Even without scientific scrutiny, our eyes tell us that a perennial forage crop like alfalfa should benefit the soil. A lush plant canopy, a deep root system, and minimal tillage comprise elements for soil-enhancing factors. Less obvious to the eye are the nitrogen-fixing capabilities that alfalfa brings to the table.

As documented in a recent issue of the Midwest Forage Association’s newsletter, Forage Focus, researchers at the University of Wisconsin have set out to evaluate the relationship between alfalfa yield on soil properties, including fertility and health.

In 2021, which was the trial’s first year, 19 farms were identified that measure on-farm yields. Farms were located in Wisconsin, Illinois, or Michigan, and 24 total alfalfa fields were evaluated.

Soil samples to a 12-inch depth were collected directly adjacent to plant crowns. The samples were analyzed for major and secondary nutrients, soil pH, physical characteristics such as bulk density and aggregation, organic matter, and biological factors, including microbial community features, respiration, and active carbon.

The first-year results showed an association between forage yields and soil chemical components with a positive association between greater alfalfa productivity and potassium, sulfur, magnesium, and calcium concentrations. High phosphorus levels in all of the soils negated any positive correlations with that element.

Carbon is key

Active carbon, soil organic matter, soil cation exchange capacity, and pH were also positively associated with higher alfalfa yields. As a component of total organic soil carbon, active carbon responds quickly to changes in crop management and is associated with organic matter.

The researchers note that the positive linear relationship between alfalfa yield and soil carbon pools shows that yields rise in a linear manner per unit of increase in organic matter. This emphasizes the importance of maintaining healthy, high-carbon soils to maximize forage yield potential.

As the project moves into its second year of data analysis, the researchers hope to identify the relationships between other soil indicators and alfalfa forage yield and quality.