USING GREEN MANURES IN POTATO CROPPING SYSTEMS

ByAndrew McGuire, Irrigated Cropping Systems Agronomist, Washington State University Extension. FS218E

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Using Green Manures in Potato Cropping Systems

Why Use Green Manures?

A green manure is a crop that is grown and then incorporated into the soil while still green. This practice was widely used to improve soils and provide nutrients to crops before synthetic fertilizers became available. Recently, innovative farmers have been giving this old technology a new look with mustard green manures (Figure 1). Washington potato producers are using green manures to produce better crops by improving the quality of their soils. In contrast to the low­input, low­management green manures of the past, mustard green manures require fertilizer, irrigation, and intensive management. They require a current understanding of soil ecology, soilborne pests, plant biochemistry, and breeding and screening techniques. And unlike synthetic fertilizers, they can improve the soil’s physical, chemical, and biological qualities (McGuire 2003).

Figure 1. A mustard green manure crop being chopped and disked in the fall before spring potato planting.

Soil physical characteristics, such as tilth, water infiltration rate, water holding capacity, and aeration, are generally improved by the addition of organic matter to the soil whether by manure, green manure, compost, or crop residues. This can lead to the growth of larger, healthier root systems, which help plants better handle stress (Magdoff and van Es 2010).

The biological characteristics of a soil, such as microbial biomass, biological activity, and biodiversity, can also be improved through green manures. These changes in the soil’s biology provide the short-term economic incentive to use green manure crops in potato cropping systems, especially for soilborne pest management (McGuire 2012).

Fungal and bacterial diseases, and parasitic nematodes have all been reduced by using green manure crops (Wiggins and Kinkel 2005; Larkin and Griffin 2007; Fourie et al. 2016).

The chemical properties of a soil can be improved by increasing nutrient and organic matter levels. This, too, comes from organic amendments to the soil.

When used in certain cropping systems, green manure crops have been able to replace expensive fumigants (McGuire 2003). However, the degree and duration of these beneficial effects depend on many factors, such as soil texture, temperature and moisture, plant age and species, climate, tillage practices, pest species and levels, and crop rotation (Larkin et al. 2011). Therefore, the benefits of green manures may differ between systems and between fields. Refer to Mustard Cover Cropping in Potatoes in the Other Resource section for case study information.

How Green Manures Help Manage Pests

The effects of green manures on soilborne pests are the result of several interacting mechanisms. These mechanisms take place in the complex environment of the soil where it is difficult to measure specific biological processes. It is not yet possible to say which mechanism is most important or how each works in conjunction with the other—we can only deduce which mechanisms may be at work. Still, it is beneficial to review these mechanisms and the strategies you can use to enhance their effects in your system.

Crop Rotation

Before advances in soil microbiology, many green manure and cover crop effects were combined under crop rotation. Crop rotation reduces pest problems by changing the environmental and biological conditions in the field (Peters et al. 2003). Each pest has a set of conditions it prefers. If pests are allowed to have their favored set of conditions for too long, they multiply rapidly and give us problems.

In general, rotating crops with different planting dates (spring vs. fall), different growing habits (annual vs. perennial; tall vs. short; fibrous vs. tap roots) or different susceptibility to pests (grasses vs. broadleaves) prevents any one pest from becoming a problem.

Strategy. Rotate crops that are as different from one another as possible, and usually, the longer the rotation, the better the pest control. With green manures, grow a crop that is not a host to the pests that affect your main crops.

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Biological Disease Suppression

A second mechanism of some green manure crops has been termed biological disease suppression. This is a mechanism put forward by scientists to account for the observed suppression of a soilborne disease even though the disease-causing organism is still present in the soil at potentially hazardous levels (Garbeva et al. 2011).

With Verticillium dahliae, researchers have observed that when certain green manures (barley, mustard, rapeseed, sudangrass, and sweet corn) are incorporated with a chisel plow or disk before planting a potato crop, the level of infection by Verticillium is low, even with high levels of the fungus still in the field (Davis et al. 2010; Ochiai et al. 2008).

