How Much Can Soil Organic Matter Increase In A Year?

Joe Christner, Water Quality Technician

05-08-18

Interest by the agriculture community in soil health has never been higher. Because soil organic matter is a primary measure of soil health it is important to monitor organic matter and try to increase it. But what is a realistic goal?

A soil test can indicate levels of organic matter, however keep in mind that it can be difficult to obtain an accurate result depending on sampling and testing methods. The following is an article by Sjoerd Willem Duiker, Penn State University Extension that was in the February 13, 2018 Corn and Soybean Digest that may be helpful to determine realistic goals for organic matter increase.

The source of soil organic matter is photosynthesis resulting in plant growth-either root or above ground. Therefore, the organic matter cannot increase more than the amount of plant growth that can be produced in a year. Let’s do some basic math assuming all the plant matter gets converted into soil organic matter.

First, we need to know what one acre of soil can produce. Let’s assume a highly productive corn crop- producing 200 bushels per acre. That is 200 bushels per acre X 56 lbs/bushel X 0.845 (to correct for 15.5% moisture in grain) = 9,464 lbs of dry grain per acre. Typically, the harvest index of corn (the proportion of stover to grain) is 1, so the amount of residue produced is also 9,464 lbs/A. The root mass produced by corn is on average 20% of the above ground. So if we add that it makes 11,357 lbs/A.

Let’s assume you also grow a cover crop of rye and that it is terminated with 5,000lbs of above-ground dry matter per acre and 1,000 lbs of below-ground root mass. The total is 17,357 lbs of plant matter from roots and stover from corn and rye. Let’s convert all that to carbon for greater accuracy. The carbon content of stover is typically 40%, so that is 6,943 lbs of carbon produced per acre in roots and stover. Is that enough carbon to increase soil organic matter 1%? Let’s calculate how much carbon is in 1% of soil organic matter. We assume one acre slice of soil (to a depth of 6.7”) weighs 2,000,000 lbs. So 1% of 2,000,000 is 20,000 lbs. Soil organic matter contains roughly 58% carbon. So one percent organic matter in soil to 6.7 inch depth equals 11,600 lbs of carbon. That is a lot more than the amount of carbon that is produced by a highly productive corn crop plus rye cover crop!

 Soil test pit in a field managed for increasing organic matter.

Soil test pit in a field managed for increasing organic matter.

Based on these calculations and this management scenario, it is not possible to increase soil organic matter at a rate of 1% per year. And we didn’t include the conversion of plant residue in soil organic matter yet! That conversion has been shown to be only 10-20%. So if you add 6,943 lbs of carbon in plant roots and stover, that would end up in only 1,388 lbs of soil organic carbon, or 2393 lbs of soil organic matter. That is 0.1% of 2,000,000 lbs of soil. Therefore, if you increase organic matter content by 0.1% per year you are doing a superb job with your management. To expect 1% increase may not be realistic.

This discussion assumes there is no input of organic matter from other fields or farms. If manure or compost have been applied that would change the story. Fred Magdoff and Harold van Es include a calculation of the effect of dairy manure application on soil organic matter in the book ”Building Soils for Better Crops (2nd Ed)”. They calculate that applying 20 T/A/yr of solid dairy manure would increase organic matter content 0.0655 per year. So if we add relatively high applications of manure to the equation it might be possible to increase organic matter content 0.17% per year.

Incorporating a combination of no-till, cover crops, residues and manure can improve soil organic matter over time. We have to be patient.

Joe Christner.JPG

Joe Christner

Joe Christner came to Holmes SWCD in 2001 with experience and knowledge drawn from 20-plus years of dairy farming. He grew up on a small farm near New Bedford, Ohio. His background interest and involvement in agriculture from the time he was a young man give him an empathy and understanding of the needs and concerns of today’s farmers. Joe can assist you with conservation plans for your farming operation, including nutrient management planning and record keeping. Water quality, soil health, and conserving the resources needed for the next generation and beyond is very important to Joe.
Contact Joe at 330-674-2811 or jchristner@co.holmes.oh.us