Considering Cover Crops - Fall Nitrogen Capture

An efficient cover crop, within the context of nutrient management, is one which will:

1. tie-up or sequester available nitrogen in the fall,
2. secure this nitrogen from leaching over the course of the fall/winter/spring when crop demand is low, and
3. release it to the corn crop in time for its phase of rapid growth.

This available nitrogen in the fall may of course be a result of leftover nitrogen that the crop did not use or came as a result of late summer or fall applications of manure. The ability of a cover crop to capture this nitrogen in the fall hinges mostly on its ability to grow rapidly after the main crop has been harvested. This has been the traditional advantage held by an underseeded cover crop like red clover compared to those cover crops that need to be seeded in August.

Cover Crops & Soil Nitrates

To assess the ability of various cover crops to sequester nitrogen following winter wheat, trials were conducted in the early-1990's by University of Guelph researchers. Red clover was spring underseeded while fall rye, oilseed radish, and oats were drilled following wheat harvest. Their findings, outlined in Table 1, indicate that all four of the cover crops ) were able to draw down soil nitrate levels compared to no cover crop. Interestingly, even red clover with its ability to fix nitrogen from the air, was as effective as the other cover crops in reducing soil nitrate concentrations by the end of the growing season.

Table 1. Influence of cover crops on soil nitrate concentrations in the surface 60 cm (24 inches) when sampled in early November of the establishment year.

Average of four site/years; Ayr, 1992 and 1993; Kirkton, 1992 and 1994.

(Vyn et al. Agronomy Journal 92:915-924)

It's All About Tonnage

A study by Isse et al. (1999) in Quebec of cover crops planted after sweet corn harvest shows that reduced leaching is dependant on cover crop biomass accumulations, and that significant benefits are possible only when dry matter accumulation is high. The study compared the effectiveness of forage radish, canola, barley, ryegrass, crimson clover and red clover.

In the first year of the study, where no cover crop had a November biomass accumulation over 650 kg / ha, little decrease in fall soil nitrate was observed. In the second year with much higher biomass accumulations, fall soil nitrate levels were significantly reduced to almost one-half compared to no cover, and January water nitrate levels were reduced to approximately one-third. The study also found little significant difference between the effectiveness of legume compared to non-legumes.

These findings suggests that the ability of a cover crop to reduce over-winter leaching may depend more on its ability to produce biomass and less on whether it is a legume or a non-legume.It is reasonable to expect that it requires one tonne of dry matter to immobilize 25 to 30 kg / ha of nitrogen.

Delay Fall Tillage

Preliminary research in Ontario also suggests that if manure and cover crop combinations are plowed early in the fall (mid-September to early October) there may be a large flush of nitrates that become available for leaching. Where manure applications are made on a vigorous stand of clover it may be best to delay tillage until late in the fall.

Of the cover crops tested that could be fall-seeded after a manure application and incorporation, oilseed radish appeared to have greater potential for biomass production and nitrate sequestration than oats, fall rye or ryegrass. With these fall-seeded options, early (August) manure applications and cover crop seeding will be essential in order to achieve high biomass production before seasons end. And again in these situations, if fall tillage is planned for these fields, it would be best to delay it until late in the fall.

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