Science into Practice: Green manures -- benefits versus costs

Green manure crops within rotations of organic and conventional systems have long been recognised as providing benefits.

But surprisingly few farm trial investigations of the true practical benefits have also included a detailed cost-benefit analysis.

Caring for the future sustainability of soil with good management is essential for organic and conventional growers. Soil management is also now becoming important to meet current and future legislation — action plans are required for meeting Water Framework Directive regulations. In organic production, there is limited availability of acceptable sources of nitrogen, so correct use of green manures to add nitrogen is essential.

HDC project FV 299 compared three grass/clover green manure mixes sown in the autumn or spring after bulb onions on four sites — two conventional, one organic and one in conversion — in East Anglia. The green manure swards were incorporated in December and followed by potatoes.

The quickest mix to establish and the one that produced the most biomass contained perennial ryegrass/red and white clover. But all added at least 120kg/ha of nitrogen at final incorporation. Bean seed fly populations were not increased by any of the treatments and volunteer weed problems from the swards were not increased in the following crops.

However, growing a green manure increased yields by up to 20t/ha. In a conventional rotation, in a year with low cereal prices (2008), substituting a green manure crop would generate a financial benefit due to the positive yield gains in the following crop and the savings on applying ammonium nitrate fertilizer, the cost of which is closely related to oil prices.


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