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In Our Field: Irrigate judiciously to preserve healthy soil properties

August 23, 2001|By KHALED M. BALI, University of California Imperial County Cooperative Extension

When it is hot there is a tendency to irrigate more frequently and apply copious quantities of water to emerging crops. However, too much water can cause as much harm to the soil and the crop as too little water. Consider the following points:

Air/water spaces: Between soil particles are void spaces called "pores" that are occupied by air and water. These pores occupy about 50 percent of the soil's volume. If water is filling the pores, then most of the air is displaced. As the water is used by plants, evaporates or drains away, the air returns. If too much water is applied then the pore spaces remain filled with water. This creates an unfavorable condition for plant roots that need soil oxygen to survive. Root hairs start to die and become nonfunctional, restricting the uptake of water and nutrients to the foliage.

Leaching of nitrogen fertilizer: Water is needed to satisfy the photosynthetic needs of plants and to drain salts from the plant root zone. Excessive amounts of water applied increase drainage volume and thus increase the loss of water-soluble nitrate-nitrogen. With fertilizer prices being high, conservation of nitrogen for plant use should be practiced. You only need to apply enough water to meet crop water requirements (evapotranspiration) during the early stages of crop growth. Additional water for leaching should be applied at or toward the end of vegetable crop season. Additional water for leaching is not needed during the growing season (vegetable crops only). For field crops, particularly alfalfa, additional water for leaching may be needed during the year. Leaching is most effective at the end of the growing season after the removal of the crop, working the soil out and applying a leaching irrigation.

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Destruction of soil structure: The soil structure starts to disintegrate when water is excessively applied to a soil. Soil aggregates melt into a puddle-like condition and pore space disappears. It becomes difficult to move water and air through soil in this condition. In addition, crusting of the soil surface occurs, restricting the emergence of crop seedlings. To prevent this condition, sprinkler water must be applied at a rate lower than the water penetration rate of the soil. Droplet size also affects the formation of "seals" on bare soil surface and restricts the water movement into the soil. Small droplets cause less pressure on the soil surface than larger droplets. Therefore, small droplets have less impact on reducing water penetration rate than larger droplets.

Nozzle opening size and operating pressure have great impact on the droplet size. Make sure you run your sprinkler system at a pressure consistent with the recommendation of the manufacturer of the sprinklers. If water starts to stand on newly planted vegetable beds, turn off the sprinklers! A good irrigator can sequence sprinkler sets across the field to keep the water moving from set to set to prevent puddling. Use same-size sprinkler heads on all sprinklers. If you use different heads on the same set, you will have poor uniformity (too much water in certain areas and not enough water in other areas).

Soil toxins: Soil microbes break down organic matter within the soil. This is a beneficial process that recycles nutrients, improves soil porosity and supports microbial life. In saturated soils, the lack of soil oxygen kills off the aerobic organisms (the ones that use oxygen) and in their place populations of anaerobic organisms develop. The anaerobes do not need as much oxygen to survive and generally fulfill their needs by extracting chemical oxygen from products such as organic matter and fertilizers. However , the anaerobes produce poisonous by-products that injure plant roots. Some of the by-products mimic herbicides in their effect.

Soil compaction: Plant roots need oxygen and must respire constantly for optimum growth and production. Poor aeration, due to soil compaction and ponded water, can induce early wilting and produce numerous chemical products related to anaerobic conditions that are toxic to plants. Soil compaction occurs when soil is subjected to pressure. The application of pressure to any soil at any soil moisture level results in expulsion of air from the soil and gradual increase in density and water content. Dry soils usually resist compaction because of their stiff structure. However, in wet soils, the resistance to deformation is reduced and the soil becomes more susceptible to deformation and compaction. Avoiding cultural practices when the soil is wet can eliminate severe cases of soil compaction.

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