F
ERTILIZER
M
ANAGEMENT FOR
G
RASS AND
G
RASS
-L
EGUME
M
IXTURES
Rich Koenig, Extension Soil Specialist
Mark Nelson, Beaver County Extension Agent
James Barnhill, Weber County Extension Agent
Dean Miner, Utah County Extension Agent
August 2002
AG-FG-03
I
NTRODUCTION
Grass and grass-legume mixtures serve
an important role in providing hay and pasture-
based forage for livestock in Utah. With proper
species selection and favorable irrigation,
fertility, and harvest or grazing management,
yields of 8 tons/acre or more have been
achieved. Management is the key to successful
forage production. Supplying the correct
amount of nutrients is one important
management factor.
F
ERTILITY
R
EQUIREMENTS
Grass grown for hay removes large
quantities of nitrogen, phosphorus and other
nutrients from soil (Table 1). Grass grown for
pasture removes lower quantities of nutrients
since as much as 85 to 90% of the nutrients
consumed in the forage are redeposited on the
pasture in the form of manure and urine. Due to the recycling effect of grazing animals the
nutrient requirements of pasture differ from the requirements of fields harvested for hay.
Legumes such as alfalfa and clover have the ability to take nitrogen from the atmosphere
and convert it into a form usable by plants. Including legumes in mixtures with grass lowers the
amount of nitrogen fertilizer required to produce forages.
Fertilizer recommendations for phosphorus, potassium, sulfur and micronutrients are
based on current soil test data. In grazed pastures, these nutrient levels remain relatively stable or
decline at much slower rates than in hay or field crop situations due to the recycling effect of the
livestock. Therefore, once a soil test is completed and the fertilizer applied, retest pastures every
three years to monitor nutrient levels. Hay fields should be tested annually.
Soil Testing is Essential!
It is difficult to generalize about the
location and occurrence of specific
nutrient deficiencies in Utah. Soils are
inherently variable due to both geologic
processes and historic manure and
inorganic fertilizer use. In addition, some
irrigation water sources add nutrients to
soil. For these reasons, soil testing is
essential to determine which nutrients are
needed and in what amounts for forage
production. Fertilizer recommendations
described in this guide are based on the
latest research results from Utah State
University trials.
Figure 1. The effect of nitrogen rate on tall fescue
and mixed fescue-clover yield.
Perennial ryegrass response to nitrogen
(photo taken in May). The plot on the left
received no nitrogen, the plot on the right
received 50 lb N/acre in April.
F
ERTILIZER
R
ECOMMENDATIONS
Nitrogen. Nitrogen is the main
nutrient needed by grass and certain grass-
legume mixtures. Both the rate and timing of
nitrogen applications will influence the yield,
quality and seasonal distribution of forage.
Grass and grass-legume mixtures
vary widely in productivity. Utah State
University research shows that responses to
nitrogen can be as high as 100 pounds of
forage produced per pound of nitrogen
applied (Figure 1). Many factors govern the
productivity of grass and grass-legume
mixtures, including soil conditions, irrigation water availability and management, and grazing
management. Nitrogen recommendations based on the production potential of the site and the
proportion of legume in the stand are summarized in Table 2. It is critical that growers identify
the production potential of the site and fertilize according to that potential. Production potential
can be estimated by assuming that grass-based forages will be comparable in yield to alfalfa on
the site.
Nitrogen will increase the crude protein content of pure grass and grass-dominated
stands. Research has shown that the crude protein content of a grass like tall fescue can be
increased from a low of <9% in unfertilized stands to a high of 18% with the application of 150
lb nitrogen/acre. Legumes also significantly improve forage quality.
The seasonal distribution of forage can also be managed with nitrogen applications. Cool
season grasses normally have a high rate of production in spring followed by a slowing of
growth during the hot summer months. Applying large amounts of nitrogen in early spring will
stimulate rapid growth and high yields. If this forage cannot be harvested for hay or grazed,
consider postponing the early spring application of nitrogen or fertilizing only part of the field.
