entry occurs over an
extended period of time. The frequency of applica-
tion will vary with the controlled-release N source,


and with environmental conditions that affect N
release. In general, SCU and the finer-textured
IBDU products release N at a faster rate than does
UF or sewage sludge, especially during cool
weather, so application frequency will be greater
for the former two products than for the latter two.
Water-soluble N sources require the shortest
application interval if turfgrass growth is to be
uniform with time, and N leaching is to be mini-
mized. But the amount of water-soluble N applied
at any one time should be kept to a minimum.
Increased application frequency generally
requires increased labor, and may increase disrup-
tion of turf usage. There are several ways to
minimize these problems, however. Nitrogen may
be applied as a liquid from a spray vehicle, and by
so mechanizing the operation, weekly application of
small amounts of N may be possible. Some golf
courses apply N to heavy-use areas in this manner.
Nitrogen may be applied even more frequently at
even lower per-application rates by applying it
through the irrigation system with each irrigation
(fertigation). Because fertigation permits turf
managers to deliver small quantities of fertilizer as
needed, this method provides an important practi-
cal approach to limit nitrate-N leaching. By
reducing the quantity of N applied with each
irrigation, the quantity of N exposed at any one
time to potential leaching events is greatly reduced.
By reducing the labor needed for fertilizer applica-
tion, fertigation makes it possible to apply water-
soluble N frequently, but at low rates per applica-
tion. It has been shown that fertigation stabilizes
the N nutrition of turfgrass and reduces N leach-
ing, relative to conventional fertilization of the
same amount of N in one application, bi-monthly
(Snyder et al., 1989).
Users of effluent (treated sewage) water in a
fashion are also fertigating. However, an impor-
tant distinction from both a horticultural stand-
point and from the standpoint of N leaching is that
actual fertigators, i.e., those injecting nutrients into
their irrigation water, control the amount of nutri-
ent application and the irrigation frequency,
whereas effluent users may be required to apply
pre-set amounts of irrigation daily, regardless of
weather, in order to dispose of effluent, and they
may have no control over the amount of nutrient,
including N, contained in the effluent. Nitrogen
leaching into the groundwater is possible under
these conditions.
Many lawn-service companies apply N in a liquid
form. Generally these applications are made only a
few time each year, and sufficient N is applied to








Table 2. Effect of irrigation method and N application on nitrate-N leaching below a turfgrass root zone.

Nitrogen application source or method'
Irrigation type Ammonium nitrate Sulfur-coated urea Fertigation
- N leaching as a % of that applied -----
Daily, at 125% of expected ET 53.6 14.4 3.5
Based on moisture sensors2 1.9 0.3 0.3
Source: Snyder et al., 1984.
'Nitrogen application rate was 1 lb./1000 sq. ft./month applied bimonthly, except in the case of fertigation for which N was applied with each
irrigation to provide the same bi-monthly rate of N.
2Moisture sensors voided pre-scheduled irrigations when soil moisture was adequate.


provide the desired yearly total in only a few
applications. In most cases, the fertilizer must be
washed off of the foliage to prevent burn since
appreciable N is distributed with each application.
This form of application is distinctively different
from fertigation. Since water-soluble N sources
generally are used, application rates should not
exceed 1 lb N/1000 sq. ft. in order to minimize N
leaching.

Irrigation
Irrigation is a management tool that can be used
to help stabilize soluble N within the turf root zone.
Water soluble nitrate-N, and to a lesser degree
ammonium-N, will move in the soil with percolat-
ing water. Since rapid percolation can occur in
coarse textured soils, both the quantity and timing
of irrigation must be carefully regulated, especially
when fertilizers have been recently applied. Re-
search at the University of Florida has demon-
strated that, during relatively dry periods, an
efficient irrigation program can reduce the quantity
of nitrate-N moving to ground water (Table 2).
Even with approximately 60 inches of rainfall in
Florida, frequent irrigation is required to produce
turfgrass. Daily turfgrass evapotranspiration (ET)
ranges between approximately 0.05 and 0.3 inches,
depending upon geographical location in Florida
and time of the year. To irrigate properly (amount
and frequency), one must know the root-zone depth,
root-zone soil water holding capacity, and irrigation
system output. In addition, an accounting must be
made for season (Augustin, 1983), and any previous
substantial rainfall. A sound irrigation program
will discourage shallow rooting and encourage a
deep root zone by providing enough water to
thoroughly wet the root zone, without causing run-
off or deep percolation.
The majority of a turfgrass root system is located
no deeper than 4 to 12 inches, depending on grass
type and cultural management. Most close-cut


