Understanding Soil Organic Matter – Grainews
The word “organic”, 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 agricultural activities are environmental, but every uninformed city dweller calls himself an environmentalist.
“Soil organic matter” is made up of a hodgepodge of organic compounds of plant, animal and microbial origin that are at any given time alive or dead. Black soil organic matter (MO) can vary across the Canadian prairies by 1-10% or more, with some soils ranging from teen to black mud that are mostly organic matter. The living fraction of an actively worked clay or sandy loam soil with an OM level of four to five percent in the best prairie soils contains an enormous load of living components. An acre of such soil can hold up to 1,000 pounds. of earthworms, 2,500 lbs. of mushrooms, 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.
These soil organisms decompose dead organic matter. If that didn’t happen, all the carbon dioxide would end up being locked up in the soil as organic matter, just like sour peat or coal. Organic matter is mainly made up of carbohydrates, lignans and proteins. The chain of organic decomposition begins with fungi, viruses (phages) and bacteria (billions per gram) which are active during the warmer months. These are fed by arthropods like worms and insects which also release crop residue nutrients into the soil.
I don’t know what a soil health test can tell you other than agricultural production soil is in continuous activity as long as it warms, and the more organic matter present, the more organic matter there is. activity in soils between pH 4 and 9.
Soil % OM is calculated when you perform a soil test to estimate the nitrogen (N), phosphorus (P), and sulfur (S) that might be available from OM during the growing season. MO levels and soil pH are also important in breeding. and herbicide degradation.
Soils formed under grasslands have higher levels of organic matter than those formed under forests. Therefore, they have nutrient reserves for higher crops.
Soil organic matter (SOM) in the prairies (excluding crop residues) grown in an average year can lose about 1% of its total (2-5% per year) for most systems crop, and perhaps only 0.5% under hay or perennial forage.
For every percent of OM present, decomposing MOS in cultivation in the top six inches of loamy or loamy soil can provide 10 to 15 pounds of nitrogen, one to two pounds of phosphorus, and about half a pound of sulfur per acre per year. SOM retains water that is released in plants. It also aggregates soil particles for drainage and reduces soil erosion.
An acre of soil six inches deep weighs an average of two million pounds (less for boggy soil and more for sandy soil). If your soil contained 2% organic matter, that organic matter would weigh about 40,000 pounds. per acre.
Ratio of C, N, P and S in SOM
Carbon 100, Nitrogen 10, Phosphorus 1, Sulfur 0.25 to 0.50.
These numbers are from the USDA Soil Organic Matter Information Kit. These levels can vary to some degree, but under cultivation or fallow these soils release or mineralize about 1.5% of their total MO per year. In the colder climates of the Canadian Prairies, this mineralization could be 1% or less, perhaps even 0.75%. This is why Canadian prairie soils have much higher SOM reserves than their southern counterparts.
When you buy cropland, a soil test from SOM will tell you about the crop’s nutrient reserves. Would you pay the same price per acre for 2% organic soil versus 6% organic soil, assuming other factors are similar?
Two percent organic soil would have 2,000 lbs. nitrogen bound per acre; six percent organic soil would have 6,000 lbs. of bound nitrogen per acre, not including phosphorus, sulfur and water holding capacity. 2% organic soil at 1% release gives you 20 lbs. of nitrogen relative to 60 lbs. six percent organic soil nitrogen release. How much is that extra 40 pounds worth. nitrogen per acre?
The conversion from parts per million to pounds per acre is based on one acre of topsoil at a depth of six inches weighing two million pounds. If you apply four pounds. of actual nitrogen or boron per acre of land, this translates to two ppm (parts per million) per acre.
I have already stressed the importance of not harvesting crop residues, so that you accumulate organic matter in the soil. Do not confuse this loose organic matter with SOM. MOS is black humus that took thousands of years to form. In addition to nutrients, SOM contains compounds like humic acids, fulvic acids, and lignans. All of these “miracle” nutrients occur naturally in your own 2-10% SOM.
Are you enlightened or even more confused?