August 2006

Nitrogen Recommendations Following Legumes

Soil testing does a good job of identifying nitrogen contributions for succeeding crops – when that N is in the nitrate form. Following harvest, much of the N in legume plant material exists in the organic form – and will be picked up by the soil test only after it mineralizes.

Following are some general guidelines for producers.

Following annual legumes (peas, beans, soybeans, faba beans)

Soil test levels do not usually detect N contribution, as seen in the following table.

Table 2. Post harvest soil nitrate levels (lb nitrate-N/ac in 0-24”) following pulse and cereal crops (average of 2 sites).

• wheat following pulses supplied with 25 kg N/ha yielded similarly to wheat after cereal with 75 kg N/ha. The “apparent contribution” of the pulse crop was 50 kg N/ha.
• the increase in N availability to the wheat crop was not a “contribution” - but was due to less N immobilization to break down legume straw versus wheat straw.
• yield potential of wheat was 10% higher following pulses IF wheat was fertilized similarly to wheat after wheat (ie 75 kg N/ha). Some of this bonus yield potential would not be realized if the producer reduced N rates according to the apparent legume credits.
• N fertilizer recommendations may need to be adjusted with the knowledge that the previous crop was a legume and it won’t be immobilizing as much soil N to decompose.

Following Forage legumes

Studies by Dr. Martin Entz have indicated that nitrogen contributions from forage legumes are dependant upon the time and method of stand termination (Table 2).

Table 2. The reduction in fertilizer N rates of crops following termination of forage legumes.

The earlier stands are terminated – the more organic N will have mineralized to the available nitrate form. This nitrate form is available for the crop or susceptible to losses to leaching or denitrification. Stands terminated with herbicide (without tillage) will make N available at a somewhat slower rate than if tillage were used.

When stands are terminated later in the season, the N release from the organic form will not coincide with crop needs – especially cereals, flax and canola. Later seeded and longer living crops like corn and sunflowers will be better able to make use of the N release in mid summer.