Getting to the root of the problem with soybean yields

Monday, January 2, 2012

When you look at all of the information, the solution is obvious. You have to apply nitrogen to some fields to get the highest soybean yields

by PAT LYNCH

While corn yields are increasing due to genetic improvements and changes in management, and wheat yields are mainly increasing due to the use of more fertilizer and more disease control with  fungicides, soys are stuck in a pattern of slow yield increases.

While we have made some strides with earlier planting, yield increase due to better genetics has been small. In the past 10 years, a lot of breeding effort has been put into developing glyphosate tolerance in soys, but there have been few other breeding advances to give higher yields.

The problem with soybeans is their root system, that part of the plant you never see but which has many problems. The root rot diseases phytophthora and rhizoctonia and a host of other minor root diseases reduce yields. And then there are soybean cyst nematodes. These critters are taking a big bite out of Ontario soybean yields. There is some genetic tolerance to this pest, but it's very limited. You can expect nematodes to overcome the main genetic tolerance in Ontario soys soon.

Experience has shown that, generally, the best soybean yield occurs the second time you plant soys in a field. This may be a case of planting back-to-back soys or the second time may be five years after the first. It appears that, by the second time you plant them, the bacteria responsible for inoculating soys are robust. The disease organisms that are feeding on the roots have not taken off.

The biggest factor affecting soybean yield is nitrogen. Soybeans fix nitrogen, but they only fix about 65 per cent of the nitrogen they need. The rest comes from the soil. We know that nitrogen-fixing bacteria do not start to work until the soil warms to 15-17 C. If you plant early for high yields, there is a good chance that the soil will be too cold to allow the nitrogen-fixing bacteria to be active when the plant needs nitrogen.

During the season, soybean plants count on getting nitrogen from the mineralization of soil nitrogen. Under warm, wet conditions, the soil releases nitrogen. Then there is a race for this nitrogen. The plant wants it, but so do the soil organisms that are breaking down the corn stalks.

The biggest part of this scenario is the reduced root system that soybeans produce.

These roots are reduced by disease, nematodes and compaction. I am convinced that one of the reasons for higher yields with tillage than with no-till is because there is a bigger root system with some tillage. This bigger root system is better equipped to forage the soil for nitrogen. If you have nematodes, you have a much smaller root system. It will not produce as much nitrogen as a bigger, healthier root system.

When you look at all of this information, the solution is obvious. We have to apply nitrogen to some fields to get the highest soybean yields. These fields include fields with nematodes (even if you plant a resistant variety), fields with a history of soys and thus more root disease, and heavy soils where soybean roots are normally reduced.

I can find no research in North America where nitrogen was added to increase soybean yields. This is because everyone knows soybeans do not need commercial nitrogen.

However, that thinking is flawed. Ontario Ministry of Agriculture Food and Rural Affairs soybean specialist Horst Bohner, and others are looking at adding nitrogen on some farms in Ontario. I have not seen the yield results, but I'm betting there will be a yield increase by adding nitrogen. The only other research I know of is from Nebraska in the 1990s and this is probably not good enough for us.

In the future, we will have better genetics. I was able to review hundreds of new soybean lines in the fall of 2011. I was really impressed with the look of the new genetics and am confident that some of these new lines will have a better root system. But, in the meantime, we have to do other things to increase soybean yields. I think one of these is adding nitrogen to some fields. BF

Consulting agronomist Pat Lynch, CCA (ON) formerly worked with the Ontario agriculture ministry and with Cargill.