By David Kringen, Extension Water Resources Field Specialist
Recently, I attended the first in a series of five tours called the I-29 Soil Health and Cover Crop Field Tours. It was a chain of tours along the I-29 corridor during the month of September that began near Sioux Falls and ended near Raymond, S.D. The main purpose of the tours was to assess cover crops for beneficial soil health properties and forage/feed value for grazing livestock. Several long-term no-till operations, however, were also evaluated with regards to soil structure and its effect on water quality and quantity on the landscape.
The first tour was on the Al Miron Farm northwest of Sioux Falls. During his introductory remarks, the owner said something that we have always held to be true with respect to water quality and quantity. Conservation practices such as filter strips and buffers along tributaries and rivers at the bottom of a landscape are certainly important to improving runoff; however, it’s what we do at the top that may make the biggest difference. Essentially, it is what can be called a “top-down” philosophy for lack of a better term.
After opening remarks, NRCS used their rainfall simulator to compare surface runoff vs. infiltration between no-till and conventionally tilled soils. If you have never had an opportunity to see the rainfall simulator for yourself, I recommend you do. It is a vivid demonstration that illustrates how soil management decisions can affect the water cycle, and the consequences those decisions can have on the environment. It also helps clear up the misconception that tillage is needed to increase water infiltration. In fact, the opposite is true.
It’s All about Soil Health & Structure
When we talk about soil health and structure, we need to define soil aggregates and aggregate stability. Soil aggregates are groups of soil particles that bind to each other more strongly than to adjacent particles. The arrangement of soil aggregates into different forms gives a soil its structure. Aggregate stability refers to the ability of soil aggregates to resist disruption and disintegration when outside forces such as tillage and water or wind erosion are applied.
Aggregates ideally possess one half solid material (mineral and organic matter) and one half pore space. Pores between aggregates can be large while those within an aggregate can be quite small. The balance between large and small pores allows for water infiltration, root penetration, and air exchange to occur — all of which are important for crop growth.
Long term no-till management allows soil aggregates and soil structure to develop over time. Residue left on the soil surface in a no-till operation is important for a number of reasons. It provides a stable source of organic matter, of which aggregate stability is highly dependent on. All of the biological activity and soil organisms that provide the “glue” to bind aggregates together rely on the slow decomposition of organic matter as an energy source.
Residue also plays an important role in protecting the soil surface. Without residue, aggregates can be broken down during a heavy rainfall event, which can encourage soil particle detachment and erosion (both wind and water).
Tillage used in crop production will destroy soil structure and aggregate stability, and accelerate the breakdown of organic matter, which results in the release of carbon dioxide into the atmosphere. Tillage will also reduce water infiltration by cutting off soil pores. This results in increased surface runoff and topsoil loss.
In terms of water quantity and quality, soil profiles are the best filters we have. With improved soil health, more rainfall will infiltrate the soil and find its way to lakes and streams through shallow groundwater than from surface runoff. Hence the top-down philosophy. The more water that is allowed to infiltrate into the soil profile at the top, the less surface runoff to be dealt with down below.
The South Dakota Soil Health Coalition is a newly-formed organization that was established in May 2015. The vision of the coalition is “a collective effort to sustain long-term agricultural production, maintain and improve soil health and diversity.” While in its infancy, this coalition (led by a farmer/rancher board of directors from across the state) will prove to be significant for both sustainable ag production and water resources in the state. As goes soil health, so goes water quality. If you would like to learn more about the coalition, visit the South Dakota Soil Health Coalition website.