In the rolling farm ground found in many parts of Missouri, no-till is often used to control soil erosion. However, no-till has also been shown to improve water infiltration. At the Soil Health Expo at the University of Missouri Bradford Research Farm in August, NRCS state soil conservationist Doug Peterson said despite the popular belief that tillage improves water infiltration in the soil, it actually reduces the porosity, therefore reducing infiltration.
Pores in the soil are created by aggregates, or pieces held together by an adhesive-like substance called glomalin, which is secreted by fungi in the soil. "The fungi that produce glomalin are a very long filamentous organism in the soil. Anything we do that chops them up or disrupts them reduces their numbers," Peterson explains. "When their numbers are reduced, it's going to reduce the amount of glue in the soil."
When there is no glue to hold the soil, the soil particles collapse when water runs through them. When the particles collapse, it reduces the ability of the water to infiltrate. "Water stable aggregates are created by having a lot of secretions in the soil," Peterson says. "Any tillage that we do that decreases the amount of fungi is going to hurt the water stability of the aggregates."
Peterson demonstrated this with two soil pedons or clods of soil: one from tilled land and the other from continuous no-till and several years of cover crops, placed in a jar of water where they were suspended by chicken wire. The soil pedon from the tilled soil dissolved and fell through the wire into the bottom of the jar. The soil pedon from the no-tilled field held together. Even the water in the jar with tilled soil turned cloudy, the other remained clear. "Look at the clarity of the water," he says. "Even though there are particles falling off, it's not really making the water cloudy."
He also demonstrated the difference between the same two soils, another sample from a mowed hay field, and another from a lightly grazed pasture with NRCS's runoff table with simulated rainfall. A section with nothing but perforated steel was used as a control group. Many were surprised by the hay field, which had about 80% runoff, not very different from the tilled soil, Peterson notes. "While we thought that grass was pretty good for infiltration, in reality, short grass hay fields that have had a lot of organic matter mined off of them really have a tremendous amount of runoff."
The height of the grass canopy is critical to water infiltration. "We're finding that having tall grass helps intercept the energy of the raindrop. It helps prevent the water from flowing off the field, and that will significantly increase water infiltration," Peterson explains. "The hay field has very little residue and very little grass height, it has had the organic matter mined off of it for a long period of time. It really limits water infiltration."