Northern Idaho Fertilizer Guide Current Information Series No. 788

Bluegrass Seed
Robert L. Mahler and Robert E. McDole

These fertilizer guidelines have been developed by the University of Idaho based on relationships between soil tests and crop yield responses to applied fertilizer. The fertilizer rates suggested are based on research results and are designed to produce above-average yields if other factors are not limiting production. Thus, the fertilizer guide assumes the use of good management practices.

The suggested fertilizer rates will be accurate for your field provided (1) your soil sample was taken properly and is representative of the area to be fertilized, and (2) the crop and fertilizer history you supply is complete and accurate. For help in obtaining a proper soil sample, confer with the Extension agricultural agent in your county.

Nitrogen fertilizer recommendations for orchardgrass, crested wheatgrass, creeping red fescue and bromegrass will be similar to those for bluegrass. Recommendations for other essential nutrients will be the same as those for bluegrass.

Nitrogen
Nitrogen (N) is the most important nutrient in bluegrass seed production, and large amounts are needed for established stands. The amount of N fertilizer required on any field depends on the following factors:

1. The bluegrass variety and its yield potential in your location. The variety Argyle generally requires less N for optimum yields than other varieties.
2. Total annual precipitation and other climatic factors.
3. The age of the bluegrass seed.
4. The amount of usable N in the soil profile, including predicted mineralizable N (released from decomposing organic matter during the growing season) and inorganic soil test N in the forms of nitrate (NO₃) and ammonium (NH₄).

Once the total amount of N needed to produce a bluegrass seed crop is know, a simple equation can be used to determine the amount of fertilizer N that will meet this need:

Total N need based on potential yield (from Table 1) - [mineralizable N (from Table 2) + soil test N (from Table 3)] = fertilizer N needed

Mineralizable Nitrogen -- Soils vary in their capacities to release N from organic matter during the growing season. The rate or amount of N released depends on factors such as amount of soil organic matter, soil erosion, available soil moisture and soil temperature during the growing season. Northern Idaho soils release mineralizable N at four different levels based on their organic matter contents (Table 2).

Soil Test Nitrogen -- The amount of inorganic N (NO₃ and NH₄) in the soil can be evaluated most effectively with a soil test. Soil samples should represent the rooting depth of the crop since nitrate-nitrogen (NO₃-N) is mobile in the soil. Bluegrass grown for seed is capable of removing N to a depth of 2 feet or more.

Soil test values include both NO₃-N and ammonium-nitrogen (NH₄-N). To convert soil test NO₃-N and NH₄-N values to pounds N per acre, add the N values (ppm) for each foot increment of sampling depth and multiply by 4 (Table 3).

Fertilizer N -- The calculation for N fertilizer needed is:

Total N needed (Table 1)		__________
Minus mineralizable N (Table 2)	-	__________
Minus soil test N (lb/acre) (Table 3)	-	__________
N fertilizer required (lb/acre)		__________

Example: With the Argyle variety in a 22-inch rainfall zone, 2.5 percent soil organic matter and soil test values from the examples in Table 3, 91 pounds N per acre are needed:

Total N needed (Table 1)		175
Minus mineralizable N (Table 2)	-	40
Minus soil test N (lb/acre) (Table 3)	-	44
Equals N fertilizer required (lb/acre)		91

Phosphorus
Phosphorus (P) is important for good root development and seed production. Plants use phosphorus most efficiently when it is incorporated into the seedbed before seeding because P is not mobile in soils. For established stands, P can be surface-applied in fall. Phosphorus needs of established grass and of seedbeds before seeding can be determined with a soil test (Table 4).

Potassium
Because potassium (K) is relatively immobile in the soil, it should be applied at seeding and incorporated into the soil. For established stands, apply K as a topdress treatment in the fall. Most soils in northern Idaho contain sufficient amounts of K for maximum seed yields. However, the soil should be tested for K every 3 years. Potassium recommendations for established stands and for seedbeds before seeding can be determined by a soil test (Table 5).

Sulfur
Without adequate sulfur (S), bluegrass appears light green to yellowish-green, resembling plants with a N shortage. Plants require S to use N efficiently. Since S is mobile in soil, it is subject to leaching. Consequently, you need to test soil for S every year. Sulfur needs based on soil test results are shown in Table 6.

Micronutrients
Grasses rarely respond to applications of micronutrients in northern Idaho. Recommendations for specific micronutrients follow.

Boron -- Boron (B) should be applied only when soils test less than 0.3 ppm B. Needed B should be surface broadcast at a rate not exceeding 11/2 pounds B per acre. For more information on B and specific fertilizer materials, refer to University of Idaho CIS 1085 (formerly CIS 608), Essential Plant Micronutrients: Boron in Idaho.

Zinc -- Although zinc (Zn) deficiencies in northern Idaho are rare, applications should be considered when soil test levels are less than 0.6 ppm Zn in the surface 12 inches of the soil. Zn fertilizer should be surface broadcast at the rate of 5 pounds Zn per acre. For more information on Zn, refer to University of Idaho CIS 1088 (formerly CIS 617), Essential Plant Micronutrients: Zinc in Idaho.

Other micronutrients -- Bluegrass has never been shown to respond to applications of chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn) or molybdenum (Mo). Therefore, application of these materials for grass seed production is not recommended.

Lime
Grass is more tolerant of low pH conditions than wheat, barley, peas, lentils and alfalfa. Bluegrass seed yields fall only when soil pH is less than 5.0. When soil pH is less than 5.0, consider lime applications of 1 to 11/2 tons per acre. Because lime must be incorporated into the soil to be effective, it must be applied before seeding. Topdress applications of lime on established sod will not effectively increase soil pH.

General Comments

1. All fertilizer sources of N, P and K are equally satisfactory for grass seed production. Sulfate (SO₄^2-) forms of S are recommended. Elemental S should not be considered a sulfur source because it becomes available to plants slowly.
2. All fertilizer N should be applied in fall. Research has shown that N applied in spring often reduces bluegrass seed yields. Phosphorus, potassium and sulfur should also be applied in fall.
3. Grass stubble burning not only causes loss of N to the atmosphere but also results in S loss. Consequently, growers must be sure to monitor soil S levels.

Issued in furtherance of cooperative extension work in agriculture and home economics, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, LeRoy D. Luft, Director of Cooperative Extension System, University of Idaho, Moscow, Idaho 83844. The University of Idaho provides equal opportunity in education and employment on the basis of race, color, religion, national origin, gender, age, disability, or status as a Vietnam-era veteran, as required by state and federal laws.

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Revised: January 3, 2002

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