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What are soluble salts and how are they measured? The term soluble salts refers to the inorganic soil constituents (ions) that are dissolved in the soil water. Pure water is a very poor conductor of electric current, whereas water containing dissolved salts conducts current approximately in proportion to the amount of salt present. Thus, measurement of the electrical conductivity of a soil extract gives an indication of the total concentration of soluble salts in the soil. The electrical conductivity measurement is usually reported in units of millimhos per centimeter (mmhos/cm), although some labs report decisemens per meter (dS/m). One decisemen per meter is equal to one mmhos/cm. Why are they important? Soluble salt levels in the soil are important, because high soluble salts can reduce water uptake by plants, restrict root growth, cause burning of the foliage, inhibit flowering, and limit fruit and vegetable yields. Sensitivity to soluble salts differs among plant species/cultivars and their stage of growth. Seed germination and seedling growth are more sensitive to salt stress than the growth of mature plants, so greenhouse growers routinely test their growing media for soluble salt levels. A soluble salt test can be useful when investigating the cause of poor plant growth, determining the suitability of a new planting site, or monitoring the quality of "black dirt" for use on landscaped areas. Where do they come from? Excessive rates of fertilizer application and placing fertilizer too close to seeds or plant roots creates soluble salt problems. Pet urine and feces can increase soluble salts in localized areas resulting in plant damage or death. The use of deicing salts (primarily in the form of sodium chloride) on streets and sidewalks frequently results in high soluble salt levels in adjacent areas that restrict the growth of turfgrass, flowers, and shrubs. Correcting Soluble Salt Problems Salts can be leached out of soil with salt-free water, and rain will naturally reduce soil salt levels in time. In soils where internal drainage is poor, prevention may be the only feasible approach for reducing salt problems. In well-drained soils, incorporation of gypsum (calcium sulfate) at the rate of 10 to 15 lbs.100 sq. ft. followed by leaching with good quality water can be an effective means of correcting soluble salt problems. Gypsum is most effective in situations where sodium is the cause of high soluble salts. High levels of sodium in the soil will tend to disperse soil particles resulting in poor soil structure and making water infiltration difficult. The calcium in gypsum will displace sodium and the sodium will then move (leach) out of soil profile with irrigation water and/or rainfall. Soils high in calcium have better structure than those high in sodium. Use of gypsum on soils where sodium is not high has generally not been shown to be effective in improving soil structure. Thus, on low sodium soils, topdressing lawns with gypsum is not an effective way to improve soil structure and therefore not a recommended practice. |
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This site was last updated 11/22/06