Corn School: How long does it take to add 1 percent soil organic matter?


You’ve just finished combining a corn field and you want to know how much organic matter all that corn residue will contribute to your soil.

RealAgriculture’s agronomist Peter Johnson tackles that question on our latest Corn School episode.

Unfortunately, as Johnson explains, all those stalks and cobs won’t have much of an impact.

When it comes to soil organic matter levels, Johnson explains that more is always better. Sand soils are typically in the 2% soil organic matter range, but higher is better; silt loam should be at 3.5% or 4%; and clay loam should be 4.5% to 5%.

Increasing field organic matter from 3% to 4% shouldn’t be that difficult, right? Unfortunately, it can take a lifetime to move the needle 1%. While standing in a freshly combined cornfield, Johnson explains that corn residue — depending on yield and other factors — could contribute 12,000 to 15,000 lb of residue to the soil. However, it takes 100,000 lb of crop organic matter to increase overall soil organic matter by 1%. Johnson adds that grain and oilseed crops typically remove the equivalent of 9,000 lb from the soil, so corn residue is barely enough to cover that loss.

Johnson also explains that soybeans contribute very little crop residue — so after corn followed by soybeans in the rotation, the field is actually in a residue deficit, and overall organic matter declines.

That’s why it’s so important to have other crops like wheat in the rotation.

So how long does it take to add 1%? “If you go to a corn/soy/wheat rotation, and you get your yields high enough, it takes 81 years to build one percent organic matter,” says Johnson. He notes that this window can be shortened with the addition of cover crops or a commitment to continuous alfalfa.

Click here for more Corn School episodes.



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