The N Benefits of
Grain Legumes in Crop Rotation
Annual legumes, such as field peas, partner with soil bacteria to make their own nitrogen (N). These crops typically produce more N than they use and the residual N may be available for subsequent crops to use, thus reducing N fertilizer requirements. The amount of N available to the crop depends on many factors such as type of legume grown and soil moisture conditions. This estimate of the amount of N added to the soil by legumes is called the N-credit.
To determine the N-credit contributed to wheat by these preceding legumes:
- field pea
- dry bean
In this experiment, wheat was grown after field pea, dry bean, chickpea, soybean or flax. Legumes were sown with the appropriate N-fixing bacteria and 55 lb/ac N was applied to the flax crop. No N fertilizer was applied to the wheat crop in year 2. All crops were harvested for grain. This experiment was conducted at Carman, MB on sandy-loam soil in 1998, 1999 and 2000; and at Brandon, MB on loam soil in 2000.
The concentration of N in the wheat kernels following legumes was determined and compared to the concentration of N in wheat after flax to determine the N-credit contributed by these grain legumes.
i.e. N credit = N yield for legume - N yield for flax
Overall, field pea provided the highest and most consistent N-credit to the following wheat crop (see table below). Only in 2000 at Carman was there no benefit. This was attributed to high background levels of N this year. The experimental area had alfalfa on it two years previous, therefore, N was in abundant supply to all crops. A low N-credit for beans was not surprising as this crop is considered a poor N-fixer under Manitoba conditions. Results for chickpea were inconsistent. At Brandon, chickpea produced the highest N-credit of all legumes, while at Carman chickpea produced very little N-benefit. Chickpea also yielded much higher at Brandon than at Carman, and was probably better suited to the drier soil conditions found at Brandon. Soybean produced very little N-benefit in this experiment. Other research has shown a greater N-benefit from soybean in rotation with corn (compared with continuous corn) but this may have been for reasons other than N fixation.
|Site and Year
|Carman - 1998
|Carman - 1999
|Carman - 2000
|Brandon - 2000
To calculate your own N benefit, try the calculator below. Note that the calculation is based on the average N-benefit for each legume from the table above. Your actual N benefit will depend on local soil conditions and environmental factors.
- Use this information as a general guideline for determining how much N your legume crop contributes to the following crop.
- Amount of nitrogen produced by legumes will be affected by soil conditions.
- Soil test and adjust N fertilizer rates in accordance with what you expect your legume contributed. Read more about Adjusting Soil Test N Recommendations for Legume N from Pulse Crops.
- Legumes must be inoculated with the appropriate bacteria for successful nitrogen fixation.
Further Reading: D.W.A. Przednowek, M.H. Entz, B. Irvine, D.N. Flaten, and J.R. Thiessen Martens. 2004. Rotational yield and apparent N benefits of grain legumes in southern Manitoba. Canadian Journal of Plant Science 84: 1093-1096.
This page created August 2004.
Updated May 2005.