State University Extension Factsheet

Ohio State University Extension Fact Sheet

Horticulture and Crop Science

2001 Fyffe Court, Columbus, OH 43210-1096

Value of Legumes for Plowdown Nitrogen


W. H. Schmidt
Area Agronomist

D. K. Myers
Extension Agronomist, Emeritus

R. W. Van Keuren
Agronomist, Emeritus

The use of legumes for plowdown green manure is an accepted practice in many parts of Ohio for improving soil tilth and erosion control. With the current interest in improving our environment, this practice could receive renewed emphasis. As the cost of inorganic sources of nitrogen increases, more farmers are considering the use of legumes in their crop rotation to supply some of the nitrogen required for row-crop production.

Red clover or red clover-sweetclover mixtures are commonly overseeded in winter wheat during early spring. Sometimes legumes for plowdown are seeded in oats. In most cases, the legume is plowed the following spring preceding corn; however, at times the soil is plowed in late fall or early winter of the seeding year.

Roots left in the soil following the removal of various crops have an effect on future soil productivity. There can be a wide variation in the yield of roots and tops within a plant species. This variation is influenced by crop management and weather. Severe cutting or pasturing of a crop reduces the amount and depth of roots. Dry weather slows top growth and tends to increase the ratio of roots to tops. Weather or any other factor that encourages top growth tends to reduce the weight of roots in proportion to that of the tops (Table 1).

Table 1. Root Weight per Acre and Nitrogen Content.
Crop Time of Sampling D.M. Weight (lb.) Nitrogen (lb.)
Red Clover Averaged 48 dates 1,000 22
Alfalfa Late fall, seeding year 1,270 35
Alfalfa Late fall, first hay year 2,680 61
Alfalfa Late fall, second hay year 3,400 78
Sweetclover November, seeding year 2,640 95
Sweetclover July, yr. following seeding 800 13
Korean lespedeza August 15 560 8
Soybeans Pods well filled 550 8
Bromegrass July, 2-year-old sod 3,740
Timothy   5,000 93
Bluegrass Old sod *6,770 142
Perennial ryegrass Sown in corn and sampled following spring, before plowing *6,000 42
*Includes all underground parts, as well as, true roots.

The soybean has a low weight of roots when compared with the red clover-sweetclover or the perennial following several years of hay production. Similar variation exists in the amount of nitrogen produced per acre.

A summation of 14 years of Ohio data indicates corn yields following alfalfa and sweetclover were equal when both crops were plowed down one year following seeding. Mammoth and medium red clover were not as productive. Table 2 shows how the nitrogen and dry matter in legume roots and tops vary with the stage of growth. Leguminous green manure crops ordinarily are plowed under sometime between April 15 and May 15 the year following seeding. At this time, the total nitrogen content per acre is highest for sweetclover, slightly less for alfalfa, and considerably less for red clover.

Table 2. Nitrogen and Dry Matter in Legume Roots and Tops Vary With the Stage of Growth Seeded in oats in Early Spring and Not Clipped After Oats Harvest. Columbus, Ohio.
Plant Part Air-dry Forage Yield/Acre (lb) Nitrogen/Acre (lb)
Redclover Alfalfa Swt.clover Redclover Alfalfa Swt.clover
October 1 of Seeding Year
Tops 2,490 1,670 2,280 66 47 63
Roots 500 1,080 1,760 13 29 58
Total 2,990 2,750 4,040 79 76 121
Early November of Seeding Year
Tops 2,050 1,350 1,550 50 39 34
Roots 940 1,270 2,640 24 35 95
Total 2,990 2,620 4,190 74 74 129
April 24 to May 12 of Year Following Seeding
Tops 2,010 2,500 2,100 58 83 78
Roots 900 1,110 1,170 20 25 37
Total 2,910 3,610 3,270 78 108 115
May 24 to June 22 of Year Following Seeding
Tops 4,360 4,040 5,400 108 103 130
Roots 1,050 1,450 1,080 22 33 25
Total 5,410 5,490 6,480 130 136 155

Table 3 indicates the nitrogen reduction incurred when sweetclover is clipped the seeding year. Sweetclover is a biennial capable of making large top growth during the season it is seeded. Any clipping or pasturing which reduced top growth of first-year sweetclover has a corresponding effect on root development.

Table 3. Reduction in Nitrogen of Sweetclover When Clipped. Harvested November 1, Columbus.
Height Clipped in July (in.) Yield/Acre* (lb) Nitrogen/Acre (lb)
Tops Roots Tops Roots Total
Not cut 2,940 3,630 47 127 174
14-16 1,900 2,760 40 99 139
7-8 1,130 1,830 25 68 93
2-3 630 1,090 14 37 51
*Air dry

Clipping sweetclover the seeding year does NOT kill it, if clipped high. Crown buds develop at the base of the plant during the early fall season. These buds grow the following spring and produce the second year's growth.

Table 4 presents some work conducted at the North Central Branch of the Ohio Agricultural Research and Development Center (OARDC) near Castalia, Ohio. These plowdown crops and mixtures are used over a wide part of Ohio. Even though these crops produce large amounts of nitrogen by June 5, plowing at this late date for corn production is not recommended. Corn yields are usually reduced so that a producer will not have a good financial return above cost of production.

Table 4. Yield and Amount of Nitrogen Produced by Four Legume Combinations at Two Dates.
Plowdown Crop D.M. Yield/Acre Nitrogen/Acre
May 15 June 5 May 17 June 5
  –tons– –lb–
20% mammoth, 80% sweetclover 0.82 2.24 60 108
60% mammoth, 40% sweetclover 0.77 2.00 56 96
Domestic mammoth red clover 1.08 2.12 78 102
Canadian mammoth red clover 0.86 2.00 63 96

Numerous agronomic conditions in Ohio could benefit from the use of legumes or other green-manure crops. The crop rotation, availability of seed and seeding equipment, and cost and availability of inorganic source of nitrogen may affect a producer's decision to use green-manure crops.

Click here to view the PDF of this Fact Sheet.

All educational programs conducted by Ohio State University Extension areavailable to clientele on a nondiscriminatory basis without regard to race,color, creed, religion, sexual orientation, national origin, gender, age,disability or Vietnam–era veteran status.

Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSUExtension.

TDD No. 800–589–8292 (Ohio only) or 614–292–1868

| Ohioline | Search | Fact Sheets | Bulletins |