We must reverse the decline in organic matter in our soils

Tuesday, March 8, 2016

Ontario is growing too many acres of soybeans and just no-tilling them will not stop the decrease in organic matter. What we need are more wheat, corn and cover crops

by PAT LYNCH

Over the last 10 to 12 years, Ontario soils have lost about 20 per cent of their organic matter (OM). This is according to the thousands of soils samples taken across the province. Jack Legg of SGS Agri-Food Laboratories in Guelph has shared the results of SGS's soil analysis from 2002 until 2104. During that time, the average organic matter level has dropped from 4.57 per cent to 3.80 per cent.

These numbers should be screaming at all of us. Jack shared the results by county and all counties are trending the same. The charts show the drop in across the province and in two typical counties, Essex and Huron. I chose these two counties because they are representative and have a large number of samples.

The drop in OM levels also explains why soil test P and K levels are dropping. OM is part of the cation exchange capacity complex, which holds nutrients such as P and K. I believe that OM levels have dropped, thus releasing P and K into the soil solution. This soluble P and K then leaves the field, either in tiles or across the top. During this time span, a lot of no-till has been used, maybe even more than 15 years ago. Clearly, no-till has not stopped the drop in organic matter. Of course, this is also what long-term tillage studies show. No-till will not change OM levels.

So what has changed? The percentage of Ontario soil that is planted to soybeans has changed over this period. During these years, Ontario has increased soybean acres from about two million to three million. That is a 50 per cent increase in soybeans and it means that, even if we stop the ridiculous amount of soybeans being planted, Ontario's soil organic matter will continue to drop.

I hate to seem like a prophet calling in the wilderness, but we are growing too many acres of soybeans in Ontario. And just no-tilling them will not stop the decrease in organic matter. We need to grow more wheat, corn and cover crops.

You know, I sometimes wonder if soybean sales people always tell the truth. When you ask them if you can grow another year of soybeans, they either do not answer or they just sell you more seed. They should be telling you to grow fewer soybeans and more wheat and corn.

This last fall, Ontario planted less than one million acres of wheat. We should have planted close to 1.25 million. When I asked numerous growers why, the reason I am given is that there is more profit in another year of soybeans than in wheat. This is partially because growers are not managing wheat as well as they might. To get the most from your wheat, you have to apply 120 pounds of actual N, preferably in two splits. Using 120 pounds per acre of N gets eight bushels an acre more than 90 pounds of N per acre. You should also add 10 pounds of sulphur. Using two fungicide applications gets another nine bushels per acre more.

If you want to increase organic matter, spread red clover into winter wheat. If you sell straw, you may remove 2,200 pounds per acre of organic matter. Adding red clover can add another 4,000-5,000 pounds of organic matter. If ever there was a time for change, it is now. We must reverse the organic matter drop.

So what has been happening to the level of organic matter in your soils over the last 12 years? I bet it's dropping. BF

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