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u <br />LJ <br />I • <br />~H. Soil reaction (or pH) of the growth medium significantly affects the availability of essential <br />nutrients in addition to the solubility of potentially toxic ions. Soils with a pH ranging from 7.5 to 8.4 <br />contain alkaline earth carbonates and may be alkali. Soils With a pH greater than 8.5 are nearly <br />always alkali and contain a large percentage ofalkaline-earth carbonates (USDA, 1951 and USDA, <br />1969). Soils with pH less than 5.0 usually contain solubilized aluminum and excessive amounts <br />of other heavy metal trace elements which are soluble under acid conditions. Total nutrient <br />availability is greatest for neutral to slightly acid soils within a pH range of 6.5 - 7.0 (Donahue et al., <br />1977; USDA, 1951; and Walsh, 1972). For these reasons, soils with a pH less than 5.0 or greater <br />than 8.5 are typically unsuitable for plant propagation. <br />Conductivity (Salinity). Salinity is normally not measured directly as a soil test. The electrical <br />conductivity (EC) of the soil solution is measured and taken as an indication of salt level. Soluble <br />salts influence the quantity of water available to plants and total dissolved solids concentration in <br />ground water. High salts cause an osmotic pressure barrier to the soil root system interface and <br />inhibit water and nutrient movement into the plant. The tolerance of plants to high soil salt levels <br />is species dependent and has been well documented (USDA, 1969 and Donahue et al., 1977). <br />Salinity levels are most critical during germination and most detrimental in hot dry climates and on <br />south facing slopes than in other landscape positions. <br />Saturation Percentage. Saturation percentage is a useful parameter for detecting low <br />water-holding capacity and clarifying the degree of sodic hazard as indicated by SAR (Merrill et al., <br />1987). The degree of saturation is highly influenced by the physical properties of the medium. <br />Soils tend to be doughty, coarse textured, and readily leached of nutrients when the water holding <br />capacity at saturation is 25 percent or less. A high saturation percentage (greater than 85 percent) <br />indicates a fine textured medium with potentially poor air and water permeability characteristics. <br />Particle Size (Texture) . Texture refers to the relative proportion of sand, silt, and clay sized <br />particles and generally relates directly to saturation percentage. Texture influences water <br />infiltration and percolation, water and nutrient retention, surface crusting, shrinkage and swelling, <br />frost hedving, compaction, and erodibility. Textural problems are generally encountered when <br />either sand contents approach 85 percent or clay levels are greater than 40 percent. Soils with <br />high silt or very fine sand content are often more susceptible to wind and water erosion. <br />REVISED August 2006 <br />2.04.6-19 <br />