Water Quality for Irrigating Vineyards
Sodium Hazard
While ECw is an assessment of all soluble salts, sodium hazard is defined separately because of sodium’s specific detrimental effects on soil physical properties—structure and permeability—in a vineyard. Damage arises as sodium displaces calcium and magnesium from soil colloids, causing them to disperse and lose structure. As soil structure deteriorates soil compaction or tightness will increase and water infiltration, water percolation, and root growth are all decreased. Fine textured soils, especially those high in clay, are more likely subject to this action than sandier soils. Sodium can also accumulate in the soil, in sufficiently large amounts, such that grapevine uptake of sodium becomes toxic to the vine.
Measuring and Classifying the Sodium Hazard
The sodium hazard is typically expressed as the sodium adsorption ratio (SAR). The sodium adsorption ratio relates the concentration of sodium to the concentration of calcium and magnesium (See equation below).
Water Salinity
The SAR is not enough by itself to predict the impact of irrigation water on soil. Other components of a water analysis also affect sodium hazard (Table 15.4). Because both SAR and EC affect water infiltration, both must be considered in estimating water infiltration hazard for the vineyard.
Adjusted Sodium Adsorption Ratio
The adjusted sodium adsorption ratio (sometimes symbolized as adj SAR) is a modification of the original SAR calculation. It serves the same purposes, but is modified to include the effects of bicarbonates and carbonates, in addition to calcium and magnesium.
Residual Sodium Carbonate
Another predictor of sodium hazard is the residual sodium carbonate (RSC) of water. RSC is the sum of carbonates (CO3 2¯) and bicarbonates (HCO3¯) ions minus the sum of the calcium (Ca2+) and magnesium (Mg2+) ions. The negative effects of bicarbonate and carbonate are negated by high levels of calcium and magnesium ions.
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