Soil Organic Matter

Resources: Christine Brown, OMAFRA Field Crop Sustainability Specialist, presentation at CCA Soil & Water Information Day, Nov. 2017 Guelph Ontario

We are losing organic matter in Ontario agricultural fields at an alarming rate. Christine Brown, OMAFRA Field Crop Sustainability Specialist presented a chart at a November 2017 meeting we had the opportunity to attend that showed the change in soil organic matter from 2002 to 2016.  During that 14-year period agricultural soils in Essex, Kent, and Lambton counties decreased soil organic matter approximately 0.8%.  That is a loss of 16000 lbs/ac of organic matter which significantly reduces the soil’s water holding capacity, water infiltration rates and aggregate stability.  We want our soils to be physically more like a sponge and not a brick because sponges have high water holding capacity, fast water infiltration rates, and a stable structure.  Organic matter is what makes our soils act more like a sponge than a brick. 


So, what can we do to build up organic matter in our agricultural soils?  As the picture below depicts, there are several activities that can help to achieve the ideal soil organic matter, including:  residue management—maintaining cover on 30 to 60% of the soil surface at planting; cover crops—growing crops outside of the typical growing season to protect and enrich the soil; adding organic amendments—adding manure or compost to the soil; crop rotation—the greater the number of different crops in your rotation on each farm, the better it is for building soil organic matter; reduced tillage—the benefits of reduced tillage can be easily understood if we think about our soil being like a skyscraper, both take a long time to build. Tillage to soil is like a wrecking ball to a skyscraper, quickly destroying what took a long time to build.

Ideal soil org mat sm c

 

 

 

 

 

 

 

 

 

 

 

Let’s take a deeper look at crop rotation and how the diversity in your rotation can build soil organic matter, or not.  Dr. Bill Deen at the University of Guelph has done extensive research about crop rotations in relation to soil organic matter. His work is summarized in the chart below:

Bill Deans Work sm c

Source: Dr. Bill Deen, University of Guelph

There were several different crop rotations studied in this work, which are shown on the left side of the chart. C=corn, Sb=soybean, W=wheat, Wrc=Wheat under-seeded Red Clover, A=Alfalfa. The right side of the chart shows how many years of each rotation it will take to build soil organic matter (SOM) 1%. This study shows it is not possible to build soil organic matter with continuous corn or with corn / soybean only rotations. It will take 81 years of a corn, corn, soybean, wheat rotation to build 1% of organic matter. You achieve the 1% SOM increase 19 years earlier if you under-seed your wheat with red clover.

The reality is most current field crop rotations will not maintain soil organic matter levels without the use of more diverse rotations, cover crops, and/or organic amendments.