Management of Vineyard Soils
Chemical Soil Properties
The grapevine root obtains essential nutrients almost entirely by uptake from the soil solution. Soil chemical properties, therefore, reflect the influence of the soil minerals and organic materials in the soil solution. The chemical interactions that occur in soil are highly complex, but understanding certain basic concepts will better help the grape grower in managing the vineyard.
Soil pH
Soil pH is the foundation of essentially all soil chemistry and nutrient reaction and should be the first consideration when evaluating a soil test. Soil pH refers to the acidity or alkalinity of the soil. It is a measure of the concentration of free hydrogen ions and hydroxide ions that are in the soil. The total range of the pH scale is from 0 to 14.
Buffer pH
In addition to soil pH, many soil tests provide a reading called buffer pH (sometimes called lime index). Soil pH is a measure of hydrogen ion (H⁺) concentration in the soil solution, which is called active acidity—an indicator of current soil conditions. However, there are hydrogen ions, referred to a reserve acidity that are released into the soil solution to replace those neutralized by the lime.
Cation Exchange Capacity
Cation exchange capacity (CEC) is a measure of a soil’s capacity to hold (or adsorb) positively charged (cations) nutrients. The major soil cations include: calcium (Ca2?), magnesium (Mg2?), potassium (K?), sodium (Na?), hydrogen (H?), ammonium (NH4?), and aluminum (Al3?).
Base Saturation
Base saturation (BS) refers to the proportion of cation exchange sites in the soil that are occupied by the various base cations—potassium (K?), calcium (Ca2?), magnesium (Mg2?), and sodium (Na?). The percent base saturation is calculated as follows:
Electrical Conductivity
Electrical conductivity (ECe) is a measure of the total soluble salt concentration in a soil (i.e., salinity). Sodium chloride is the most common salt others include bicarbonates, sulfates, and carbonates of calcium, potassium and magnesium. A high ECe value corresponds with high amounts of soluble salts, and vice versa.
Sodium Adsorption Ratio (SAR)
Soil sodium adsorption ratio (SAR) is analogous to the irrigation water SAR discussed in Chapter 15, Water Quality for Irrigating Vineyards. SAR, expresses the proportion of sodium (Na?) relative to the proportions of calcium (Ca2?) and magnesium (Mg2?).
Exchangeable Sodium Percentage
Sodium levels are evaluated based on exchangeable sodium percentage (ESP). ESP is the percentage of soil exchange sites occupied by sodium (Na?) and is calculated by dividing the concentration of sodium cations by the total cation exchange capacity—ESP = (exchangeable sodium ÷ CEC) x 100.
Macronutrients
Macronutrients that may be tested in your soil include N, phosphorus (P), potassium (K), sulfur (S), calcium (Ca), and magnesium (Mg). Nitrogen, P and K are considered "primary" macronutrients, because they are required in higher quantities than S, Ca and Mg ("secondary" macronutrients), and because vines develop N, P, and K deficiencies more often.
Nitrogen
Nitrate nitrogen is most commonly measured in standard soil tests because it is the primary form of nitrogen available to grapevines and, therefore, an indicator of nitrogen soil fertility.
Phosphorous and Potassium
Phosphorus (P) and potassium (K) are reported as ppm (parts per million) P2O5 or ppm K2O, respectively.
Micronutrients
Iron (Fe), manganese (Mn), copper (Cu), boron (B), zinc (Zn), nickel (Ni), chlorine (Cl), and molybdenum (Mo) may also be listed on a soil test report. Micronutrients are required by the grapevine in small quantities with the availability directly dependent on the soil pH.
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