Organic Broadcaster

Research looks to cover crops to alleviate soil compaction, suppress weeds

By Rachel Welch, John Masiunas, Dan Anderson, and María Villamil, University of Illinois

Rotary hoeing and in-row cultivation during the grain growing season help suppress weed populations. But, tillage is weather-dependent, and can be economically and physically detrimental. Extensive tillage encourages organic matter decomposition, breaks down soil aggregates, weakens soil structure and can eventually lead to compaction. Not only are compacted soils physically difficult and costly to reverse, compaction has a multitude of negative consequences to soil quality and crop productivity. Compaction interferes with water infiltration, nutrient cycling, root development, and aeration which in turn can negatively affect crop growth and yield. Compacted soils in organic grain production present a serious issue to the efficiency and success of the system.

Cover crops might be a solution to soil compac­tion and weed suppression. Research in other states has suggested that incorporating deep-rooted cover crops minimizes compaction and improves soil quality (Chen and Weil, 2009). Introducing deep-rooted cover crops, such as forage radish, into organic grain production systems has the potential to alleviate compac­tion, improve soil quality, and suppress weed populations. Though the potential benefits from deep-rooted cover crops are multiple, the results are highly dependent upon factors such as agronomic management, length of the growing season, plant species, subsequent cash crop, soil type, and weather conditions.

The Agro-Ecology team at the University of Illinois Urbana-Champaign, led by Dr. Maria Villamil, conducted collaborative research with three farmers who own certified organic farms located in Malta, Cerro Gordo, and Pana (two locations), Illinois. The objective of this research was to identify the best cover cropping practices to alleviate soil compaction, improve nutrient cycling, and suppress weeds, while partnering with organic farmers to develop a deeper knowledge on how to effectively use these multifunctional cover crops.

Figure 1. Soil bulk density values during fall (before cover crops) and spring (after cover crops) for compacted and non-compacted areas across all depths. Letters indicate statistically significant differences detected.

When forming the study, the farmers specifically expressed compaction as a concern. They identi­fied two areas of their farms as compacted and two areas as non-compacted. These areas were planted in four variations of cover crops: forage radish (FR), forage radish/buckwheat (FRbw), forage radish/hairy vetch/rye (FRhvr), and a control area that was left fallow.

The study started in fall 2011 and concluded this past fall 2013. Cover crops were planted in early fall and tilled under at least two weeks before the spring grain crop planting at all sites. Soybeans were planted the first spring, and corn the following spring. Soil sampling was con­ducted four times at each site down to 50 cm of depth, before each respective cover crop or grain crop planting to observe trends in soil physical and chemical properties. Two physical properties are reported here that provide complementary information: soil bulk density, which is the ratio of voids to solids in a given volume of soil, directly affecting aeration and water movement; and penetration resistance, which provides a proxy for the effort that a root tip has to exert to penetrate the soil layers. Additionally, weed and cover crop counts and biomass, and grain crop yields were recorded.

Figure 2. Weed biomass during springtime under each cover crop treatment within compacted and noncompacted areas. The treatments were control fallow; FR forage radish; FRbw, forage radish and buckwheat; and FRhvr, forage radish, hairy vetch, and rye. Different letters indicate statistical differences detected.

Our results indicate that the compacted areas had higher bulk density (Figure 1) and penetra­tion-resistance values than the non-compacted areas. These difference were still present in the following spring after the cover crop season. This trend was witnessed down to 40 cm of depth, and is attributed to the densification of the soil. Compacted areas had significantly higher pH values, and were also richer in nutrients in comparison with the non-compacted counterparts, yet there were important seasonal differences. Total mass of phosphorus and nitrates (Mg/ha) were higher in compacted areas in the fall season, but not in the spring— an effect that could be attributed to the cover crops improving the nutrient cycling and the efficiency of these systems. During springtime, we observed a higher concentration of nitrates in the surface soil of rotations that included the mixture of forage radish, hairy vetch, and rye, which helps support the previous statement.

Our spring weed biomass data shows that, in both compacted and non-compacted areas, the rotation including the mixture of forage radish, hairy vetch, and rye significantly reduces weed pressure (Figure 2). Yet this same mixture resulted in a reduction of soybean yield in non-compacted areas, though yield of soybean from compacted areas did not show any effect of cover crop (Figure 3).

Figure 3. Soybean yields following each cover crop treatment within compacted and non-compacted areas. The treatments were control fallow; FR forage radish; FRbw, forage radish and buckwheat; and FRhvr, forage radish, hairy vetch, and rye. Different letters indicate statistical differences detected.

Our corn yield in 2013 ranged from 151 to 167 bu/acre, and did not show any effects from compaction or cover crop treatment. Soybean yields were collected in 2012, an especially dry year, and the observed trend could be due to the cover crops drawing valuable water resources away from the cash crop.

These preliminary findings support the abil­ity of overwintering cover crops to suppress weed populations and retain nutrients in their biomass. We continue to collect data from this study, and hope to have more insight on the effect of these crops on soil properties and yield.

This project was funded by the Ceres Trust Organic Research Initiative. The authors are from the Department of Crop Sciences at the University of Illinois.

References

Chen, G., and Weil, R. R. (2009). Penetration of cover crop roots through compacted soils. Plant Soil, 331(1), 31-43.

From the May | June 2014 Issue

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