Soil organic matter is an indicator of soil health and performs many important functions. The higher the organic matter, the better the soil. OM, as we commonly call it, comes from plant residues and roots that have been broken down and decomposed by soil microorganisms. Only about 10% of organic matter is converted into stable MO, so it takes a lot of plant residues, roots and time to dramatically increase the amount in the soil. This is especially true in hotter, drier climates where the amount of material that can be grown is limited and OM is mineralized faster by microorganisms.
Originally, the soils of western Kansas had fairly high MO levels, between 3.5 and 4%, because they formed under grassland vegetation that provided organic matter from both aerial and root growth. We don’t usually think of roots as providing organic matter, but they actually provide about four times as much as the plant matter on the soil surface.
OM is a reservoir of nutrients that are mineralized by microorganisms and released into the soil for plants to use. For every percentage of MO in the soil, approximately 25 pounds of nitrogen, 5 pounds of P2O5, and 3 pounds of sulfur become available to plants each year. In soils with high MO content, this can result in significant fertilizer savings. The higher the soil temperature, the more nutrients are released, so summer crops benefit more from OM mineralization than winter crops.
OM behaves like a sponge and increases the soil’s water retention capacity. It can hold up to 90% of its weight in water. This is especially important in drier climates where soil moisture is almost always the most limiting factor when it comes to crop yields.
Soils rich in MO have a higher percentage of stable soil aggregates that do not break down, consolidate or condense when it rains. This improved soil structure increases the pore space in the soil, which helps the soil absorb more water. It also decreases runoff and erosion.
It’s not easy and it takes years to see significant improvement, but there are ways to increase MO levels in the soil. Here are a few.
Reduce tillage. Tillage has been used on most of our prairie soils since they were first cultivated over a century ago. As a result, soil MO levels were reduced to about half of what they were originally. Tillage aerates the soil and causes a surge of soil microbial activity which accelerates OM decomposition. Tillage also increases erosion, so when the soil leaves the field, organic matter goes with it, because most organic matter is near the surface of the soil.
Test the soil regularly and fertilize accordingly. Proper fertilization, based on soil analysis and realistic yield goals, encourages plant growth and root development. This can help build or maintain MO levels, even when top growth is removed for forage.
Where possible, intensify crop rotation with additional cash or cover crops. Without a green growing plant that provides plant residues and roots, there is nothing to build organic matter on.
It makes economic sense to do what you can to maintain and improve organic matter levels in your soils. The higher the organic matter, the less fertilizer you will need, as more plant nutrients are released into the soil. Soils rich in OM can help keep a crop growing and healthier longer during prolonged dry spells due to higher water holding capacity. In some years, this can mean the difference between growing a profitable crop and not growing one, which can affect the bottom line.
Consider using these practices on your farm to increase and maintain organic matter levels in your soil.
For more information on this or other soil health practices, you can contact me at [email protected] or any local NRCS office.
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