Phosphorus Mobilization and Weed Suppression by Buckwheat.

Final Report for LNC00-168

Project Type: Research and Education
Funds awarded in 2000: $50,000.00
Projected End Date: 12/31/2005
Region: North Central
State: Minnesota
Project Coordinator:
Jim Stordahl
University of Minnesota Extension Service
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Project Information


This project looks at the uses of buckwheat as a cover crop in the Red River Valley area of North Dakota and Minnesota. A variety of results were obtained from the experiments, many of them creating additional questions in the process.


In the Red River Valley of Minnesota/North Dakota all the major crops have become problematic due to a serious disease or insect problem: scab (Fusarium head blight) in barley and wheat, white mold and midge in sunflowers, and numerous diseases in sugar beets. Furthermore, incompatibilities between these crops and various alternative crops in rotation with them have left many farmers with fallowing their land as the only alternative to high pesticide inputs. Besides helping with diseases, a black fallow with frequent disking reduces the weed seed bank. This has traditionally become a standard weed control practice for the area's organic farmers. However, fallowing has problems of its own: the frequent tillage burns off soil organic matter and leaves the soil vulnerable to wind and water erosion, no residues are returned to the soil to maintain the soil's microbial life or to hold on to soil nutrients, and it requires large amounts of fossil fuels to run the tractors, not to mention the long hours the farmer must spend in the field away from family and civic life. If summer cover crops suitable to the Red River Valley could be identified that do not harbor the same pests as the cash crops, protect the soil from erosion, build organic matter, maintain or improve soil fertility, and help control weeds, insects and diseases, they would add some much-needed diversity to the farming system of the Red River Valley.

Buckwheat (Fagopyrum esculentum) is one such candidate for a green manure crop. In "Managing Cover Crops Profitably" (SAN, 1998) buckwheat is lauded as an outstanding smother crop for its ability to mobilize soil P, especially from calcareous soils such as are found in the Red River Valley. Anecdotal information from Minnesota farmers supports this: many have remarked on the absence of weeds in a healthy buckwheat stand, and some have observed increases in soil test P following buckwheat plow down. However, there is little scientific research to translate this into practical recommendations. For example, although the ability of buckwheat to scavenge phosphorus from extremely low-P soils has been well documented, we need to know how well this translates into increased availability of P to subsequent crops. This would be valuable because livestock production is rare in the Valley and therefore organic farmers are currently supplying P by importing large quantities of manure from elsewhere, at great cost both monetarily and energetically.

Likewise, we do not know how well buckwheat's smother crop capabilities carry over to better weed control in subsequent years. There are other questions: many farmers appreciate the way buckwheat mellows the soil, making it easier to work; however there are concerns that, because it returns relatively little residue to the soil, this may actually increase the erosivity of the soil, unless it is followed by another cover crop that protects the soil over the following winter.

Project Objectives:

To assess the value of buckwheat as alternatives to black fallow in the crop rotations of the Red River Valley to verify their value for 1) mobilizing soil P and other nutrients, 2) suppressing weeds, and 3) providing habitat to beneficial insects.


Materials and methods:

Buckwheat was established in 2001 as a green manure crop and incorporated after flowering but before seed set. Soil samples were taken prior to buckwheat establishment in 2001 and from the same sites (within 1 meter) during the next three years. Soybean or wheat plant samples were collected as the same time and analyzed for P and several other common elements from the two treatment areas. In the fall of 2002, grain yield was also measured. No other soil amendments were added during the trial.

Soil Type: Fargo Clay, Bearden Loam
Tillage: variable
Previous Crop: Buckwheat as a green manure crop in 2001, soybean in 2002, wheat in 2003, soybean in 2004
Row Width: soybean: 22" wheat: 6"
Fertilizer: none on plot areas
Herbicide: None, both fields are certified organic
Experimental Design: Randomized Complete Block with three or six replications.

Research results and discussion:

Soil conditions were extremely wet in 2001, the year for buckwheat establishment. Floods or excessive rainfall destroyed eight of our ten research sites. In addition, it delayed planting and reduced the buckwheat biomass at the remaining three locations; we were able to salvage two locations. Wet conditions reduced the buckwheat biomass at the Olsgaard location, while the buckwheat stand was excellent at the Thomas location.

