Chapter 15

Water Quality for Irrigating Vineyards

Alkalinity

Alkalinity is a measure of the total carbonates, bicarbonates and hydroxyl ions. Alkalinity increases as the amount of dissolved carbonates and bicarbonates rises. The dissolved carbonates and bicarbonates increase the media pH over time by neutralizing H? ions in the media solution. Irrigation water containing high levels of bicarbonates and carbonates can worsen the soil sodium hazard by causing calcium and magnesium to precipitate, thereby becoming unavailable to counteract the negative effects of sodium.

Hardness

The term hardness refers to the combined concentration of Ca and Mg in the water and is expressed as the total amount of calcium carbonate in milligrams per liter of water (mg/L). Other cations such as iron, manganese, aluminum and zinc can also contribute to hardness.

Measuring and Classifying Alkalinity

There are two measurements used for assessing the carbonate level of irrigation water: 1) direct measurement of carbonates and bicarbonates and 2) the residual sodium carbonate equation.

Total Carbonates

Since bicarbonates and carbonates are the major components of water alkalinity, most laboratories assume that Total Carbonates (TC = carbonates + bicarbonates) equals alkalinity. Among these components, bicarbonates are normally the most concern to the grower. At pH values between about 6.5 and 10, bicarbonates dominate.

Residual Sodium Carbonate

The residual sodium carbonate (RSC) equation compares the concentration of carbonates and bicarbonates to the concentration of calcium and magnesium ions. The negative effects of bicarbonate and carbonate are negated by high levels of calcium and magnesium ions.

Managing Irrigation Water High in Carbonates

Acidification and adding gypsum are two ways to decrease the amount of bicarbonates (and carbonates) in water. Adding sulfur products only makes sense when a soil is sodic and has free lime present or when a soil is basic (high pH).

Acidification

The common acids used for reduction of alkalinity in irrigation water are: phosphoric (75 and 85%), sulfuric (35 and 93%), and nitric (61.4 and 67%). Phosphoric acid is the safest and nitric the least safe of the acids. The most commonly used is sulfuric acid. It is inexpensive, moderately safe and provides sulfur for grapevine growth.

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