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Soil pH and drainage

  • Soils in California typically range from pH 5 (acidic) to 8.5 (alkalline).  Most plants do best when the soil pH is slightly acidic to neutral (pH 5.5 to 7.5) because absorption of minerals can occur in this pH range. At higher and lower pH some nutrients are not soluble, and therefore not available to the plant roots.

    Soil pH affects the availabililty of essential nutrients and also of toxic mineral elements.

    Very acidic soils (low pH) may contain inadequate levels of plant-available nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, molybdinum and may contain toxic levels of aluminum and manganese.

    Very alkaline soils (high pH) are deficient in plant-available nitrogen, phosphorus, iron, and manganese but also may contain excessive concentrations of soluble salts or sodium, both of which are detrimental or toxic to plant growth.

    Low levels of calcium, magnesium and potassium in the soil can lead to low soil pH. Low soil pH can be adjusted by addiiton of lime (calcium carbonate, for example). High soil pH can be adjusted with additions of sulfur. These materials need to be incorporated into the root zone of the soil, and the process can take some time, especially for sulfur treatments.

    A saline soil is a soil that has excessive soluble salts and (usually) a high pH in the root zone inhibiting plant growth.  Saline soils can develop in the arid climates of California because evaporation of water vapor from the soil surface occurs more often than leaching, leaving the salts behind in the root zone. Saline soils develop a surface crust of whitish material (salts).

    Plants tend to show wilt when growing in saline soils. Saline soils should be irrigated depply to flush away the excess salts. Mulch on the soil surface helps prevent surface water evaporation, reducing the accumulation of salts in the root zone.

    A sodic soil is a soil that has a high pH (greater than 8.5) because of sodium accumulation alone without the presence of excessive concentrations of other salts.  High sodium levels are toxic to plants.

    Some soils are both sodic and saline.

    Gypsum (calcium sulfate) and sulfur are used to reclaim sodic soils. A common misconception is that gypsum lowers soil pH, but its major effect is to improve water infiltration of sodic soils or high-sodium irrigation water. Large amounts of water must be added to gypsum-treated sodic soils to leach away sodium displaced by gypsum. If excessive sodium is not present, adding gypsum will not improve water infiltration (drainage).

    Review information on testing your soil pH.