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Crop residues potential for damaging plants

January 10, 2002

In agricultural fields, last season's crop residue can result in poor stands, inconsistent plant sizes or yellow to light green foliage in recently sown crops.

As the plant material left in the soil from last season's crop decomposes, it releases many chemicals that are toxic to growing plants. If a crop is planted in a field before residue decomposition is complete, the crop may be damaged. The effect of toxic substances produced by one plant on another plant is called allelopathy.

Roots of affected plants will be discolored. The number and length of the roots also will be reduced. If the crop residue is toxic, roots will be discolored where there is contact with crop residue.

Distribution of symptoms within an affected field may be consistent with patterns associated with soil movement by farming equipment or may have a random distribution throughout the field. The condition will be more severe in lighter-textured soils, so symptoms may be more pronounced in areas with a sandier soil within a field.

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Sudan grass and corn residues are particularly damaging, however residues of other crops can also release toxic compounds.

Crops susceptible to this disorder include onions, lettuce, sugar beets, corn, carrots and tomatoes. However, crops like broccoli, cabbage and cauliflower don't appear to be affected.

Injury from crop residue can be avoided by allowing sufficient time for residue decomposition. Fine chopping of the residue coupled with high temperature and good moisture levels increases the rate of residue decomposition. The amount of time that a crop residue and associated toxins will remain in the soil will depend upon temperature, moisture, the size of the residue fragments and the type and quantity of microorganisms present. In most cases, the crop will break down in 30 days if the soil is moist and well aerated.

Growers are motivated by economics to quickly replace one crop with another to capitalize on an early market. To conserve water, they irrigate no more than absolutely necessary. Many times, even given the quick replacement of one crop with another, the crop escapes damage. However, when a large amount of organic material in the soil and when soil conditions do not favor decomposition, a quick turnover between crops can cause substantial damage to a susceptible crop.

>> Thomas Turini is the plant pathology farm adviser at the University of California-Imperial County Cooperative Extension.

>>The Cooperative Extension serves all residents of the Imperial Valley.

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