Make sure you use cover crops for the right reason
Sunday, June 7, 2015
The main function for cover crops is to reduce erosion. Don't expect them to cut your fertilizer rates or do much to improve soil characteristics
by PAT LYNCH
I sometimes just shake my head when I hear some agronomic advice. It falls somewhere between "it's too good to be true" and "you have got to be kidding."
Some of the information on cover crops this winter falls into that category, specifically the advice that you can cut your fertilizer rates because the cover crops will be pulling up fertility from deep down and building soil test levels. Yeah, right.
This advice is being mixed up with the idea that you can reduce fertilizer rates by using cover crops. You might be able to cut fertilizer rates and maintain yields, but this has nothing to do with using a cover crop. You can reduce fertilizer rates and 'mine' the nutrients from the soil. But your soil test levels will drop. Soil test phosphorus will not drop a lot or fast in one year, but it will drop.
How can I best explain about cover crops not bringing soil nutrients from down deep? First, this is because most crop roots are in the top six inches of soil, so they don't go deep enough to bring nutrients up. Secondly, if you have ever soil-tested in a bush where trees have been bringing nutrients up for years, you know that the soil there tests very low. Those deep roots do not build soil test levels.
Why am I upset? Simply because I have seen too many great ideas ignored because the benefits of a new system were either exaggerated or oversold without full details being provided by a knowledgeable advocate. I do not want that to be the fate of cover crops in Ontario.
The main function of a cover crop is to cover the ground, mainly to reduce erosion. A cover crop holds the soil together. When rain falls, it will hit the plants and not the soil. Rain droplets do a great job in destroying soil structure. Cover crops slow water movement across the field, thus reducing erosion. Cover crops aid soil structure and organic matter to hold more water. This gives better yields. So cover crops may actually reduce soil test levels because you are getting higher yields.
Cover crops also reduce the weed seed bank and help in building a diversity of soil microorganisms. They do help water to filtrate through the soil, which may lessen the phosphorus leaving your farm.
Our time-tested cover crop is red clover in winter wheat. Only a fraction of what could or should be seeded is actually seeded. We have some growers looking at cover crops in corn, but it is not really needed there. The biggest place for cover crop use and soil improvement is in or after soybeans.
Soybean crops are a negative soil-building crop. Even when you no-till soybeans into soybeans, there is a reduction in soil tilth due to limited residue return. About the only good thing you can say about soybeans when it comes to soil health is that they allow you to plant wheat. (I do realize you grow soybeans to make money. But do you have to grow so many of them?) If you want to enhance soil productivity, use as many crops in rotation as possible. For most, that is corn, beans and wheat underseeded to red clover.
The main point of this article is to let you know that cover crops will give a very slow improvement of soil characteristics. They will quickly reduce soil erosion from both wind and water. With the exception of red clover, cover crops do not add nutrients to the topsoil. And we need to be working on cover crops in soybeans. BF
Consulting agronomist Pat Lynch, CCA (ON), formerly worked with the Ontario agriculture ministry and with Cargill.