Avoiding Monoculture

Published in the May 2009 Issue Published online: May 03, 2009 Nancy I. Butler
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During the time of the Dust Bowl, growers did not realize that the devastation was due in part to the lack of crop rotation. Similar results are blamed on the Irish Potato Famine, where that historic farming mistake has been labeled "monoculture."

Monoculture in the agriculture industry is defined as growing a single crop on the same piece of land season after season.

The potato demise in Ireland was caused by planting the same potato variety over and over in the same place. In addition, there was no resistance to potato blight in that variety and the nutrients in the soil were exhausted.

According to the USDA-ARS, an intensive research program was started following the disastrous "Dust Bowl." The mission of the USDA in conjunction with Kansas State University was to ".increase understanding of wind erosion processes, develop reliable predictive tools, develop control practices and transfer technology for sustaining agriculture, protecting the environment and conserving natural resources."

Part of that program includes determining Best Management Practices. Practicing monoculture negatively affects the soil and the crop. It is recognized that one crop will pull specific nutrients from the soil while other crops will replenish nutrients.

Utilizing BMPs has become a common practice as it is designed specifically and individually. For no two fields or growers are alike.

ROTATION CROPS

Rotating crops and fields are the best bets in producing a healthy, high-yield crop.

Cover crops are often considered soil-building as their purpose is to be grown specifically to be worked back into the soil toward the end of the season.

A lot of oilseed radish and a fair amount of mustard are planted as cover crops. Choosing the variety is often up to personal preference and seed salesman. It is advisable to consult with your agriculturalist as well.

By using green manure as a cover crop, or plow-down crop organic matter, nitrogen and other nutrients are added to the soil.

In a presentation given at the Idaho Potato Conference in 2001, Brian F. Finnigan, discussed green manures in cropping systems.*

Finnigan's report said benefits include, "improved soil condition, increased organic matter, improved water penetration, reduction of some diseases, reduced nematode population and increased availability of nutrients."

His report also highly recommends that "Adapting green manures into potato-cropping systems requires knowledgeable management of several crops."

"The most common or conventional rotation is wheat-potatoes or wheat-wheat-potatoes with fumigation during the potato-cropping year. Green manure crops of sufficient tonnage incorporated before potatoes, have shown to improve potato yields and quality without fumigation in the major production areas of southern Idaho."

EFFECTS OF GREEN MANURE

According to Finnigan's report, there are numerous benefits in east Idaho when radish or other green manures have been incorporated into the soil ahead of potato rotation. They include:

  • Reduction of Verticillium wilf (early die) following radish green manures;
  • Oil radish suppressed nematode infections in potatoes;
  • Increased quality of potatoes by increasing percent of U.S. No. 1s;
  • Legume green manures provided fertilizer replacement values of 50 to 120 pounds of nitrogen per acre;
  • Barley, wheat and corn incorporated as green manures reduced early die and increased yields
  • Green manures reduced weeds and need for some herbicide applications;
  • Soil organic matter increased with benefits of better water infiltration, soil-water holding capacity and better availability of some soil nutrients, especially in white soils;
  • Reduced the need for soil fumigation.

Basically, Finnigan's study says that green manures can reduce the need for fumigation and other soil-applied pesticides, thus reducing costs. Ongoing field demonstrations and research on green manures will continue to provide evidence of the benefits for producers. There is much potential and encouragement to implement this practice in crop rotation.

ROTATIONS & DISEASE

Continuing the notion that rotations may reduce costs, J. Reed Findlay presented a report at the University of Idaho 2003 conference that said "Crop rotations are becoming more popular for control of potato disease."**

Also, "Rotations are taking the place of some chemical control measures because of the significant risks and costs associated with fungicides and insecticides."

Findlay's report stated that "Potato diseases that can be controlled through some form of crop rotation include: verticillium wilf, root-knot nematode, common scab, powdery scab, phytophthora wilt, pythium, alternaria, southern bacterial wilt, fusarium and silver scurf."

For instance, in nematode control Findlay suggests "growing rapeseed (Brassica napis and Brassica rapa) prior to a potato crop reduced root knot nematodes 70 to 80 percent."

By tilling in grass clippings, streptomyces scabies (common scab) may be reduced while long rotations reduce inoculum of powdery scab. Also, Findlay says that green manures with high glucosintilates can also reduce scab.

Reducing rhizoctonia is possible by growing rapeseed prior to the potato crop.

REPLACING METHYL BROMIDE

When the announcement came that methyl bromide was going to be phased out, alternative applications have been a constant study at universities.

Increasing the need to try mustard greens, Andrew McGuire, a Lauzier Agricultural Systems Educator, Washington State University Extension, Grant-Adams Area, said that the mustards can successfully replace metam sodium in some cropping systems.

McGuire reported that they are trying to figure out what factors determine if the mustard gives equal control, or not at all. When they figure out those factors they will then need to determine threshold levels.

Also, they need to measure how and when those levels turn out to be, perhaps from a soil test. As more proof is generated, less risk will be associated with this process and growers will become more confident in using the mustard to replace metam sodium.

Jeff Miller at Miller Research LLC in Rupert, Idaho, has been doing research on early dying and fungal disease. Miller says, "Green manure products have proven to be very effective and economical in some circumstances."

Some of the earliest references to the use of green manures are from China in the 12th century B.C. Also, there is proof that colonists brought the practice to North America.

With the introduction of synthetic inorganic fertilizers, the use of green manures declined.

"Green manures," says Miller, "can actually act as biofumigants."

"They release the same active ingredient as metam sodium when they are plowed into the soil and when they decompose they release these products that kill the fungi. But green manures do much more than this. They can increase the percentage of decaying organic matter in the soil. This changes the composition of organisms in the soil. The late Dr. Jim Davis showed that incorporation of green manure crops reduced potato diseases without actually decreasing the disease-causing organisms," said Miller.

There is a looming concern with the possibility of wiping out all the "good guys" in the soil. Fumigants are not selective; they are a broad spectrum killer.

Green manures can reduce disease differently, without killing organisms in the soil.

What university scientists and researchers are shooting for is to stimulate a diverse micro-population to reverse disease.

Through many trials, tests and experiments, they can compile results and create improved methods to make green manures the most effective. Miller explains how eager they are to learn and work with growers. He admonishes growers to contact him or any of the University of Idaho specialists to set up trials and tests.

"However, we don't want growers to abandon what they have been doing. Call U of I specialists to set up experiments on small-acre basis first, we are more than happy to help set up trials and tests." The University of Idaho has a complete listing of specialists on their "Idaho Center for Potato Research & Education" website, www.ag.uidaho.edu/potato/ or call Miller at 208-531-5443.

Sources: *University of Idaho Report titled: The Value of Green Manures in Potato Cropping Systems, by Brian F. Finnigan. *University of Idaho Report titled: Rotations and Potato Disease Control in Idaho, by J. Reed Findlay.

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