This month let’s take a closer look at how improving soil health on our farmland acres can have a dramatic effect on reducing soil erosion and rainfall runoff that result in downstream flooding and property damage. During the past few years, as a result of climate change, rainfall totals have increased in north-central Iowa. We also seem to experience more intense rainfall events where several inches of rain falls in a short period of time. Both of these occurrences result in soil damaging runoff and flooding.
One of the most challenging farming myths to dispel continues to be that for the soil to absorb the maximum amount of water it needs to be tilled. All farmers in my generation were told by their fathers and grandfathers that you had to till the soil to allow the water to soak in. This notion was disproved many years ago. For soil to absorb the most water, therefore reducing runoff from large rain events, it needs to be healthy. Healthy soil has a higher organic matter percentage, it has good soil particle aggregation that allows for air spaces in the soil profile, and it has a thriving soil biology. Tillage harms all these parameters and results in degraded soil.
First, soil organic matter (OM) percentages in our area have been steadily decreasing since the native prairie was first tilled over 150 years ago. In the past 50 years, the dramatic reduction in pasture and small grain acres has accelerated this loss. When most of my generation started farming, it was common to have 5-7% OM in our soils. Now most fields are at 3% or less. Intensive grain cropping and tillage reduce this decaying plant material in the soil profile. Soil organic matter is critical in forming soil aggregates and aggregates are critical for allowing a soil to absorb water and to hold on to it for crop plants to take up. In a 3% OM silt loam soil, increasing the OM by 1% through less tillage and cover crops results in a 20,000 gallon/acre increase in water-holding capacity.
The popular soil slake test is a great illustration of the advantages of a well aggregated soil. To do your own soil slake test, take a soil clod from a field where long-term no till and cover crops have been used and a clod from a consistently tilled field. Put each clod in a container of water. The differences are dramatic. The clod from the tilled field (that has no stable aggregates) crumbles, dissolves, and turns the water brown. The clod where soil health practices have been consistently used maintains its shape, does not crumble into small pieces, and does not disperse into the water. Soils with poor aggregation will crumble, seal up the soil surface, and force rainfall to run off.
To explain all the reasons why this occurs requires a lengthy discussion, but primarily cover crops keep a living root in the soil most of the year. Living root helps develop a healthy soil biology that produces exudates, or glues, that help bind soil particles together. Also, cover crop root channels and the huge increase in the number of earthworm tunnels that result when tillage is eliminated help the soil to absorb more water. Finally, it has been shown over and over that a healthy soil is able to absorb two to three times as much water during a rain event than a degraded soil, thereby reducing runoff and downstream flooding.