bermudagrass roots will be in the top 6 inches of
soil. The majority of St. Augustinegrass,
centipedegrass, and bahiagrass roots will be in the
upper 12 inches of soil.
Florida sand soils are particularly drought,
with approximately 0.08 inches of water-holding
capacity per inch of soil depth. The total soil-water
reservoir for a St. Augustinegrass lawn that is
grown in a sand soil will be approximately 1.0 inch
per foot of root depth.
The most efficient way to irrigate lawn turf is to
apply water when there are visual signs of plant
water stress, such as leaf blade folding, when
footprints remain after walking, or the turf takes
on a blue gray color.
Irrigation duration depends upon the rate at
which the system delivers water. For effective
irrigation scheduling, managers should calibrate
each irrigation zone in order to estimate how long it
takes to put out a unit amount of water (see IFAS
Extension Fact Sheet EH-61).
Since the goal of irrigation is to restore soil
moisture after a certain amount of depletion has


Irrigation, fertilization, and fertigation experimental plots at
FLREC.








occurred, the same amount of irrigation will be
required regardless of the season. The interval
between irrigations, however, will change with the
season, since during warm weather it will take less
time for depletion to occur than is required during
cool weather.
In some instances, professional managers
irrigate daily to replace water lost by evapotranspi-
ration and maintain the soil water content near
field capacity. To do this without causing percola-
tion, the irrigation amount must be constantly
monitored and adjusted. Soil moisture sensors,
such as tensiometers, offer a method to automati-
cally schedule irrigation based upon soil-water
status. Tensiometers for this purpose have been
commercially available for many years, and a
variety of other devices for scheduling irrigation
on the basis of soil moisture content have been
marketed periodically. A tensiometer is essentially
a water-filled tube that at one end has a
water-porous ceramic cup and at the other end has
a vacuum gauge for measuring soil-moisture
tension, which is related to soil moisture. The
ceramic cup is installed directly into the turf root-
zone. After the water in the sensor equilibrates
with the surrounding soil water, water will be
removed through the ceramic cup as the surround-
ing soil dries, creating a tension on the water
column, which is in turn recorded on the vacuum
gauge. On sand soils, irrigation generally is needed
when the dial reaches 10 centibars. Although
moisture sensors can be used to automatically
activate an irrigation system, it usually is best to
schedule irrigation daily and then let the tensiom-
eters void pre-scheduled irrigations when soil
moisture is adequate. In that way, irrigation can
occur at a time of the day that will cause minimal
disturbance of those using the turf and during that
time of day when irrigation can be most efficiently
applied (generally early morning). Research at the
University of Florida's Ft. Lauderdale Research
and Education Center has shown that during
relatively dry periods, soil-moisture sensor based
irrigation can effectively reduce the loss of nitrate-
N, even when soluble sources are used (Snyder et
al., 1984). However, during periods of heavy
rainfall, irrigation scheduling alone, either manu-
ally or automatically, may not prevent nitrate-N
losses from conventionally-applied water-soluble
sources.

Practices for better root systems
Deep, active root systems are better able to
recover applied N and thereby prevent N leaching.


A number of cultural practices promote deep, active
root systems (Beard, 1973).
Root-zone depth is directly related to the height
of cut; as the mowing height is reduced root-zone
depth is decreased. Thus, turfgrass should be
mowed no closer that is required for the intended
turf usage. Furthermore, turfgrass should be
mowed frequently enough such that no more than
one-third of the leaf blade is removed in a single
mowing. When turfgrasses are "scalped," i.e. when
excessive leaf blade is removed, root systems are
adversely affected.
Root systems are also adversely affected by soil
compaction and by the presence of different tex-
tural layers in the soil profile. Both of these
conditions can reduce oxygen penetration into the
root zone. To function properly, roots need oxygen.
Various forms of mechanical aeration can be used
to increase air movement in the soil, and root
growth and activity often respond markedly to
mechanical aeration.
Thatch is an accumulation of undecomposed and
partially decomposed organic matter on the soil
surface. Some thatch is found in most stands of
turfgrass, but it can build up to excessive levels.
Excessive thatch may be caused by high rates of N
fertilization. In extreme cases, the majority of the
turfgrass roots may be located in the thatch layer,
and not in the soil. Thatch can be difficult to keep
moist, and it does not retain nutrients well. Thus,
when excessive thatch is present, additional irriga-
tion and fertilizer may be needed to maintain the
turfgrass, thereby increasing the opportunity for N
leaching. Thatch can be removed mechanically,
which results in improved turfgrass growth.
Turfgrass growth and appearance are adversely
affected by diseases, insects and by nematodes.




a .
".*


2CM

Controlling pests are an important aspect of turf management.


