Use of Legumes in No-till and Minimum Till

Chapter 6 – Fertility, No. 2, October-November 1984

Roger Veseth

Interest in the use of legumes in crop rotations is in creasing throughout the Palouse, not only from a soil conservation standpoint, but also from other important considerations. Rising nitrogen fertilizer prices make nitrogen fixation by legumes increasingly attractive. Economical alternative crops are needed to lengthen rotation between winter wheat crops for control of disease and weed problems. Recent Cephalosporium stripe problems illustrate the point. Alternative crops are also needed for farm program set-aside acreages.

Seed legume crops and plowed-down green manure legumes crops have long been part of the conventional farming system in this region. But the selection and management of legumes under conservation tillage is relatively new. To address this new aspect, Dave Bezdicek, Washington State University soil microbiologist, and other STEEP researchers are studying the use of legumes under no-till and minimum tillage farming systems. Their project objectives include evaluation of four aspects of legume-cereal rotations in conservation farming.

1. No-till and minimum tillage techniques for the direct planting of cereals into legume residues as compared to plowing.
2. Effect of legumes in rotation on soil water depletion for individual legume species and the legume-cereal rotation.
3. Effect of both grain legumes and forage legumes on cereal yield response in the rotation.
4. Potential nitrogen fixation of legumes in rotation, as well as improvement of soil organic matter and reduction of weed populations and cereal diseases.

The amount of legume nitrogen fixation varies consider ably with the type of legume, as well as soil nitrogen level present and other factors. Table 1 shows the results of 3 years of research on the amount of nitrogen fixed from the air by different legumes. This is not total nitrogen in the plant, which includes nitrogen taken up from the soil. Some yearly variations in nitrogen fixations are evident.

Table 1. Amount of nitrogen fixed by Legumes in 1981-83 at Pullman, WA, and Genesee, ID (Smith, Turco and Bezdicek, WSU).

Table 2 compares winter wheat yield after fallow, green. manure legumes and spring barley with and without additional nitrogen, The number in parenthesis is the percent of winter wheat yield after spring barley with 80 pounds per acre nitrogen. For example, winter wheat yield after peas with no nitrogen fertilizer was 93 percent of the yield after spring barley with nitrogen fertilizer. This illustrates some of the yield benefit that can be obtained from legumes in the crop rotation.

To determine how quickly nitrogen is released from legumes as compared to cereal residue under different tillage practices, the researchers labeled plants with an isotope of nitrogen (N-15). Differences will be evaluated in 1985 in terms of the amount of residue nitrogen that is available to the cereal crop the following season. Legumes being studied include peas, chickpeas and fababeans.

At Dusty, WA, forage legumes (annual sweet clover,medic and rose clover) and seed legumes (fababeans, lentils, spring peas and chickpeas) are being evaluated for seasonal soil moisture depletion, nitrogen fixation, total plant nitrogen and yield. Yield of winter wheat planted in the fall of 1983 will be evaluated in relation to moisture depletion by the legume and nitrogen contribution from the legume.

A 3-year study is underway at the Palouse Conservation Research Station near Pullman involving four forage and seed legume-cereal rotations. These rotations include: (1) winter wheat-grain legume; (2) spring wheat (interplant red clover)-red clover green manure; (3) winter wheat-Austrian winter pea green manure; and (4) winter wheat-winter vetch green manure. Three tillage treatments consisting of shallow tillage, chemical kill and moldboard plow will be tested for each rotation. Winter wheat will be planted in fall of 1985 under three nitrogen fertilizer rates to evaluate winter wheat production in terms of type of legumes, tillage and nitrogen rate. A no-till drill will be used that places the bulk of fertilizer below the seed in the chemical kill treatment. The shallow (minimum) tillage and moldboard plow treatments will be conventionally planted.

Table 2. Yield of winter wheat after fallow, green manure Legumes plowed or spring barley with 0 and 80 pounds per acre nitrogen, 1984, Pullman, WA (Kirby and Bezdicek, WSU).