Soluble Salts in Compost and Their Effects on Soil and Plants: A Review

The chemical, physical, and biological conditions of soil and growing media can be substantively improved by the addition of compost. Compost contains many plant essential nutrients (e.g. N, P, and K) and can also be a source of organic matter. However, concerns persist over composts with a high concentration of soluble salts and their effects on soil fertility, plant growth, and yields. Soluble salts refer to soluble ions such as Ca²⁺, K⁺, Mg²⁺, and Na⁺ in compost and are measured indirectly and cumulatively through electrical conductivity (EC). Specifically, compost salinity is commonly measured using a method referred to as EC₅ whereas soils are measured using an EC_e. The use of a variety of non-standardized methodologies often make interpreting results between studies difficult. A compost with an EC₅ >5 dS m⁻¹ could be the result of high concentrations of Na⁺ or other ions, which can be detrimental to plants due to their ability to accumulate in plant tissue and interfere with root uptake of water. Thus, reducing soluble salts, specifically Na⁺ and Cl⁻ in composts is of high importance. Other soluble salts present in compost (e.g. K⁺ and Ca²⁺) are mineral nutrients required for plant growth and can aid in reducing soil sodicity. In appropriate proportions, quality compost with a high EC₅ mixed with soil or media can enhance plant growth and yields. Composts with a high EC₅ are adept at aiding in soil remediation by facilitating soil particle flocculation, helping leach Na⁺ deeper into the soil profile and raising soil cation exchange capacity in support of increased soil fertility. Labeling composts and developing appropriate application methods could allay concerns associated with composts where EC₅ > 5 dS m⁻¹and promote compost use and sustainable farming practices.