The green manure serves as an energy source for beneficial microorganisms. It is suspected that these organisms out-compete Verticillium for energy and subsequently increase in number. Then, after the potato crop is planted, they possibly exclude Verticillium from the area along the potato roots, called the rhizosphere. This is the only place where Verticillium can infect potato plants.

There are probably other mechanisms at work resulting from a green manure crop. These may include predation, parasitism, and the interference of chemical signals between pathogens and plants. However, little is known about the specifics of these mechanisms and how they might affect control of soilborne pests.

Strategy. Manage your green manure crop to produce maximum biomass. The more biomass produced, the better, as long as it does not affect your ability to establish the following cash crop. This is where fall incorporation can be helpful; large amounts of biomass will have longer time to break down before you need to plant your potato crop. However, in some conditions, fall incorporation may also increase wind erosion and winter leaching of nitrate. Choose the most productive crop for your available growing window.

For this mechanism, dried crop residues do not work as well as fresh plants. Therefore, incorporate the crop while it is still green (Davis et al. 1996).

Biofumigation

This term was coined to describe the effects of Brassica rotation crops or green manures on soilborne pests (Matthiessen and Kirkegaard 2006).

Brassica crops such as rapeseed and mustard contain biologically active chemicals, called glucosinolates. In the soil, certain glucosinolates in the roots of rotation crops, or in the roots, stems, and leaves of green manures, break down into isothiocyanates (ITCs) and other chemicals (Figure 2). ITCs are known to kill or suppress some soilborne diseases, nematodes, and weed seeds. There are many types of glucosinolates, some of which produce different types of ITCs, which vary in their toxicity to different pests. Methyl ITC is the active chemical produced when metam sodium, a common synthetic fumigant, is applied to the soil; hence the name biofumigation when ITCs are produced by plants.

Figure 2. Biofumigation reaction in Brassica crops.

In the laboratory, these chemicals have suppressed growth of silver scurf, white rot, powdery scab, and pink rot (Manici et al. 1999). Field tests have shown mixed results and research is ongoing.

Sorghum-sudangrass and sudangrass green manures have been shown to be effective against root-knot nematodes by a very similar mechanism (Widmer and Abawi 2002). These crops produce dhurrin that, like glucosinolates, produce a toxic compound when tilled into the soil; in this case, hydrogen cyanide (HCN).

Strategy. First, select species and varieties that produce large amounts of biomass with a high concentration of glucosinolates. Generally, the concentration of glucosinolates peaks just before flowering. However, biomass continues to increase until the plants begin to dry. The time of incorporation for maximum biofumigation is not yet known, however, to prevent the green manure from becoming a weed in the following crop, incorporate before viable seed production.

Second, to produce ITCs, the glucosinolates must be exposed to specific enzymes, which are normally separated from the glucosinolates in the plants. This is also true for producing HCN from dhurrin in sudangrass.

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WSU EXTENSION | USING GREEN MANURES IN POTATO CROPPING SYSTEMS

Figure 3. Green manure crop that has been incorporated into the soil.

Current practice is to chop the green manure before incorporating to ensure that this mixing occurs (Figure 3). A high-speed flail chopper, such as those used by some grass seed and asparagus growers, may be the best implement for this. See Mustard Green Manures (McGuire 2016) for more information on green manure incorporation.

Finally, ITC production is greater in wet soils than in dry soils, so if possible, irrigate following incorporation.

Systemic Acquired Resistance

This mechanism, related to biological disease suppression, occurs when organisms are stimulated by the addition of organic matter to the soil. Some of these organisms then secrete chemicals that come in contact with plant roots and activate the plant’s natural defense systems. (Navarre et al. 2002)

Strategy. Although many of the details of this mechanism are known in other situations, we do not know how important it is with the use of mustard green manures. Therefore, there is no clear strategy for enhancement. Incorporating a green manure crop may or may not provide the active organic matter in the soil that this mechanism depends upon. Research is ongoing.

Factors to Consider

Goals for a Green Manure

Before you add a green manure crop to your rotation, you should decide what you want to achieve. It may be that you want to control a certain nematode, a disease, a problem weed, or just improve the soil’s tilth. Once you have decided on a primary goal, then your management decisions should be made to maximize the effects that take you towards that goal. Often you may have secondary goals, but because there are different ways to manage green manure crops, you should always have your primary goal in mind.

