Northern Idaho Fertilizer Guide Current Information Series No. 448

Peas and Lentils
R. L. Mahler

These fertilizer guidelines were developed by the University of Idaho and Washington State University based on relationships between soil tests and crop yield response. The guidelines are based on research results and are designed to produce above-average yields if other factors are not limiting production. Thus, this fertilizer guide assumes the use of sound management practices.

The suggested fertilizer rates will be accurate for your field provided (1) your soil sample was properly taken and represents the areas to be cropped and (2) the crop and fertilizer history you supply is complete and accurate. For help in obtaining a proper soil sample, refer to University of Idaho Extension Bulletin 704, Soil Sampling, or consult the Extension agricultural agent in your county.

Nitrogen
Peas and lentils are legumes that can obtain or "fix" a portion of the nitrogen (N) they require from the atmosphere. The fixing is done by bacteria (Rhizobium leguminosarum) that form nodules on the roots of peas and lentils. These bacteria are present in adequate amounts in most northern Idaho soils.

Seed should be inoculated with this bacterium when (1) peas and/or lentils have not been grown in the field for 5 or more years before planting or (2) the soil pH is less than 5.7. Peas and lentils are most commonly inoculated with Rhizobium in a peat-based carrier using a custom inoculation, seed-applied system. Follow the inoculum manufacturer's recommendation for the inoculum rate per bushel of seed. If custom seed inoculation is not available, a peat-based carrier can be used with the planter box, seed-applied system. For additional information on inoculation and methods of inoculum application, see University of Idaho CIS 838, Inoculation of Legumes in Idaho.

In addition to fixed N, the soil often supplies some residual N to peas and lentils as well as N from decomposition of organic matter. Thus, N applications on peas and lentils in most cases have not been profitable. However, peas and lentils need to obtain some N from the soil early in their growth, before effective N-fixing nodules have formed.

Phosphorus
Phosphorus (P) needs can be determined effectively with a soil test (Table 1). Incorporate P into the seedbed by whatever method is most convenient. Acceptable methods include (1) broadcast and plow-down or disk-in, (2) band, and (3) drill with seed. Do not allow direct contact between the seed and any fertilizer containing more than P.

Germinating peas and lentils are extremely sensitive to salts contained in fertilizer N, K, and S. If heavy P applications are required to correct nutrient deficiencies, apply fertilizer before or during seedbed preparation.

Potassium
Potassium (K) needs can be determined with a soil test (Table 2). Incorporate K into the seedbed by whatever method is most convenient. Acceptable methods include (1) broadcast and plow-down or disk-in, (2) band, and (3) drill with seed. Do not allow direct contact between the seed and the fertilizer because peas and lentils are sensitive to salts during germination. If heavy K applications are required to correct nutrient deficiencies, apply fertilizer before or during seedbed preparation.

Sulfur
Without adequate sulfur (S), pea and lentil plants are unable to fix enough atmospheric N to meet their needs. Consequently, soils testing at less than 10 ppm (SO₄-S) should receive 15 to 20 pounds of S per acre. Avoid using granular elemental S on peas and lentils because this form of S becomes available only slowly. Elemental S also greatly reduces soil pH.

Micronutrients
Boron -- Peas and lentils grown in northern Idaho respond to boron (B) applications. Boron need can be determined by a soil test. Soils testing at less than 0.5 ppm B should receive 1 pound of B per acre. Boron can be toxic at excessive rates or when concentrated near seedlings. Boron fertilizer should always be broadcast, never banded. 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.

Molybdenum -- Peas and lentils grown in northern Idaho also respond to molybdenum (Mo). Because Mo is present in soil in only small amounts, soil Mo analysis is not commercially available. Consequently, base Mo fertilizer applications on cropping history and soil pH. Apply Mo as a seed treatment on peas and lentils at the rate of 1/8 to 1/2 ounce Mo per acre when (1) the soil pH is less than 5.7 or (2) every third time peas and/or lentils are grown in a field. Do not exceed 1/2 ounce Mo per acre because at higher rates the N-fixing bacteria may die. For more information on Mo, refer to University of Idaho CIS 1087 (formerly CIS 589), Essential Plant Micronutrients: Molybdenum in Idaho.

Zinc -- Response of peas and lentils to zinc (Zn) applications is extremely rare. Zinc applications of 5 pounds per acre should be considered only where Zn soil test levels are less than 0.6 ppm. For more information on Zn, refer to University of Idaho CIS 1088 (formerly CIS 617), Essential Plant Micronutrients: Zinc in Idaho.

Other micronutrients -- Peas and lentils have not been reported to respond to applications of chlorine (Cl), copper (Cu), iron (Fe), or manganese (Mn). Therefore, applications of these materials in northern Idaho is unnecessary.

Lime -- Consider lime applications of 1 ton per acre on fields with pHs of 5.3 and below. Reduced pea and lentil yields may occur at soil pHs of 5.4 or lower. However, the yield response from liming at pH values near 5.4 may not always be economical. For additional information on the impact of soil pH on peas and lentils, refer to University of Idaho CIS 811, The Relationship of Soil pH and Crop Yields in Northern Idaho.

Starter fertilizers
Starter fertilizers have not produced a consistent economic advantage in pea and lentil production. Starter fertilizers are most beneficial in years of particularly cold, wet soils, which are unfavorable for peas and lentils. If used, starter fertilizers should be placed below and to the side of the seed at planting, not with the seed. Do not allow direct contact between the seed and the fertilizer because peas and lentils are sensitive to salts during germination and early seedling growth.

General comments

• Peas and lentils take up residual soil nitrates (nitrates not used by preceding cereal crops) and therefore reduce the potential for N loss by leaching. Thus, peas and lentils can have a positive impact on groundwater quality.
• Early planting of both spring and winter varieties is critical for maximum economic yields.
• Using peas and lentils in a crop rotation can reduce disease and weed problems in grain crops.
• Spring-planted peas and lentils have generally been planted in seedbeds having a minimum of straw residue on the soil surface. However, spring peas grown under conservation tillage systems with moderate levels of surface residue typically produce similar or higher yields than peas grown under low-residue, intensive tillage systems. The greatest yield benefits are in relatively dry years.
• To prevent soil compaction, avoid tillage at high soil moisture levels. Also avoid over-working the soil and creating a finely pulverized surface that is vulnerable to erosion and prone to sealing and crusting.
• Avoid planting in poorly drained areas.
• If you need further information on cultural practices contact the Extension agricultural agent in your county.

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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