By Sjoerd Willem Duiker, Penn State University Extension
Soil organic matter is a primary measure of soil health. The source of soil organic matter is photosynthesis resulting in plant growth – either root or aboveground. Therefore, the organic matter content cannot increase more than the amount of plant growth that can be produced in a year. Let’s just 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 5,000 lbs of above-ground dry matter per acre and 1000 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!
I hope you agree that this calculation shows that it is not possible to increase soil organic matter at a rate of 1% per year with current production constraints. 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 is unrealistic.
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 0.065% 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.