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University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Adjusting Soil Test N Recommendations
for Legume N from Pulse Crops

Background

As fall approaches and the soil testing season begins again, one of the questions that some farmers and agronomists will ask is, "should soil test N recommendations be adjusted for the expected release of legume N from a previous pulse crop?"

Soil tests generally measure only the nitrate N in soil and not the organic N that is present in crop residues, but which is not immediately available to subsequent crops. Therefore, the organic N in the crop residues from annual grain legumes, such as field peas, is not detected in most soil analyses.

However, under appropriate conditions, annual grain legumes increase the amount of plant available N in the soil directly, by the microbial decomposition of legume residue N and indirectly, by reducing the amount of soil and fertilizer N that is immobilized by soil microorganisms during the decomposition process. In both cases, the process of increasing the soil N status is biological, depending on soil microorganisms that are, in turn, sensitive to environmental conditions such as soil temperature and moisture.

Given the relatively cool and/or dry conditions that typically occur after harvest in most regions of the Prairies, many research trials in Alberta and Saskatchewan have demonstrated that this increase in plant available N after growing pulse crops is delayed until the subsequent growing season. However, Southern Manitoba's climate is both warmer and wetter than most other areas of the Prairies and has the potential to encourage rapid decomposition of pulse crop residues immediately following harvest. Therefore, in this region the N benefit expected from pulse crops may already be evident in the conventional nitrate-based soil test recommendations.

Research Results

Part of a recent study conducted by David Przednowek, Martin Entz and myself from the University of Manitoba and Byron Irvine from the AAFC Research Centre in Brandon provides evidence of the rapid increase in soil test N values after pea crops. Soil samples collected in early spring showed that soil nitrate N concentrations were significantly higher where field peas were grown, compared to flax (Table 1).

Table 1. Average Amount of Nitrate-N in the Soil Profile Prior to Planting Wheat as Affected by Previous Crop at Three Sites in Manitoba.
Previous Crop Nitrate N
(lb N/ac to 4 feet)
Flax 80
Pea 100

However, during the growing season for the subsequent wheat crop, more N appeared to be released by flax stubble than by pea stubble, especially later in the growing season, between flowering and harvest (Table 2). The lower apparent N benefit of the pea stubble during the subsequent growing season was probably due to rapid decomposition of the pea residues after harvest, as illustrated by the high soil test N values in Table 1. So, the slow, steady N release pattern of legume residues during the subsequent growing season as observed by other Prairie researchers was not observed under the conditions of this study.

Table 2. Average Amount of N Apparently Released During the Growing Season for a Subsequent Wheat Crop as Affected by Previous Crop at Three Sites in Manitoba.
Previous Crop Amount of N Released
(lb N/acre)
Planting to Flowering Flowering to Harvest Planting to Harvest
Flax 18 33 51
Pea 17 25 42

Therefore, this study showed that under Southern Manitoba conditions, if the post-harvest period in late summer or fall is reasonably warm and moist, the residues from pulse crops, such as field peas, may increase nitrate N concentrations very rapidly, perhaps prior to soil testing and freeze-up.

Recommendations

  • The N benefit from pulse crop residues may already show up in conventional soil nitrate tests and be accounted for in soil test recommendations from those tests. Under these conditions, no further N benefit, beyond that already measured in the soil test should be expected and N fertilizer recommendations should not be reduced.
  • Given the higher soil nitrate concentrations of field pea stubble prior to planting the subsequent crop, the potential for leaching and denitrification losses are elevated, both of which are of environmental concern. Fall crops, relay crops or cover crops should be considered in environmentally sensitive areas.

The resources for this study were provided by the Agriculture and Agri-Food Canada, the Canadian Wheat Board, Agrium, ARDI and the University of Manitoba.

By: Don Flaten, Department of Soil Science, University of Manitoba

Copyright and Liability

This page created August 2004.