The most important step in organic potato production is planning a crop rotation scheme that allows a few years between potato crops on the same land. For organic production, a lengthy rotation from four to seven years generally assures good plant and soil health. A lengthy rotation also reduces long-term reliance on expensive inputs and increases the percentage of marketable potatoes.
Longer rotations can be thought of as a form of crop insurance because the rotations help prevent plant pathogens in the soil from building up to economically damaging levels. Growers must consider rotation plans with crops that are not hosts for potato pathogens or insects. The key consideration for the long-term viability of organic production is preventing problems through maintaining good soil quality.
Rotations that include cover crops have the advantage of adding organic matter and nitrogen to the soil. This generally will reduce input costs over time. Organic matter helps soils resist compact ion, al lows for better root penetration, stores more soil moisture and allows more water penetration. Cover crops and green manures may include legumes, sudangrass, and mustards.
Mustards also have been shown to play a positive role in soil pest management.
Useful characteristics for a cover crop or green manure in a potato rotation include:
• The ability to tolerate frost and grow well under cool fall conditions
• The ability to quickly produce substantial amounts of biomass as a weed suppressant
• The ability to fix nitrogen and suppress soilborne potato pests
• A compatibility with the management requirements of other crops in the rotation
• The availability of seed and a lack of planting restrictions, such as the restriction of rapeseed production in canola districts
• The ability to avoid producing and shedding seed, which leads to problems with volunteer plants.
A good rotation includes crops that are not hosts to common potato pests, as well as green manures that add nutrients and organic matter to the soil.
The ATTRA publications Overview of Cover Crops and Green Manures and Intercropping Principles and Production Practices provide more detailed information about these subjects. Small grains, corn and sorghum sudangrass may benefit a potato crop that follows. In Maine, some growers have used Japanese millet as a cover crop in the year prior to potatoes in an effort to reduce Rhizoctonia. The skin of potatoes with the Rhizoctonia fungus appears to be covered in dirt that won’t wash off. In parts of the West, producers rotate potatoes with mustard cover crops to prevent root knot nematode and Verticilium outbreaks.
It is important to note that legumes such as peas, beans, and crimson clover are hosts to some races of Rhizoctonia and can encourage scab in certain regions. Red clover may be a host of Rhizoctonia as well.
As often happens in agriculture, there is no clear-cut answer to the question of what rotation a farmer should use. It is a matter of evaluating the costs and benefits of a particular practice or combination of practices. In this case, producers must weigh the risk of these crops hosting and possibly increasing Rhizoctonia against the soil fertility advantages and other benefits of planting a legume.
The ATTRA publication Potatoes: Organic Production and Marketing includes a table that provides some examples of potato rotations used around the country. It is not meant to be exhaustive, simply because there are so many factors that influence the choice of rotations, including economics of the crops in the rotation, available land, weather and climate, farmer skills and knowledge, pest management and soil quality goals.
Since many of these factors are moving targets, implementing a good crop rotation is as much an art as a science because so much depends on the knowledge, skill and creativity of the farmer. When making rotation decisions, it is helpful to have additional information from local experts — be they farmers, extension agents or researchers — who know about the pest pressures and soil and climate considerations for your particular region.
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