Table 1. Average nutrient concentrations and
removal by grass hay.
Nutrient
Dry matter
concentration
Removal per
ton of hay
Nitrogen
2.0 % N
40 lb N
Potassium
3.0 % K
2
O
60 lb K
2
O
Phosphate
0.65 % P
2
O
5
13 lb P
2
O
5
Sulfur
0.25 % S
5 lb S
Table 2. Nitrogen recommendations for irrigated grass and grass-legume mixtures.
Yield potential of the site
Stand composition
1-2 tons/acre
2-4 tons/acre
4-6 tons/acre
6-8 tons/acre
----------------- nitrogen recommendation (lbs/acre) -----------------
100% grass
50
75
1
100-150
1
150-200
1
75% grass, 25% legume
25
50
75-100
100-150
1
50% grass, 50% legume
0
25
50
75
25% grass, 75% legume
0
0
25
50
1
For pasture, split the total nitrogen rate into two or three separate applications. Apply 1/3 to
1/2 of the nitrogen in early spring, 1/3 to 1/2 in June, and the remainder in late August.
Schedule mid- and late-season nitrogen applications to coincide with irrigation or rainfall
events. For hay-pasture systems, apply 2/3 of the nitrogen in early spring and 1/3 after the
hay crop is removed to stimulate regrowth for grazing.
Phosphorus. Low phosphorus soils are common in Utah. Research shows that grass and
grass-legume mixtures will respond to phosphorus fertilizer applications when soils are deficient.
Phosphorus applications also favor legumes and can enhance the proportion of legumes in a
mixed stand.
Phosphorus recommendations based on current soil test results are summarized in Table
3. These recommendations are adequate for 2 years of hay production or approximately 4 years
of grazed pasture production.
Phosphorus movement in soil is very
limited; therefore, where possible apply and
incorporate fertilizer prior to pasture
establishment. Surface broadcast
applications are also effective and should be
made in the fall or early spring.
Various sources of phosphorus are
available, including triple superphosphate (0-
45-0; 45% P
2
O
5
), monoammonium
phosphate (11-52-0; 52% P
2
O
5
), and fluids
such as 10-34-0
(34% P
2
O
5
). Comparisons
indicate that, when applied at the same rate
of P
2
O
5
, the materials are equally effective.
Select a phosphorus source based on local
availability, ease of application, and cost per
unit of P
2
O
5
.
Potassium. Potassium deficiency is
less common than phosphorus deficiency,
Table 3. Phosphorus recommendations for
grass and grass-legume mixtures. Soil test
phosphorus is based on a 12 inch sample
depth and sodium bicarbonate soil extract.
Soil test phosphorus
(mg/kg soil or ppm)
Recommendations
(lbs P
2
O
5
/acre)
0 to 3*
100-125
4 to 7
75-100
8 to 10
50-75
11 to 15
0-50
> 15
0
*Low soil test levels are severely limiting
yield. Test soil annually until levels are
adequate.
A mixed tall fescue-clover stand. Managing
the stand with nitrogen or phosphorus can
shift the balance to favor either the grass or
legume.
but is frequently found on sandy soils, fields
irrigated with clean waters low in potassium, and
high elevation sites. Limited research has shown
that grass and grass-legume mixtures respond to
potassium when soils are deficient. Fertilizer
should not be applied unless a soil test indicates
a need since fertilizing high potassium soils will
produce undesirably high potassium levels in the
forage. Potassium recommendations based on
current soil test results are summarized in Table
4.
Commonly available potassium sources
are potassium chloride (0-0-60; 60% K
2
O) and
potassium sulfate (0-0-50; 50% K
2
O). Select a
potassium source based on local availability,
ease of application, and cost per unit of K
2
O.
Sulfur. In Utah, sulfur deficiency has been identified on sandy, low organic matter soils,
fields irrigated with clean waters and high elevation locations. Sulfate-sulfur (SO
4
-S) soil test
values less than 8 mg/kg soil (ppm) indicate the need for sulfur fertilization.
