Building soils for the future

By Dusty Sonnenberg, CCA (Adapted from Crop & Soils Magazine, May-June 2022 by Jerry Hatfield and Wayne Fredericks)

It is often recommended to farmers when adoption a new practice that they start on a small scale so they can make mistakes and learn on a small scale before adopting it across a large number of acres. Over time, much is learned about the practice they are implementing and ways to modify it to best fit the operation. Over that same time period, the soils will also exhibit changes. While not all the changes are understood, many can be explained and managed.

Understanding the soils on your farm is the first step to measuring any change. Soil health and weather work in tandem to produce crops. We cannot control the environment, however as we improve soil health we can mitigate the risk of adverse weather conditions, and hopefully increase crop yields.

In an on-farm study conducted in Mitchell County, Iowa, a farm was gradually transformed over 30 years by transitioning from conventional tillage in all crops to no-till soybeans in 1992, 10 years later adopting strip-till for the corn in 2002, and finally rye cover crops were added in 2012 with full implementation in 2017. Today the soybeans are directly seeded into a standing rye cover crop, and strip-till corn is planted directly into the green crop. They have found that planting green allows for timely operations in the spring to take advantage of the entire growing season.

Can organic matter levels change?

In the study, soil samples were collected at nine locations across each of three fields on the farm. In all three fields, soil organic matter increased over a 30-year period from 1984 to 2015. There were differences among the fields in the initial organic matter levels, however, all three fields increased over that time with a doubling of the organic matter over that 30-year period. The initial reduction of tillage showed the greatest increase in organic matter. All the fields continued to increase the level of organic matter over the time measured, however not all at the same rate. For the fields that showed slower changes in organic matter levels later in the study, an increase in field uniformity was observed.

It was noted that the results demonstrate the potential across the upper Midwest to adopt conservation practices and use agricultural soils to capture carbon.

Do different soil types respond at different rates of change?

When reviewing the yield data from the fields in the study, two common soil types in the area were identified and investigated. In both soils, there were two evident changes. First, over time, the “skewness” of the distribution changed, so the low yielding parts began to decrease; and second, the tightness around the mean increased. Changes in the skewness of the distribution represents an elimination of low yielding areas within each soil type. While not dramatic, it brought the average yield up across the entire soil type. Observations during harvest from the 2020 and 2021 growing seasons have shown the uniformity in the fields continued to increase as lower yielding areas caught up with the higher yielding areas. There is still variation within the field, but it continues to be less.

What is the impact of increased soil organic matter on the field and water holding capacity?

There was an increase in water use efficiency over time. There is improved soil structure that allows for traffic on the fields after rains in the spring. The practice of planting green and the use of cover crops increases the trafficability of the field. This allows for timely planting in the spring and the capability to take advantage of the longer growing season. Other farms that have adopted no-till and strip-till and cover crops have shown a similar increase in efficiency in the use of seasonal rainfall compared to county levels. 

The observations regarding water dynamics in reduced tillage and cover crops showed three positive outcomes. The presence of the crop residue protected the soil surface from the direct impact of raindrop energy and maintained the infiltration rate of water into the soil. The crop residue reduced the water evaporation rate from the soil surface making more water available to the crop. The presence of crop residue creates a favorable microclimate near the soil surface for crop roots to grow and be able to take advantage of small rainfall events.