University of Manitoba: Natural Systems Agriculture: Green Manure Crops for P Availability
University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Green Manure Crops
for P Availability


Incorporating green manure crops at Carman, MB. August 20, 2005.

Farmers have used green manure crops in organic rotations for their N contribution to following crops.The N benefit from green manure crops is well documented (see The Fertilizer Replacement Value of Legume Cover Crops).

But what about phosphorous? It is known that some organic cropping systems drain soil of “soil test” P. For example, in the Glenlea long-term rotation study, sodium bicarbonate extractable P is very low in rotations that contain alfalfa hay crops (see Glenlea Research Results - Soil Nutrients).

There is a great deal of interest in using green manure crops to make the unavailable forms of P available to crop plants. Crops such as buckwheat and also legumes are thought to make soil P available through the acidification of the plant root zone. However, few studies have been conducted in Canada to test this theory.

There are also many questions about whether rock phosphate, an allowable P fertilizer source for organic production, works in prairie soils.

The Current Research Project

Field trials were initiated in 2004 and again in 2005 at three Manitoba locations to test the effect of six green manure crops on soil P uptake by following crops. The Glenlea site was conducted on soil that had been farmed organically for 12 years and was known to be low in available soil P.

Green Manure Crops

The following images show the green manure crops growing at the Carman site in 2005. Results from 2004 showed little significant difference in total P uptake of the different green manure crops.

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Oat green manure crop Fababean green manure crop Fababean green manure crop Buckwheat green manure crop Mustard green manure crop


Organic Wheat Following Green Manure Crops

The following images show organically grown wheat in 2005 at the Glenlea site, seeded the year after the green manure crops were grown. Clearly, the wheat after oat and sorghum sudangrass green manure crops show signs of N deficiency. The wheat after berseem clover and fababean green manure crops look lush and green and show few signs of N deficiency. Surprisingly, wheat grown after buckwheat and mustard also look very healthy, with few N deficiency symptoms.

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Wheat after oat Wheat after sorghum sudangrass Wheat after berseem clover Wheat after fababean Wheat after buckwheat Wheat after mustard

Organic Wheat Following Soil Amendments

The following images show organically grown wheat after the green manure treatments, treated with various soil amendments. Few visual differences are evident. So far, results appear to suggest that green manure crop was more important than soil additive.

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No soil amendment Manure in 2003 Rock phosphate Jumpstart Rock P and Jumpstart

Organic Potatoes Following Green Manure Crops

The following images show results of the organic potato production trial. Clearly, potato growth was least vigorous after sorghum sudangrass and best after legumes.

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Potatoes after alfalfa Potatoes after buckwheat Potatoes after peas Potatoes after sorghum sudangrass

Brenda Frick (Organic Agriculture Centre of Canada, University of Saskatchewan) and Jackie Pridham (U of M graduate student) discuss greem manure research trials.

Ongoing Research

We are currently analyzing the test crops (wheat at Glenlea; potatoes at Graysville) for P uptake after the various green manure and soil amendment treatments. This will help us determine the value of specific green manure crops and soil amendments in increasing the availability of phosphorus to the following crop.

Copyright and Liability

This page created September 2005.