Crop Rotations Impact Soil Quality
By Jay Norton, UW Extension Soils Specialist
Two underlying strategies toward improving soil quality include conserving soil organic matter by reducing tillage and adding organic material by applying composted manure or other amendments. Crop rotation also builds soil quality by supplying diverse crop residues, especially if perennial nitrogen-fixing legume crops, such as alfalfa, are included.
Starting in 2009, we conducted a four-year experiment on irrigated fields at the University of Wyoming Sustainable Agriculture Research and Extension Center (SAREC) near Lingle to evaluate how rapidly combining crop rotations, reduced tillage and manure application would improve soil quality on depleted soils that had been under intensively tilled, continuous corn for many years.
We divided the area into 12 one-acre plots and converted four plots to a conventionally-managed rotation of dry beans, corn, sugarbeets and corn, four plots to reduced-tillage under the same rotation, and four plots to an organically managed rotation of alfalfa, alfalfa, corn and dry beans.
Tillage was similar among the conventional and organic plots, and fertility management was similar among the conventional and reduced-tillage plots. Tillage on the conventional and organic plots included moldboard plow, disk and harrow for seedbed preparation and cultivation as needed for weed control. The reduced-tillage plots were tilled once with a Landstar. Conventional and reduced-tillage plots were fertilized with commercial fertilizers based on soil test-based recommendations, while the organic plots were fertilized with composted and fresh cattle manure applied based on crop needs for soil nitrogen at about five tons per acre.
Yields were comparable among the three systems, and economic analysis indicates that the organic rotation is more profitable than the other two if premiums for organic-certified crops are considered.
To assess effects on soil quality, we measured properties that respond rapidly to management changes, including soil microbial populations and functional groups like fungi and bacteria, soluble organic carbon and nitrogen compounds and easily decomposed organic materials.
Results indicate that converting from mono-cropped corn to crop rotations had the largest positive impacts of the three strategies, regardless how they were managed, and that reduced tillage, manure and alfalfa in rotations added to the positive effects. At the end of the four-year study, total microbial biomass in the soil had tripled or quadrupled in all three systems, with the largest increase in the organic system.
The most striking increases in microbes were among fungal species that break down crop residues, with five- to eight-fold increases in that group. Mycorrhizal fungi also increased in all three systems by factors of two to four. Mycorrhizal fungi form symbiotic relationships with plants and support improved uptake of water and nutrients, especially phosphorus. Both types of fungi form extensive networks of microscopic hyphae that hold soil particles together and improve soil structure, porosity and water infiltration. Their presence indicates more stable and resilient soils.
Overall microbial diversity also increased by 50 to 70 percent over the four years, indicating establishment of a rich soil biological system with abundant beneficial organisms.
These increases in microbial numbers and diversity correspond to increases in decomposable organic material that serves as substrate – or food – for microbes. Dissolved organic carbon, which is essentially sugar derived from initial decomposition of plant residues, increased six-fold by the end of the fourth year, with larger increases under organic and reduced-tillage than conventional management. Dissolved organic nitrogen, however, did not increase appreciably, meaning that the carbon-to-nitrogen ratio of the most microbially available substrates doubled or tripled in the three systems. This favors beneficial fungi over bacteria and probably results from diverse types of crop residues.
The amount of easily decomposable organic material more than doubled in the organic system but stayed relatively stable in the reduced-tillage and conventional systems, indicating that the combination of manure and alfalfa in rotation began to build soil organic matter more rapidly than either the crop rotations alone or reduced tillage.
Results of this four-year study emphasize the importance of rotating crops for maintaining soil quality and productivity. Even with two years of corn and one of sugarbeet – both highly consumptive crops – in the four-year rotation, soil microbial activity and organic matter components increased significantly compared with conditions after years of continuous corn.
Combining rotation with reduced tillage further boosted microbial activity, as did including alfalfa and manure applications in the rotation, but rotation itself had the largest impact on soil quality.
Results of this study were published in the open access scientific journal PLoS ONE and can be found on line by searching on PLoS ONE 9(8): e103901. Parts of this article were published previously in the online Western Nutrient Digest Newsletter.