- Cover Crops and Manure Help
- Rebuild and Improve Soils
- By Judith Nyiraneza and Sieg Snapp
Michigan State University
- Potatoes have shallow root systems and require substantial amounts of fertilizer, especially nitrogen (N). However, N applications need to be managed carefully because while deficiency will limit yield, excessive N can cause a number of problems. These include potential environmental impacts from leaching and reduced profits from lower quality tubers and increased vine-kill costs. If N levels are too high, growers may run into quality problems such as increased percentage of misshapen and non-marketable tubers, decreased specific gravity and reduced storage life.
A challenge in producing potatoes and managing N can be the persistent decline in soil organic matter. This is related to limited return of residues to the soil as well as the widespread use of sandy and irrigated ground that is subjected to intensive tillage. Applying organic sources such as manure and cover crops will help rebuild the soil and improve its structure. At the same time these organic sources provide a slow-release form of N that can optimize yield and tuber quality.
Organic sources build soil N reserves, but short-term availability of N from organic materials can be difficult to predict. Nitrogen in manure and residues needs to be mineralized to be available for crop uptake and the mineralization rate depends on environmental conditions, as well as the quality of the organic amendment.
Using models and soil monitoring we have found, under North Central and Upper Midwest climate conditions, that the quality of the manure or residues incorporated is the key factor determining how fast N becomes available. Environmental factors have a limited impact. A cool spring will tend to slow down mineralization, but will only reduce N availability by about a week compared to a warm spring. Quality of organic matter is related generally to the C/N ratio. A readily mineralizable plant material for example is one high in N content, frequently with immature tissues that are characterized by sugars and starch rather than rigid cell materials such as lignins and cellulose. High quality tissues with a low C/N ratio, in the range of 20 to 25 or below, will generally be a reliable source of N within about one month of incorporation.
Cover Crops
Using winter cover crops as a N trapping crop that builds soil organic reserves requires cover crops with specific characteristics. These include rapid germination, aggressive growth and extensive root systems. This is the case for many winter cereal and brassica crops. These cover crops improve the N supply to the following crop if 1) conditions allow cover crop growth, root exploration and N uptake during the fall, winter or early spring and, 2) if the mineralization rate of cover crop residue incorporated is synchronized with the timing of cash crop N demand. Time of planting is important and this will influence the biomass harvested. Nitrogen accumulated during wintertime was decreased by more than 50% when a series of cover crops were planted in September comparing to those planted in August. To choose the most suitable cover crop it is important to review its characteristics; i.e. cereal rye is an appropriate winter cover for USDA Plant Hardiness Zones 4 through 6, whereas hairy vetch is most productive in zone six or in zone five if sown early enough in the fall.
Characteristics of Rye Cover Crop:
• Germinates in cold temperature 34°F to 36°F but vegetative growth requires warm, wet weather.
• Has rapid growth, and produces larges amount of biomass.
• Flowers in long days and, when flowering starts vegetative growth stops.
• Extensive root systems; can cover one millimeter radius and two millimeter depth of soil.
• Tolerates a wide range of soil condition but does best in well-drained, light soils.
• Can have allelopathic effects on other grasses.
• Seed is inexpensive and available.
To optimize total amount of biomass produced and the quality of that biomass, a mixed cover crop can be grown such as a 1:1 or 2:1 mixture of a cereal rye and a legume. This allows delayed killing of the cover by a few weeks, as the legume will increase N availability and soil quality. Although management can be difficult in cool or wet spring, if a cover crop is allowed to grow until 12 inches high, it will allow a legume more time to fix N and a cereal more time to scavenge potentially leachable N.
The bottom line is that a winter cover crop does more than protect soil from erosion, it provides immediate yield advantages for the following cash crop. Our research suggests that even a six- inch high rye cover crop increased yields of a subsequent crop, compared to a bare winter fallow (both treatments were fertilized at recommended rates, which was 200 pounds N/acre after a bare follow and 190 pounds N/acre after a winter cover crop).
Manure
Animal manures have been used as fertilizers for a long time. Increases in soil organic matter and aggregation, or tilth, have been reported after single applications of compost or manure to potato rotations. Poultry manure is considered among the most desirable because of its high nutrient content, although its soil-building benefits are likely more limited than dairy or swine.
Application Strategies
An ideal approach to applying manure in a potato or vegetable rotation is to target manure to a rotation crop, such as corn, wheat or a forage. This will protect against any possible disease risk associated with some manures, such as common scab in potatoes. If this is not possible, consider carefully the trade-offs related to fall versus spring application of manure before a cash crop. On the one hand, fall application is suitable because the manure will have enough time to mineralize and it may be possible to match N released with crop demand in spring. However a fall application may be accompanied by losses over the winter.
Mixing Sources
The use of chemical fertilizers along with organic manures is probably the best way to optimize yield while minimizing environmental concerns. Our recent studies show that an aged poultry manure (2.5 tons/acre) or a winter cover crop (34 N lb) combined with fertilizer can increase potato yields, compared to crops grown with fertilizer alone.
The take-home message is that cover crop and manure are good management options that can be integrated into potato systems to revitalize soils. The benefits of these organic sources depend on quality, type and the quantity applied. The key to optimizing manure and fertilization strategies is to analyze organic amendments for N and organic carbon content. A list of manure testing laboratories is available from county MSU Extension offices, or by visiting www.maeap.org/cnmp.htm.
Additional information about manure and cover crops is found at www.kbs.msu.edu/extension/covercrops/home.htm.web2.msue.msu.edu/manure.
