Reducing Tillage with Sweetclover Green Manure

By Brenda Frick, Ph.D., P.Ag.

Producers may be able to “manage weeds with sweetclover in a reduced tillage system”, according to a new study out of the Lethbridge Research Centre of Agriculture and Agri-Food Canada. Scientists Jim Moyer, Bob Blackshaw and Henry Huang found that killing sweetclover with a wide blade cultivator and leaving the residues on the surface could suppress weeds, and in some cases, virtually eliminate them for the rest of the season.

Legume-based green manures are a vital tool for improving soil fertility and suppressing weeds. Traditionally, organic producers have disked or cultivated to terminate green manures and incorporate them into the soil. The Lethbridge study asks if green manures could fit into a system with less soil disturbance.

Sweetclovers were terminated with a wide blade cultivator that minimizes soil disturbance and leaves the killed crop standing. Sweetclover residues were either removed, or left on the soil surface. Five types of sweetclover were used in the study, including both yellow and white, and both high and low coumarin levels. The sweetclover was seeded with wheat in its first year. It was terminated in the second year at either the bud to 20% bloom stage (early June) or at the 70% to full bloom stage (late June or early July).

In the establishment year, yellow sweetclovers produced greater biomass than did white sweetclovers. Common sweetclover was the only variety that significantly suppressed weeds in its establishment year; weed biomass with common sweetclover was less than half of what it was without sweetclover. White sweetclovers did not suppress wheat yield in their first year, but yellow sweetclovers suppressed wheat yield 12 to 20%.

Weed populations in the Lethbridge tests included flixweed, stinkweed, prickly lettuce and wild oats. Most of these weeds were suppressed by the growth of most of the types of sweetclover, but some combinations were more effective than others. For instance, Norgold sweetclover suppressed prickly lettuce, but not flixweed or stinkweed.

Sweetclover residues left on the soil surface after termination at the 70% bloom stage suppressed the growth of new annual weeds, especially lamb’s-quarters, and fall emerging winter annuals. Weed suppression was best when there was either a dense growth of live sweetclover or a thick layer of sweetclover residue.

Sweetclover in this test did not increase available soil nitrogen above the amount available in cultivated (black) fallow plots. Generally, more nitrogen is available after sweetclover killed by cultivation. This suggests that residues laying on the surface do not provide as much available nitrogen to the soil as those incorporated into the soil by tillage.

In the Lethbridge study, the sweetclover did not increase the availability of soil phosphorus. The results may have been different if a low phosphorus soil had been used for the test.

Moisture use by the sweetclovers differed among years. In some cases the sweetclover used up to 10 mm of soil moisture compared to checks with no sweetclover. Yield of wheat following sweetclover was not reduced by this moisture loss, even in a drought year. The moisture used by the growing sweetclover was gained back by reduced moisture loss from the soil due to the layer of residues. Other studies suggest that green manures should be terminated before mid-June to avoid excess moisture loss in low moisture years.

Weeds in the wheat crop that followed green manure were generally fewer if the sweetclover had been terminated at the 70% bloom stage, with residues left on the soil surface. Yukon, the high coumarin, yellow sweetclover, produced large biomass and consistently suppressed weeds both in the sweetclover years, and in the following year.

An ideal green manure would suppress weeds, provide sufficient nitrogen for the following crop, provide cover to protect the soil from erosion, and recapture any moisture used during its growth. It would also fit into the desired cropping system. Traditionally, sweetclover has been the green manure most commonly used by organic producers on the prairies. The Lethbridge study suggests that organic producers may find new ways of incorporating sweetclover into their reduced tillage systems.

References
Moyer, J. R., Blackshaw, R. E. and Huang, H. C. 2007. Effect of sweetclover cultivars and management practices on following weed infestations and wheat. Can. J. Plant. Sci. 87: 973–983.


Brenda Frick, Ph.D., P.Ag., is the Senior Research and Extension Associate for Organic Agriculture Centre of Canada at the University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email at organic@usask.ca.


en français


Posted February 2008