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Q: With record-breaking heat waves all over the country, water is very precious. How can we manage soil to best use water?
A: When it comes to soil moisture management, we often focus on improving the physical qualities of the soil. After all, soil water infiltration and holding capacity are directly tied to portion and distribution of the space between mineral particles filled with air or liquid, known as pore space. But just as important to moisture management is soil biology, the living organisms within soil, which improves the structure and resilience of pore space in the soil.
Ideal Soil Conditions for Moisture Management
As a quick refresher, an ideal soil composition has 50% solids and 50% pore space with a nice distribution throughout the soil profile. Also, soil organic matter can hold 20 times its weight in water, so every pound of soil organic matter can hold 2.4 gallons of water. Practices that build toward these fundamentals will improve not only water holding capacity, but also infiltration and drainage of excess moisture to prevent saturated conditions.
Tillage Creates Moisture Management Problems
Tillage may create artificial pore space, but often the result is fractured macropores around large, compacted soil chunks, so while the ratio is 1:1, the distribution is poor, and the size of the pores limit water infiltration and holding capacity. Tillage disturbs the natural processes of soil biology to create porous soil structure. When you reduce or eliminate tillage, it protects the natural creation of pore space through soil biology. It also allows plant root mass to increase to create more pore space.
Soil Biology Builds Soil Structure
Soil biology plays two main roles in building and maintaining soil structure: 1) creating soil organic matter and pore space, and 2) providing the glue and structure that binds soil particles.
Let’s look at a few examples.
Larger soil organisms such as earthworms and beetles create macropores which provide water infiltration channels—think drains in a bathtub—to allow moisture to easily enter the soil profile before it runs off. This is critical in drier environments to maximize rainfall capture. These same organisms also improve deep infiltration, allowing excess moisture to drain into the subsoil and away from the rootzone, reducing the incidence of saturated soil conditions.
Soil organisms also provide the glues vital to holding together soil particles. Fungal hyphae act like rebar in concrete to hold together soil particles while glomalin, a substance used by plant roots and fungi to transfer nutrients, acts like glue to coat and bind particles. The more fungal hyphae and glomalin you have in your soil, the more pore space available. Just as important as their role in building soil structure, fungal hyphae (tiny, root-like structures fungi use to connect with plant roots to exchange nutrients) and glomalin (proteins produced by fungi to help exchange nutrients with plant roots) improve the resistance of soil particles to break apart or compact together. This means the more you have of these ingredients, the more resistant your soil is to soil particle structure loss and, the more quickly it can bounce back and regain good soil structure.
If improving water management and availability is a top goal for a farm, building soil health can help a farmer reach that goal faster. The practices that best encourage soil biology are the same practices that help build soil physical structure: reducing tillage, adding more living roots throughout the year via cover crops, and allowing livestock to graze.