Common sulfur sources in Utah include ammonium sulfate (21-0-0-24S; 24% sulfur),
potassium sulfate (0-0-50-18S; 18% sulfur), gypsum (17% sulfur), and elemental sulfur (0-0-0-
90S; 90% sulfur). Elemental sulfur is a slow release form that will last 3 to 4 years. Sulfate forms
are more soluble and readily available, but may leach out of the soil in 1 to 2 years. Where sulfur
deficiency has been identified, the application of 25 to 50 lbs sulfur/acre is recommended.
Micronutrients. Deficiencies of zinc, iron, copper, manganese and boron are rare in grass
and grass-legume mixtures. Soil testing will indicate whether these micronutrients are needed.
Soil test interpretations are summarized in
Table 5. If soil tests indicate a deficiency,
apply 5 (for marginal levels) to 10 (for low
levels) lbs of zinc, manganese, or iron, or 1
(for marginal) to 2 (for low) lbs of copper or
boron per acre. Sulfate salts are common
sources of zinc, manganese, iron, and
copper. Sodium borate and boric acid are
common sources of boron per acre.
A
DVANCED
T
ECHNIQUES
Fertigation. The application of liquid
fertilizers through the irrigation system is an
efficient way to supply nutrients, and allows
growers to make in-season or split
applications if desired. Liquid sources of
most nutrients are available. Carefully
compare the cost and convenience of using
Table 4. Potassium recommendations for
grass and grass-legume mixtures. Soil test
potassium is based on a 12 inch sample depth
and sodium bicarbonate soil extract.
Soil test potassium
(mg/kg soil or ppm)
Recommendation
(lbs K
2
O/acre)
0 to 40*
180-220
40 to 70
140-180
70 to 100
80-120
100 to 150
40-60
> 150
0
*low soil test levels are severely limiting
liquid sources relative to dry fertilizer
materials.
On-farm testing. Conducting on-
farm tests allows growers to customize
management programs for specific
situations. On-farm testing should be used
to evaluate new fertility programs. To
conduct an on farm test, treat several
strips in a field with the new fertility
practice and alternate with strips of the
standard practice. Where possible, try to
make several test strips across a field
instead of just splitting a field in half and
treating each half differently.
Keep records. Forages respond differently to fertilizer applications due to soil, variety,
and other management differences across farms. Keeping individual records of soil and tissue
test values, fertilizer applications, and yield for each field allows the grower to customize
fertility programs for specific situations.
A
CKNOWLEDGMENTS
Funding for the research to develop these fertilizer recommendations was provided by the
Utah Agricultural Experiment Station through the Utah State University Pasture and Forage
Initiative.
Utah State University is committed to providing an environment free from harassment and other forms of
illegal discrimination based on race, color, religion, sex, national origin, age (40 and older), disability, and veteran's
status. USU's policy also prohibits discrimination on the basis of sexual orientation in employment and academic
related practices and decisions.
Utah State University employees and students cannot, because of race, color, religion, sex, national origin,
age, disability, or veteran's status, refuse to hire; discharge; promote; demote; terminate; discriminate in
compensation; or discriminate regarding terms, privileges, or conditions of employment, against any person other
wise qualified. Employees and students also cannot discriminate in the classroom, residence halls, or in on/off
campus, USU-sponsored events and activities.
This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914,
in cooperation with the U.S. Department of Agriculture, Jack M. Payne, Vice President and Director, Cooperative
Extension Service, Utah State University. (EP/DF/08-02)
Table 5. Micronutrient soil test values (mg/kg
soil or ppm) and interpretations.*
Nutrient
Low
Marginal
Adequate
Zinc
<0.8
0.8-1.0
>1.0
Iron
<3.0
3.0-5.0
>5.0
Copper
<0.2
>0.2
Manganese
<1.0
>1.0
Boron
<0.25
0.25-0.5
>0.5
*DTPA extractable zinc, iron, copper, and
manganese; hot water extractable boron.