Water Quality for Irrigating Vineyards
Irrigating with water of higher salinity than grapevines can be detrimental to vine growth, affect yields, and ultimately threaten the plant’s survival. The first of sign of salinity is usually stunted growth, with plant leaves often having a bluish-green color. As salt levels in the soil increase to more toxic levels, scalding or burning on the tip and edges of the older leaves occurs. The leaf dies and falls off and finally, the plant dies. In other cases, the younger leaves may appear yellow, or the vines may show signs of wilting, even though the soil appears adequately moist. Saline irrigation water can affect grapevine growth in two ways: salinity effect and toxicity effect.
The most serious problem caused by water salinity is the decreased osmotic potential of soil water. Plants absorb water from the soil through their roots. A key process that allows the roots to do this is osmosis. Dissolved substances within the roots such as salts and sugars attract water through the root membrane, from where it moves to the rest of the plant.
Excessive concentrations of sodium and chloride ions in irrigation water can cause can cause ions to accumulate in the woody tissue and eventually in the leaves (See Figure 15.1). These ions can be taken up either by the roots or by direct contact on the leaves.
Factors Affecting Grapevine Damage
The extent of plant yield loss when irrigated with saline water depends on a number of factors. In well drained sandy soils, irrigation water can readily flush salts out of the root zone but this is less successful on poorly drained, heavy clay soils. Salt concentration in the root zone continually changes following irrigation. As the soil dries, the salt concentration in the soil solution increases and this reduces the moisture available to the plant. Frequent, light irrigations increase salt concentrations in the topsoil and should be avoided.
Soluble Cations and Anions
One critical factor in water quality is the concentrations of dissolved salts such as (sodium chloride, NaCl), calcium chloride (CaCl2), calcium sulfate (CaSO4), magnesium sulfate, (MgSO4), and sodium bicarbonate (NaHCO3). Salts dissolve in water and form positive ions (cations) and negative ions (anions).
Measuring and Classifying the Salinity Hazard
There are two common water quality measurements that characterize the salinity of irrigation water. The salinity of irrigation water is sometimes reported as total dissolved solids (TDS) but more often as electrical conductivity (ECw). TDS is not useful in evaluating agricultural salinity because crop tolerance thresholds are correlated with ECw and ECe rather than with TDS. Subscripts are used with the symbol EC to identify the source of the sample. ECw is for irrigation water and ECe for soil. The higher the recorded values for TDS or ECw, the higher the salinity hazard of the water.
Managing Irrigation Water High in Salts
Managing irrigation water high in salts requires constant attention. Practices which aid in remedying salinity problems are:
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