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).
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.
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.