I








Nematodes, and certain insects and diseases, are
particularly damaging to root systems. In some
cases, these problems are masked by using extra N
fertilizer and irrigation, thereby increasing the
opportunity for N leaching. To avoid this, appropri-
ate measures should be taken to control diseases,
insects, and nematodes. Turf damaged by improper
herbicide application also has reduced capacity for
absorbing N, as does turf suffering from a nutrient
deficiency. Healthy, actively growing turfgrass can
scavenge for soil N better than diseased, damaged
turf.
Grasses are used as biological filters to cleanse
contaminated waters (Younger, 1974). Turfgrasses
need to be maintained in a healthy, active condition
to maximize their ability to serve as 'living filters,'
and soluble N in the soil should not exceed the
ability of the turfgrass to absorb N. These goals are
compatible with the objective of maintaining
quality turfgrass.

Literature cited
Augustin, B. J. 1983. Water requirements of
Florida turfgrasses. Bul. 200, Univ. Fla. (IFAS),
Gainesville.
Alexander, M. 1972. Accumulation of nitrate.
National Academy of Sciences, Washington,
D.C.
Beard, J. B. 1973. Turfgrass: Science and practice.
Prentice-Hall, Englewood Cliffs, N. J.
Sartain, J. B. 1989. Fertilizers and fertilization.
Bulletin 183-D, Univ. Fla. (IFAS), Gainesville.
Sartain, J. B., and L. B. McCarty. 1990. Under-
standing soil analysis and fertilization. p. 31-36
In L. B. McCarty, R. J. Black, and K. C. Ruppert
(eds.) Florida Lawn Handbook. Univ. Fla.
(IFAS), Gainesville.


Snyder, G. H., E. O. Burt, and B. L. James. 1976.
Nitrogen fertilization of bermudagrass turf in
south Florida with urea, UF, and IBDU. Proc.
Fla. State Hort. Soc. 89:326-330.
Snyder, G. H., E. O. Burt, and G. J. Gascho. 1979.
Correcting pH-induced manganese deficiency in
bermudagrass turf. Agron. J. 71:603-608.
Snyder, G. H., E. O. Burt, and J. M. Davidson.
1980. Nitrogen leaching in bermudagrass turf:
Effect of nitrogen sources and rates. p. 313-324
In R. W. Sheard (ed.) Proc. 4th Int. Turfgrass
Res. Conf., Guelph, Canada.
Snyder, G. H., B. J. Augustin, and J. M. Davidson.
1984. Moisture sensor-controlled irrigation for
reducing N leaching in bermudagrass turf.
Agron. J. 76:964-969.
Snyder, G. H., B. J. Augustin, and J. L. Cisar. 1989.
Fertigation for stabilizing turfgrass nitrogen
nutrition. p. 217-219 In H. Takatoh (ed.) Proc.
6th Int. Turfgrass Res. Conf. (Tokyo), Japanese
Soc. Turfgrass Sci., Tokyo.
Starr, J. L., and H. C. DeRoo. 1981. The fate of
nitrogen fertilizer applied to turfgrass. Crop
Science 21:531-536.
Turgeon, A. J. 1985. Turfgrass management.
Reston Pub. Co. Inc. Reston, VA.
Younger, V. B. 1974. Using effluent water for
irrigation. California Turfgrass Culture.
24(4):27-28.


















































































COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, John T. Woeste,
director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose of the May 8 and June
30, 1914 Acts of Congress; and is authorized to provide research, educational information and other services only to individuals and institutions that
function without regard to race, color, sex, age, handicap or national origin. Single copies of extension publications (excluding 4-H and youth
publications) are available free to Florida residents from county extension offices. Information on bulk rates or copies for out-of-state purchasers
is available from C.M. Hinton, Publications Distribution Center, IFAS Building 664, University of Florida, Gainesville, Florida 32611. Before publicizing
this publication, editors should contact this address to determine availability. Printed 7/91.




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