Soil organic matter is important to soil health and profitability

Soil organic matter is an indicator of how healthy a soil is and serves many important functions. The higher the organic matter the better the soil is. OM, as we typically call it, comes from plant residue and roots that have been broken down and decomposed by soil microorganisms. Only about 10 percent of organic material is converted to stable OM, so it takes a lot of plant residue, roots and time to increase the amount in the soil significantly. This is especially true in warmer and drier climates where the amount of material that can be grown is limited and the OM is mineralized quicker by the microorganisms.

Originally western Kansas soils had fairly high OM levels, between 3.5 to 4 percent, because they formed under prairie vegetation that supplied organic material from both the top growth and the roots. We usually don’t think about roots supplying organic material but they actually provide about four times more than the plant material on the soil surface.

OM is a reservoir of nutrients that are mineralized by microorganisms and released in the soil that plants can use. For each percent of OM in the soil about 25 pounds of nitrogen, 5 pounds of P2O5, and 3 pounds of sulfur become plant available annually. In high OM level soils this can lead to significant fertilizer savings. The warmer 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 water holding capacity of the soil. It can and hold up to 90 percent 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.

High OM soils have a higher percentage of stable soil aggregates which don’t break down, consolidate or condense when it rains. This improved soil structure increases the pore space in the soil which helps the soil take in more water. It also decreases runoff and erosion.

It is not easy, and it takes years to see a significant improvement, but there are ways that we can increase OM levels in the soil. Here are just a few.

Reduce tillage. Tillage has been used on most of our prairie soils since they were first cropped over a century ago. As a consequence soil OM levels have been reduced by about half of what they originally were. Tillage aerates the soil and causes a flush of soil microbial activity that speeds up the decomposition of OM. Tillage also increases erosion so when soil leaves the field organic matter goes with it, since most of the OM is near the surface of the soil.

Soil test on a regular basis and fertilize accordingly. Proper fertilization, based on a soil analysis and realistic yield goals, encourages plant growth and more root development. This can help build or maintain OM levels, even when top growth is removed for forage.

Where possible intensify the crop rotation with additional cash or cover crops. Without a green growing plant that provides plant residue and roots, there is nothing there to build organic matter.

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 because more plant nutrients are released in the soil. High OM soils can help keep a crop growing and healthier longer during prolonged dry periods because of the higher water holding capacity. In some years this can be the difference between growing a profitable crop and not growing one so it can affect the bottom line.

Consider using these practices on your operation to build and maintain organic matter levels in your soil.

For more information about this or other soil health practices you can contact me at [email protected] or any local NRCS office.