Northern Idaho Fertilizer Guide Current Information Series No. 851

Legume and Legume-Grass Pastures
R. L. Mahler

These fertilizer guidelines have been developed by the University of Idaho and Washington State University based on relationships between soil tests and crop yield responses to applied fertilizers. The suggested fertilizer rates are designed to produce above average yields if other factors are not limiting production. Thus, the fertilizer guidelines assume good management.

These suggested rates will be accurate for your field provided that (1) the soil samples are properly taken and are representative of the field to be fertilized, and (2) the crop and fertilizer history supplied is complete and accurate. (For help in obtaining a proper soil sample, see University of Idaho Extension Bulletin 704, Soil Sampling, or consult your Extension agricultural agent.)

Harvested legumes and legume-grass mixtures remove large quantities of nutrients from the soil. Incorporate fertilizer into the soil as the seedbed is prepared, and apply additional amounts periodically over the life of the stand.

Established non-irrigated legumes and legume-grass mixtures
Nitrogen (N), phosphorus (P), potassium (K), sulfur (S), boron (B), and molybdenum (Mo) are nutrients essential for plant growth but often deficient in northern Idaho pastures. Deficiencies of other essential plant nutrients -- calcium (Ca), magnesium (Mg), copper (Cu), chlorine (Cl), manganese (Mn), iron (Fe), and zinc (Zn) -- are rare in northern Idaho pastures.

Nitrogen -- Pure stands of legumes such as alfalfa, birdsfoot trefoil, and clovers should not require N fertilizer because legumes have the capacity to fix their N requirement when sufficiently nodulated with rhizobia. Nitrogen fixation efficiency depends on adequate levels of other nutrients (especially S) and acceptable soil pH (nontoxic levels of aluminum and manganese). Excessive soil acidity can interfere with N fixation.

Nitrogen fertilization is often beneficial when the pasture contains a legume-grass mix. When the legume stand is sparse and the grass stand is dense, apply 30 to 50 pounds of N per acre in the early spring. Thirty pounds per acre should be used on sandy soils, while 50 pounds per acre is recommended for finer-textured soils, such as: loams, silt loams, silty clay loams, silty clays, clay loams. When legumes compose up to 60 percent of the stand, apply 10 to 25 pounds of N per acre. Excessive N applications in this situation will lower the legume percentage in the stand.

Phosphorus -- Conduct a soil test to assess the P status of pastures. Apply P while preparing the seedbed for pasture establishment. On established stands, fall or winter surface P applications are most beneficial. Phosphorus may be applied in sufficient quantity on established stands to last 2 or 3 years (table 1).

Potassium -- Legume and legume-grass pastures remove large amounts of K from the soil. On established stands, fall or winter topdress applications of K are most beneficial. Most northern Idaho soils contain enough K for optimal forage production, but deficiencies occur in localized areas. Determine K needs with a soil test (table 2).

Sulfur -- Northern Idaho soils are often S deficient. A S deficiency in plants appears as a yellowing of the entire plant early in the growing season. This symptom is indistinguishable from a N deficiency. Sulfur deficiency can reduce both forage yield and quality.

Soils testing less than 10 parts per million (ppm) sulfate sulfur (SO₄-S) need 20 pounds of S per acre. Sulfur can be applied as gypsum or with liquid or dry fertilizer materials containing S. Use materials containing sulfate (SO₄). Since S is mobile and subject to leaching in soils, make S applications early spring. Applying S in the fall is not recommended.

Boron -- Legumes have a greater B requirement than grasses. Consequently, in B-deficient soils, legume forage production decreases in relation to the grass yield. Legumes grown in northern Idaho will respond to B applications when soils are deficient. Use a soil test to determine the need for B. A soil testing less than 0.5 ppm of B requires 1 to 2 pounds of B per acre. Do not exceed the 2 pounds per acre rate. Boron should be broadcast and not banded. High concentrations of B are toxic and could damage the legume.

