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March 24, 2012

Restoring Your Soil.

Yard and Garden
March 24, 2012

Q.

We have a 12” x 8” plot that has been used to grow tomatoes. Each year our crop has seriously dwindled. Is there something we can do to restore our soil?

Dale C.
Rio Rancho

A.

The decline of a crop in the garden is often due to growing the same crop in the same soil for several years. This may result in the buildup of plant diseases targeting the crop grown repeatedly or to the depletion of nutrients that the crop needs.

Diseases may be obvious or subtle in the expression of their presence. If there is a disease, it often displays itself as obvious symptoms exhibited by the plants. Fungal and some bacterial diseases my express themselves as foliar symptoms. Fungal diseases are more common in New Mexico and result in necrotic (dead) spots in the leaves, or yellowing, browning, then dying of the leaves. You may also notice a constriction and discoloration of the stem near the soil line. There may be vascular diseases that block movement of nutrients and water in the vasculature of the plant stems. This is then made evident by wilting and dying back of the leaves. If the plant is infected late in the season, the result may be decline of the plant and poor yield, but some crop produced. The longer a crop is grown on the same soil, the more the disease organism accumulates and the earlier the plant is affected. Eventually, there may be no harvest.

Each crop has certain nutrients that it needs in greater quantities than other crops. This depletes the soil of the specific nutrient needed in greater amounts by that crop. The longer a crop is grown on the same soil, the greater the depletion of the nutrient and the poorer the plant growth and yield. Soil tests allow determination of the availability of nutrients in a soil and targeted nutrient replacement.

In both cases, nutrient depletion and disease accumulation, crop rotation reduces the problem. When rotating crops, it is important to choose a crop that has distinctly different nutrient requirements and diseases than the preceding crop. Tomatoes and fruit producing crops need higher levels of phosphorous. Corn and some crops utilize more nitrogen. Root crops often need more potassium. Closely related crops, such as tomatoes and chiles, can harbor the same diseases, so they are not good rotational crops. Beans and legumes are good plants for crop rotation because, although they do have some potential disease problems, they also supply nitrogen to the soil through a symbiotic relationship with a bacterium that lives in nodules on the roots of legume plants. For this to work to best effect, the soil should be inoculated with the appropriate rhizobium bacteria before the legume crop is planted. The crop rotated to that location the following year benefits from the nitrogen added by the legume crop.

In a small garden like yours, crop rotation is difficult, but still important. You can replace depleted nutrients, but accumulation of diseases will still occur.

For more gardening information, visit the NMSU Extension publications Web site at http://aces.nmsu.edu/pubs/_h, or to read past articles of Yard and Garden go to http://aces.nmsu.edu/pubs/periodicals.html.

Send your gardening questions to :

Yard and Garden, Attn: Dr. Curtis Smith
NMSU Agricultural Science Center
1036 Miller Rd.
SW, Los Lunas, NM 87031.

Curtis W. Smith, Ph.D., is an Extension Horticulture Specialist emeritus with New Mexico State University's Cooperative Extension Service. NMSU and the U.S. Department of Agriculture cooperating.