Your Field’s Combination of Pests

Goal setting should consider the unique mixture of crops, soils, and pests on your farm or even in each field you manage. Because certain green manures will be better in certain situations, you should investigate the advantages and disadvantages of each type of green manure crop and try those that seem to best fit your situation.

Green Manure Management

Success or failure in reaping benefits from a green manure crop can be a matter of management details. Factors such as planting method, planting date, seeding rate, fertilization, weed control, and incorporation method and timing can all help or hinder your goals. Before you buy any seed, make sure you have all the information available on the green manure you have chosen. See the resources at the end of this publication.

Cost

Green manure crops, in certain situations, can be costly. However, green manures produced by using the cheapest seed, residual soil nutrients, and minimal irrigation will not normally produce satisfactory results, especially in terms of pest management.

When looking at the cost of green manures and comparing them with the benefits, it is important to consider all the benefits. While it is relatively easy to figure the benefits if you can reduce or eliminate pesticide applications, it is far more difficult to estimate the benefits of improving soil physical characteristics, or other longer-term benefits.

Also, remember that some of the money you spend on a green manure crop would also have been spent if you did not grow the green manure. This is the case with fertilizer applied to green manures as much of it will be recycled into following crops, thus reducing the amount of fertilizer needed.

Management and Labor Requirements

When considering a green manure, take into account the time and labor required to obtain good results. Will you have these resources available when they are needed? Do you have other activities at that time of year that may keep you from paying due attention to the green manure crop? You may have to supply additional machinery or labor if you want to successfully produce a green manure crop.

Short-term vs. Long-term Benefits

While many of the pest management benefits may come soon after producing a green manure crop, other benefits will not be evident for years.

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Regular use of green manure crops can give you benefits that increase every year. One the other hand, if you only grow a green manure crop every four or five years, you may find that the short-term benefits are all you see.

In the short term, adding a cover crop to a cropping system will impact nitrogen dynamics. The nitrogen released from the crop residues will affect the timing and amount of fertilization needed. Additional soil and tissue sampling may be needed to optimize this nitrogen.

Other Cropping System Factors

You can affect the success of your green manure cropping through other management decisions. In general, soil quality will improve or be maintained if you:

Minimize tillageAvoid overwateringAvoid soil compactionPrevent wind and water erosion

Changing System Components

Remember that you are working within a system and changing the components of a system may change the outcomes.

For instance, one successful cropping system has a mustard green manure crop following wheat. (Refer to Mustard Cover Cropping in Potatoes in the Other Resource section for case study information.). In this system the mustard is planted without disturbing the wheat straw (Figure 4). In late October, both the mustard and wheat straw are tilled into the soil together.

You may want to change this system by tilling the wheat straw before planting the mustard. This may seem like an insignificant change. However, it may be that the incorporation of the wheat straw with the mustard is important in the success of this system. By incorporating the wheat straw, you can increase the weed pressure, increase the nitrogen requirements of growing the mustard, or increase the risk of nitrate leaching and wind erosion in the following winter and spring.

This should not prevent you from developing your own system, but be aware of the possible complications when changing system components.

Figure 4. Green manure crop growing in standing wheat straw.

Getting Started with Green Manures

When done correctly, on-farm testing is a good way to evaluate green manures. However, a single side-by-side comparison, although easy to conduct, will not tell you if your observations were the result of the practices you were comparing or the result of other varying conditions. For best results:

Start with a small part of a larger fieldLeave areas that are managed as normalUse replication and randomization

Call your local Extension office for help and resources to conduct on-farm tests.

Other Resources

McGuire, A. 2016. Mustard Green Manures. Washington State University Extension Publication FS219E. Washington State University.

Mustard Cover Cropping in Potatoes. REACCH Case Studies, Dale Gies System profile.

On-farm research results and other information on mustard green manures. WSU Center for Sustaining Agriculture and Natural Resources.

This is a revision of EB1951E, written by Andrew McGuire and published by WSU Extension in 2003. Portions of this publication have been included in Collins et al. (2006).