Using borated gypsum is an effective and economical method of applying needed B and S. An application of 100 pounds of borated gypsum (1 percent B and 20 percent S) per acre supplies 1 pound of B and 20 pounds of S per acre. (For more information on B and availability of specific fertilizer materials, see University of Idaho Current Information Series 608, Boron in Idaho.)

Lime -- On highly acid soils (pH less than 5.6), apply lime to obtain maxiumum legume yields. A highly acid soil reduces the nitrogen-fixing potential of legume root nodules. Legume yields are highest when soil pH values are more than 5.8. Yet, grass production in pastures is not reduced until soil pH values fall below 5.1. (For more information on acid soils, see University of Idaho CIS 787, Liming Materials, and CIS 811, Relationship Between Soil pH and Crop Yields in Northern Idaho.)

Established irrigated legumes and legume-grass mixtures
The P, K, S, B, and lime recommendations for irrigated legumes and legume grass mixtures are similar to recommendations for nonirrigated legumes and legume-grass mixtures. Nitrogen recommendations, however, are different. When the legume composes less than 60 percent of the stand, N fertilizer applications will improve both forage quality and yield. It is best to apply between 60 and 80 pounds per acre of N annually. This recommended rate of N should be applied in split applications with one-half applied in the fall, and the rest between mid-May and the middle of June. If late growth is desired, apply an additional 20 to 30 pounds of N per acre in late July.

Where forage yield potential exceeds 5 tons per acre annually, it may be desirable to apply 80 to 120 pounds of N per acre. These high N rates will decrease legume competitiveness, however, and may increase the potential for grass tetany (magnesium deficiency) in the spring.

New forage seedings
Consider soil fertility needs before establishing new forage crops. Both P and K are particularly important because these nutrients are immobile in the soil, and more available when worked into the seedbed before seeding. Prior to seeding, incorporate 80 pounds of P₂O₅ per acre and appropriate amounts of K (based on soil test; see table 2) into the seedbed. Add S according to soil test levels. Sulfur can be, but does not need to be, incorporated. In addition, 20 to 30 pounds of N per acre will aid legume establishment until the plants are able to fix their own N.

Inoculate legume seed with the compatible strain of rhizobia inoculant and coated with Mo just prior to planting. (For additional information on Mo, see University of Idaho CIS 589, Molybdenum in Idaho. For additional information on inoculation see University of Idaho CIS 838, Inoculation of Legumes in Idaho.)

General Comments

1. Phosphorus, S, and B are the elements northern Idaho legume and grass-legume pastures most often need. At times, applying N and K will also improve plant growth.
2. Since P and K are relatively immobile in soils, it is best to incorporate these nutrients into the seedbed before seeding.
3. When the pasture is established, apply Mo as a seedcoat on legumes. Apply rhizobia inoculant to legume seed just prior to planting.
4. Legumes grow poorly in soils with pH values less than 5.6. Lime applications may be necessary to correct soil acidity. Apply and incorporate lime into the soil before the forage is established.
5. Acid soils and high levels of soil N will favor grass growth at the expense of legumes.
6. Using N will tend to reduce the proportion of legumes in a grass-legume mixture, while using P will tend to increase the proportion of legumes.
7. Seed the best-adapted legume and grass varieties for your area.
8. Legume-grass mixtures often yield higher than grass alone, have higher protein content, and are less prone to grass tetany problems.
9. Sulfur and N can increase protein (quality) content of forages.

Further readings

EXT 704	Soil Sampling (50 cents)
CIS 1085 (formerly CIS 608)	Essential Plant Micronutrients: Boron (40 cents)
CIS 787	Liming Materials (25 cents)
CIS 811	The Relationship of Soil pH and Crop Yields in Northern Idaho (35 cents)
CIS 1087 (formerly CIS 589)	Molybdenum in Idaho
CIS 838	Inoculation of Legumes in Idaho (35 cents)

To order copies of this or other University of Idaho College of Agriculture publications, contact your Extension agricultural agent or write to Agricultural Publications, Idaho Street, University of Idaho, Moscow, Idaho 83844-2240, or call (208) 885-7982.

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.

1,000, June 1993 (revised) 50 cents per copy

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Revised: January 3, 2002

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