Soil Quality in Vineyards

The soil must provide a favorable physical environment for the growth of vines—their roots and beneficial soil organisms. Some of the important properties con­tributing to this condition are infiltration rate, soil strength, available water ca­pacity, drainage, and aeration. Ideally, the infiltration rate IR should be >50 mm/hr, allowing water to enter the soil without ponding on the surface, which is predisposed to runoff and erosion. The range of infiltration rates for soils of different texture and structural condi­tion is shown in table 7.1. Typically, the soil aggregates should have a high de­gree of water stability so that when the soil is subjected to pressure from wheeled traffic or heavy rain, the aggregates do not collapse, nor do the clays deflocculate. Some of the problems associated with the collapse of wet aggregates and clay de-flocculation, and the formation of hard surface crusts when dry, are discussed in section 3.2.3. Pans that develop at depth in the soil profile, as a result of remolding of wet aggregates under wheel or cultivation pressure, can be barriers to root growth. Soil strength is synonymous with consistence, which is the resistance by the soil to deformation when subjected to a compressive shear force (box 2.2). Soil strength depends on the soil matrix potential m and bulk density BD, as illustrated in fig­ure 7.1. In situ soil strength is best measured using a penetrometer, as discussed in box 7.1. The soil strength at a ψm of −10 kPa (FC ) should be <2 MPa for easy root penetration and should not exceed 3 MPa at –1500 kPa (PWP). As shown in figure 7.1, when ψm is between −10 and −100 kPa, the soil strength increases with BD. The BD of vineyard soils can increase, particularly in the inter-row areas because of compaction by machinery, such as tractors, spray equip­ment, and harvesters. Typically, compaction occurs at depths between 20 and 25 cm and is more severe in sandy soils than in clay loams and clays (except when the clays are sodic; see section 7.2.3). Figure 7.2 shows the marked difference in soil compaction, measured by penetration resistance, under a wheel track and un­der a vine row on a sandy soil in a vineyard.