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Understanding soil organic matter

Do you know your crop residues from your soil organic matter?

The word “organic,” just like “environmental” has become confusing over the last 20 or 30 years. Organic food for example? All the food we eat is organic (except salt or other minerals). All farming activities are environmental, but every misinformed urbanite calls him or herself an environmentalist.

“Soil organic matter” is made up of a hodgepodge of organic compounds from plant, animal and microbial origins that are at any given time living or dead. The black organic soil matter (OM) can range on the Canadian Prairies from one to 10 per cent or more, with some soils in the teens to black muck soils that are mostly organic matter. The living fraction of an actively worked clay or sand loam soil with an OM level of four to five per cent in the better soils of the Prairies has a huge load of living components. An acre of such soil may have up to 1,000 lbs. of earthworms, 2,500 lbs. of fungi, 1,500 lbs. of bacteria and up to 1,000 lbs. of protozoa and insects — most fully active in June and July. This living biomass is equal to the biomass of about eight to nine cows per acre.

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Closeup of a plowed field, fertile, black soil.

These organisms in the soil break down dead organic matter. If this did not occur, all carbon dioxide would eventually be locked up in the soil as organic matter, just like acidic peat or coal. Organic matter is primarily carbohydrates, lignans and proteins. The chain of organic decomposition starts with fungi, viruses (phages) and bacteria (billions per gram) that are active in the warmer months. These are fed upon by arthropods like worms and insects that also release crop residue nutrients into the soil.

I do not know what a soil health test can tell you other than that crop production soil is in continuous activity as long as it warms up, and the more organic matter present the more activity in soils between pH 4 to 9.

Soil OM percentage is calculated when you take a soil test to estimate the nitrogen (N), phosphorus (P), and sulphur (S) that could be available from the OM during the growing season Soil’s OM levels and pH is also important in selection and degradation of herbicides.

Soils formed under grass prairies have higher organic matter levels than those formed under forests. Consequently, they have higher crop nutrient reserves.

Soil Organic Matter (SOM) on the Prairies (not including crop residues) under cultivation in an average year can lose around one per cent of its total (two to five per cent annually) for most cropping systems, and perhaps only 0.5 per cent under hay or perennial forage.

For each one per cent of the OM present, the SOM decomposition under cultivation in the top six inches of silt or silt loam soil can provide 10 to 15 pounds of nitrogen, one to two pounds of phosphorus and around a half-pound of sulphur per acre per year. SOM holds water that is released to plants. It also aggregates soil particles for drainage and reduces soil erosion.

An acre of soil six inches deep, on average, weighs two million pounds (less for peaty soils and more for sandy soils). If your soil was two per cent OM that organic matter would weigh about 40,000 lbs. per acre.

Ratio of C, N, P and S in SOM

Carbon 100, nitrogen 10, phosphorus 1, sulphur 0.25 to 0.50.

These figures come from the USDA’s Soil Organic Matter information kit. These levels may vary to some degree, but under cultivation or fallow such soils will release or mineralize about 1.5 per cent of their total OM per year. In the Canadian Prairies’ colder climates, this mineralization could be one per cent or lower, perhaps even 0.75 per cent. This is why Canadian Prairie soils have much higher SOM reserves than their southern counterparts.

When you buy cropland, a soil test of SOM will tell you about the crop nutrient reserves. Would you pay the same price per acre for a two per cent organic soil versus a six per cent organic soil, assuming other factors are similar?

A two per cent organic soil would have 2,000 lbs. of bound-up nitrogen per acre; a six per cent organic soil would have 6,000 lbs. of bound-up nitrogen per acre, not including phosphorous, sulphur and water-holding capacity. A two per cent organic soil at a one per cent release gives you 20 lbs. of nitrogen versus a 60 lbs. release of nitrogen from the six per cent organic soil. What is the value of that extra 40 lbs. of nitrogen per acre?

Converting parts per million to pounds per acre is based on one acre of topsoil to a depth of six inches weighing two million pounds. If you apply four lbs. of actual nitrogen or boron to an acre of land it translates to two ppm (parts per million) per acre.

I’ve previously stressed the importance of not harvesting crop residues, so you build up soil organic matter. Don’t confuse this bulk organic matter with SOM. SOM is the black humus that has taken thousands of years to form. As well as nutrients, SOM contains compounds like humic acids, fulvic acids and lignans. All these “wonder” nutrients are naturally present in your very own two to 10 per cent SOM.

Are you enlightened or further confused?

About the author

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Dr. Ieuan Evans is a forensic plant pathologist based in Edmonton, Alta. He can be reached at [email protected]

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