GardenLine | Porpourri - Miscellaneous | Common Sense Gardening Part 1: Crop Rotation and Green Manures

Williams was a graduate student with the Department of Horticulture Science. This column is provided as an extension service by the Division of Extension and Community Relations and the Department of Horticulture Science, University of Saskatchewan.

Some people call it natural or organic gardening; a more trendy term for it is "sustainable agriculture." It might more simply be called "common sense gardening. It involves returning to the soil what is taken from to restore its structure and nutritional content. This can be achieved, for example, by adding organic matter, often referred to as "green manuring." Common sense gardening also involves avoiding or reducing the use of toxic or persistent pesticides by such approaches as: the use of resistant varieties of plants; mechanical barriers and other cultural methods of insect and disease control; and crop rotation to reduce the prevalence of insects and diseases.

If you practice common sense gardening, you may have to tolerate some imperfection in your produce - a flea beetle bite here, a scab mark there. Since the home gardener does not have to meet government grading standards or make a profit, a pinch of imperfection is a small price to pay for a reduction in pesticide use.

Crop Rotation

Crop rotation is a regular schedule of planting in which different crops are planted in different parts of the garden each year. Put more simply, the rule of thumb might state: "Don't plant the same veggies in the same spot two years running."

What are the benefits of crop rotation? Rotating crops will prevent the continued depletion of certain nutrients in particular areas of the garden which would occur if the same crops were planted there year after year.

Vegetables differ in their nutrient or fertilizer requirements. Corn and members of the cabbage family (cabbage, cauliflower, broccoli, Brussels sprouts) require a great deal of nitrogen. If these are planted in the same area of the garden year after year, they will soon use up the available nitrogen. By grouping together crops which have similar nutritional needs and planting them in a different area of the garden each year, you allow the the soil to replenish lost minerals.

Plants also differ in the extent and distribution of their root systems. Some are shallow rooted (lettuce and radishes), while others (potatoes, tomatoes, parsnips, carrots, turnips, and beets) are deeper rooted. Those with shallow roots will absorb nutrients from the upper layers of the soil; those with more extensive root systems will absorb minerals from the lower depths of the soil. These types of plants should also be rotated.

If some vegetables are "depleters" and use a lot of minerals, others are "replenishers". Peas and beans are members of the legume family. Due to a unique relationship with certain soil bacteria called rhizobium, legumes are able to "fix" nitrogen from the air, utilize it for their own growth, and still produce some in excess for the crops to follow. If your garden is new or if legume crops have never before been grown there, it may be necessary to "inoculate" or introduce the rhizobium. Inoculants are available commercially in granular form and are usually sprinkled in the furrow along with the seed as it is being sowed. Once peas or beans have been grown in a garden, the rhizobium will be present naturally in the soil and it will no longer be necessary to apply the granular inoculum.

Green Manures

If your space permits, green manures can also be part of the crop rotation. Green manures are crops such as fall rye, oats, alfalfa, peas, lentils, wheat, millet, or buckwheat which are seeded but then plowed into the garden while still green rather than harvested. (Alfalfa is a deep-rooted perennial and will take considerable more effort to plow under than the other crops.) Plowing should be done prior to the crop setting seeds, or weed problems will result. Green manure crops improve soil fertility and structure. Buckwheat and alfalfa have vigorous and far-reaching root systems which penetrate the soil deeply. and take up nutrients. These nutrients become part of their plant body and are subsequently released into the soil for the next crop (your veggie garden) upon decay of the cover crop.

Under Saskatchewan conditions, the use of green manures usually means taking a section of the garden out of production for a season, or alternating between two garden areas. Our growing season is simply not long enough to permit the growth of a green manure in the fall or spring, and then have six weeks of warm weather for the cover crop to decay prior to planting the garden.

Green manure crops may be left to stand for an entire year. In this way they may act as cover crops, securing the soil against erosion and penetrating into the soil to bring up nutrients which might be inaccessible to most vegetables.

Fall rye should be seeded from Sept. 1 to 20 (at the rate of 2-3 lbs/1000 sq. ft.) It will germinate in the fall, remain dormant over winter, and resume growth early the following spring. It should be plowed down a few weeks prior to seeding the garden. At that time it may be necessary to add some nitrogen fertilizer.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.