Cover crops can do a lot more than just protect the soil surface to prevent water and wind erosion. Many cover crops perform more than one job and have characteristics that make them better suited to some uses than others. The many uses of cover crops and some recent research in this area will be examined over the next few months.
Cover Crops as a Catch Crop
A catch crop reduces the amount of nitrogen left in the soil that can be either leached or lost as nitrous oxide. There are three ways that the cover crop does this:
– The cover crop takes nitrogen out of the soil solution, reducing the concentration. Any water that leaches through the soil will carry less nitrogen. Under dry soil conditions this will have little effect on leaching.
– Nitrogen is captured by the crop roots and transported upward. Even if the crop dies, the captured nitrogen is now higher in the soil profile and more likely to be captured again by following crop.
– The water use of the cover crop will dry the soil slightly, reducing the amount of leaching or downward movement of water.
Deep-rooted cover crops like oilseed radish and mustards can be expected to be more effective in chasing the nitrogen down the soil profile earlier than grass cover crops or weakly rooted broadleaves. Research done in Denmark estimated that 1000 day degrees after planting, crucifer catch crops like oilseed radish and fodder radish would have an average root depth of 1.5 metres. Winter rye and oats would have 0.9 to 1.0 m of root while ryegrass would lag behind with only 0.6m of root depth. Further work estimated root depth development for grasses like ryegrass, rye and oats was approximately 1 mm per day per degree C while crucifer crops like winter rape had root development rates of more than 2 mm per day per degree C.
Selecting the right catch crop species is important for rooting depth and speed of rooting but planting date plays a role too. The longer a cover crop has been growing, the greater the rooting depth.
Capturing nitrogen is only one part of the catch crop role. Releasing the nitrogen to subsequent crops is key. For example Ontario research has shown that winter killed cover crops like oats and oilseed radish are "leaky". Once dead the crop residue readily looses much of the nitrogen in the plant tissues, usually before the next crop is established. Cover crops like rye that need to be killed in the spring often break down slowly and it is difficult to estimate the amount and timing of the nitrogen release for the next crop.
A compromise that some growers are using is to plant a combination of cover crop species. Oilseed radish or fodder rape works well with a frost and shading tolerant grass like winter rye. The oilseed radish establishes quickly in August and early September with rapid root growth and shading of the soil surface. The rye grows more slowly under the radish, tillering but not elongating until vernaliza-tion. The cold temperatures in late fall and early winter (colder than –8C) kill the radish. The rye will grow up through the dying radish residue, capturing at least some of the nitrogen as it is released.
Getting a better handle on the uptake, timing and release of nitrogen from cover crops is the goal of two demonstration projects funded through the Greenhouse Gas Mitigation Program of the Ontario Soil and Crop Improvement Association.
One project looks at cover crops and manure after wheat harvest in field crop rotations, while the other examines the effectiveness of cover crops after early harvested vegetable crops. Preliminary results from these projects will be reported later in the winter.
From the Archives:
OMAF Update 1, December 2003