The soil P concentration increased significantly from 2001 to 2002 on both the buckwheat and fallow treatments. Buckwheat did not significantly increase the measurable soil P concentration at either location. 'Cluck' (4-4-2) was applied at both locations for the crop year 2000 and may explain the precipitous increase in P concentration between years.

In 2002, the P concentration in soybean biomass increased following buckwheat despite no measurable differences in soil P concentration at the Olsgaard location. At the Thomas location, buckwheat reduced the plant K concentration but caused an increase in Na and Zn concentrations. These differences had no effect on grain yield at either location

In 2003 and 2004, the soil P concentration at the Thomas location was significantly greater (p<.01 and p<.06) where buckwheat was planted in 2001. The trend was similar at the Olsgaard location in 2003, but the increase was not statistically significant (<.14). This difference may be explained by the difference in buckwheat biomass production, which was significantly greater at the Thomas location. In 2004, the Olsgaard plot area was contaminated. Buckwheat is very competitive and effectively eliminates weed competition if an adequate stand is established (data not shown). This was clearly the case at both locations.

Research conclusions:

Buckwheat is inexpensive to establish and offers many other benefits over black fallow. The potential benefits include quick establishment to prevent soil erosion, easy to destroy, and is an excellent soil conditioner.

Furthermore, buckwheat increases soil phosphorus concentration with the greatest concentration apparent during the second year after incorporation. The increase in soil phosphorus appears to be dependant upon the amount of buckwheat biomass incorporated.

Buckwheat is an effective competitor with weeds if an adequate stand is established under normal growing conditions. Weed pressure, however, was not heavy in the experimental locations.

Buckwheat is very effective in attracting various species of beneficial insects. However, their effect on the following soybean crop was not determined.

Economic Analysis

Buckwheat is a rather inexpensive cover crop alternative. Although an extensive economic analysis was not attained it can be said that using buckwheat as a cover crop is a definite profitable alternative to many other traditional methods.

Farmer Adoption

Buckwheat is often used as a green manure crop on organic farms in the Red River Valley. Buckwheat did increase soil P concentration when an adequate amount of biomass is incorporated into the soil. Its use as a phosphorus scavenger, however, may have secondary, though still relevant, importance.

Nitrogen remains the most elusive nutrient on organic farms in the Valley. To meet the nitrogen requirements, organic farmers depend on poultry manure, which is typically phosphorus rich. To balance nitrogen crop needs with poultry manure, excessive amounts of phosphorus are an unintentional side effect which negate the need for buckwheat as a phosphorus scavenger.

On organic farms that include legume cash crops, such as alfalfa and red clover, buckwheat is often used as a green manure crop is used to supply part of the phosphorus crop needs when alfalfa is used to balance nitrogen requirements. Purchased manure often supplies the remaining needs.

Buckwheat may have the greatest value to farmers that do not have ready, or economically feasible, access to manure or those that find benefit in its ability to attract insects and smother weeds.

Participation Summary

Educational & Outreach Activities

Participation Summary:

Education/outreach description:

The results of this study have been published in the 2003 and 2004 editions of the University of Minnesota Extension Service, "On Farm Cropping Trials," which is disseminated to approximately 2500 farmers, industry representatives and Extension personal.

An oral presentation has been shared with approximately 350 producers at the Northern Plains Sustainable Agriculture Society Annual Symposium and the Minnesota Organic Conference. In 2004, the preliminary results were displayed at the Poster Session at the National Association of County Agricultural Agents (NACAA) annual meeting in Orlando, Florida.

Project Outcomes


Areas needing additional study

The ability of buckwheat to sequester soil phosphorus, and other elements, was demonstrated when incorporated as a green manure crop. However, will buckwheat have similar effect if the crop is allowed to mature and be harvested as a grain crop? This question was beyond the scope of this project and would have definite practical implications to farmers and so it may be worthy of further investigation.

Additionally, although buckwheat effectively attracts beneficial insects, it is unknown whether they provide meaningful benefit to subsequent crops; a question that may also be worthy of further investigation.

Finally, could buckwheat be grown in a binary combination with soybean for the sole purpose of enhancing beneficial insect population? In this hypothetical scenario, perhaps buckwheat could be planted, along with soybean, at a very low density to provide the necessary nectar source. The buckwheat seed would, however, have to be separated from the soybean seed before marketing the crop. Could this strategy be an inexpensive, safer, more sustainable method of managing soybean aphid populations?

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.