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References

Collins, H.P., A. Alva, R.A. Boydston, R.L. Cochran, P.B. Hamm, A. McGuire, and E. Riga. 2006. Soil Microbial, Fungal, and Nematode Responses to Soil Fumigation and Cover Crops under Potato Production. Biology and Fertility of Soils 42 (3): 247–57.

Davis, James R., O.C. Huisman, Dale O. Everson, P. Nolte, L.H. Sorensen, and A.T. Schneider. 2010. Ecological Relationships of Verticillium Wilt Suppression of Potato by Green Manures. American Journal of Potato Research 87 (4): 315–26.

Davis, J.R., O.C. Huisman, D.T. Westermann, S.L. Hafez, D.O. Everson, L.H. Sorensen, and A.T. Schneider. 1996. Effects of Green Manures on Verticillium Wilt of Potato. Phytopathology 86 (5): 444–53.

Fourie, H., P. Ahuja, J. Lammers, and M. Daneel. 2016. Brassicacea-Based Management Strategies as an Alternative to Combat Nematode Pests: A Synopsis. Crop Protection 80 (February): 21–41.

Garbeva, P., W.H. Gera Hol, A.J. Termorshuizen, G.A. Kowalchuk, and W. de Boer. 2011. Fungistasis and General Soil Biostasis – A New Synthesis. Soil Biology and Biochemistry 43 (3): 469–77.

Larkin, R.P., and T.S. Griffin. 2007. Control of Soilborne Potato Diseases Using Brassica Green Manures. Crop Protection 26 (7): 1067–77.

Larkin, R.P., C.W. Honeycutt, and O.M. Olanya. 2011. Management of Verticillium Wilt of Potato with Disease-Suppressive Green Manures and as Affected by Previous Cropping History. Plant Disease 95 (5): 568–76.

Magdoff, F., and H. van Es. 2010. Building Soils for Better Crops. J. Mason, ed. 3rd ed. Beltsville, MD: SARE Outreach.

Manici, L.M., O. Leoni, L. Lazzeri, S. Galletti, and S. Palmieri. 1999. Fungitoxic Activity of Four Thio-Functionalised Glucosinolate Enzyme-Derived Products on Ten Soil-Borne Pathogens. Pesticide Science 55 (4): 486–88.

Matthiessen, J.N., and J.A. Kirkegaard. 2006. Biofumigation and Enhanced Biodegradation: Opportunity and Challenge in Soilborne Pest and Disease Management. Critical Reviews in Plant Sciences 25: 235–65.

McGuire, A.M. 2003. Mustard Green Manures Replace Fumigant and Improve Infiltration in Potato Cropping System. Crop Management.

McGuire, A.M. 2012. Mustard Green Manure Use in Eastern Washington State. In Sustainable Potato Production: Global Case Studies. Z. He, R. Larkin, and W. Honeycutt, eds. 117–30. Springer Netherlands.

Navarre, D.J., P. Thomas, C. Brown, and P. Kachroo. 2002. Systemic Acquired Resistance in Potato. In XXVI International Horticultural Congress: Potatoes, Healthy Food for Humanity: International Developments in Breeding, 619, 177–81.

Ochiai, N., M.L. Powelson, F.J. Crowe, and R.P. Dick. 2008. Green Manure Effects on Soil Quality in Relation to Suppression of Verticillium Wilt of Potatoes. Biology and Fertility of Soils 44 (8): 1013–23.

Peters, R.D., A.V. Sturz, M.R. Carter, and J.B. Sanderson. 2003. Developing Disease-Suppressive Soils through Crop Rotation and Tillage Management Practices. Soil and Tillage Research 72 (2): 181–92.

Widmer, T.L., and G.S. Abawi. 2002. Relationship between Levels of Cyanide in Sudangrass Hybrids Incorporated into Soil and Suppression of Meloidogyne Hapla. Journal of Nematology 34 (1): 16–22.

Wiggins, B.E., and L.L. Kinkel. 2005. Green Manures and Crop Sequences Influence Alfalfa Root Rot and Pathogen Inhibitory Activity among Soil-Borne Streptomycetes. Plant and Soil 268 (1–2): 271–83.

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Use pesticides with care. Apply them only to plants, animals, or sites as listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. It is a violation of the law to disregard label directions. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock.

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