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Soil and Water Management
Rice Irrigation - Water Quality

While ample irrigation water is necessary for a productive rice crop, poor quality water can cause soil-related problems that negatively impact rice. Some of the predominant soil-related problems that affect rice include salinity (high soluble salts), zinc deficiency, phosphorus deficiency and excessive sodium, which causes poor physical soil conditions.

Salinity is most often associated with arid or semi-arid regions of the world, such as in the southwestern USA. However, salinity problems are common in the rice producing regions of Arkansas in some circumstances. The poor drainage characteristics of the soils in Arkansas that allow them to be efficient for rice production also contribute to the problems associated with salinity.

Salinity results from adding salt to soils, usually in irrigation water, faster than it is removed by natural processes, such as surface runoff and downward percolation. Irrigation water is the major contributor of soluble salts in Arkansas but excessive nutrient additions from fertilizers, manures or waste materials may also contribute to the accumulation of salts. The types of soluble salts that usually contribute to salinity problems include calcium, magnesium, sodium, chloride, sulfate and nitrate.

In addition to the effect on rice production, irrigation water that contains excessive levels of chloride can lead to chloride toxicity in soybeans. Rice is most sensitive to chloride and nitrate salts at the seedling growth stage. Sodium problems are usually native to particular soils such as the Foley, Lafe, Hillemann and Stuttgart soil mapping units. However, isolated cases of water containing excess sodium have been observed. Excessive sodium may cause poor physical conditions of the soils which can interfere with crop stand establishment.

Zinc and phosphorus deficiencies are usually associated with alkaline (high pH) soils, particularly on silt loam soils. Alkaline soils are created by irrigating with water that contains high concentrations of calcium and magnesium bicarbonate. When the water enters the field, the bicarbonates are converted to calcium and magnesium carbonate (lime) which are then deposited in the field. The soil pH increases in the field where the carbonates (lime) are deposited. A soil pH gradient is usually created such that the soil pH is high near the inlet and decreases down the slope. It is possible to develop both salinity and alkalinity problems in the same field.

Correct diagnosis of problems concerning irrigation water quality are critical for effective management. Water quality testing is an important step in diagnosing existing problems and identifying potential problems. Several values are helpful in evaluating the quality of a particular water source. These include calcium concentration, bicarbonate concentration, chloride concentration, electrical conductivity (EC) and sodium absorption ratio (SAR) (Table 7).

Table 7 provides a brief guide for evaluating water quality. The calcium and bicarbonate concentrations provide an estimate of the amount of lime that will be deposited and predictions can be made concerning the change in soil pH with long-term use. Electrical conductivity is a measure of the total salts that are dissolved in the water, which allows an estimate of the potential for salinity injury to rice with use of the water. Chloride concentration is important because of the potential for chloride toxicity to soybeans and because it often is the major contributor to high electrical conductivity. The SAR is a ratio of sodium to calcium and magnesium. This number provides an estimate of how much sodium is in the water relative to calcium and magnesium. The SAR allows the prediction of whether sodic (high sodium) soils are likely to develop with long-term use of the water.

The University of Arkansas conducts water quality testing for a small fee that includes a computer prediction of any long-term effects that may result from using the irrigation water. This analysis includes effects of various crop rotations, soil texture, and water management alternatives. Once a water source has been tested, retesting is usually not necessary for at least five years. However, earlier retesting may be necessary when crop problems develop that may be related to water quality or when the pumping rate or depth changes significantly.

For more information on management of saline or alkaline soils, refer to Section 8, "Efficient Use of Fertilizer," or to University of Arkansas Soil Test Note No. ST003, Management of Soils with High Soluble Salts.

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Last Date Modified 10/22/2009
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