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Soil Organic Matter is Key to Soil Water Capacity
by LuAnn Rolling - NRCS District Conservationist

One benefit of increasing soil organic matter is to store more water in your soil because soil organic matter creates pores in a range of sizes. Exactly how much more water is stored due to soil organic matter will depend on soil texture.

Soil organic matter is a mix of fragments of last year’s stalks and roots, earthworm casts, and living microbes and invertebrates, to name just a few. These materials are broken down by physical and biological processes. For example, freezing and thawing causes plant residue to lose its structure. Hungry invertebrates, fungi, and bacteria consume complex living and dead organic material and excrete nutrients they don’t need in a smaller, simpler form. These small organic molecules can stick to clay surfaces. Clay surfaces covered with organic material grow like snowballs, and soil aggregates are formed.

Soil aggregates are critical for holding water in the soil for two reasons. First, a well-aggregated soil has large pores between aggregates to let water enter the soil profile. Second, small pores within aggregates hold water tightly enough to keep it around, but loosely enough for plant roots to take it up. It’s critical that soil both let water flow through and hold water for later. If your soil doesn’t let water infiltrate, you’ll have ponding, runoff and soil loss, and lower plant water supply. If your soil doesn’t hold water, plants suffer from drought.

So, soil organic matter is critical for forming aggregates, and aggregates are critical for holding water

During the past century, through an input-dependent, industrial business model, we have mined the life and resilience out of our soil.  Overall, our soils are less able to store water or absorb heavy rainfall and, as a result, they’re more susceptible to periods of drought or flooding. Our soils are also increasingly dependent on synthetic fertilizers and pesticides just to sustain current levels of productivity. During heavy rains, many of those chemicals are carried into our rivers. Weather extremes are just proving that most of our soils are devoid of diverse microbial life and unable to function at anywhere near their intended capacity.

We know we can heal our soils relatively quickly and profitably, with practices that have been around for years. These include no-till planting, the use of cover crops and diverse cropping rotations  that feed and protect soil microbes, which in turn, feed and protect the crops that feed and nourish us.

Management focused on protecting soil structure and building soil organic matter can build organic matter and improve soil function.

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