The effect of municipal solid waste compost (MSW) on the replacement of sodium in sodic soil models

Compost extracts or intact compost suspensions, with or without CaCO3. were equilibrated with Na+-saturated cation exchange resin used as a model of sodic soil. A release of cations from the solid phases (compost or CaCO3) and subsequent exchange with Na+ was evaluated through the apparent deviation from electro-neutrality in the solution. (Electro neutrality is defined here as the deviation from an increase of Na compared with the decrease of the sum of other cations (Ca + Mg + K + NH4+) in the solution). This approach enables experimental evaluation of the contribution of solids, compost, or other organic amendments to the reclamation of sodic soils. The compost used contained 3.7 eq/kg of Ca, Mg, and K. About 50% of the total amount of these potentially Na+-displacing ions in the compost were released during a 3-day incubation period and effectively replaced Na+. Compost suspension, unlike filtered compost extract, increased the release of Ca+2 from CaCO3 significantly. The release of cations from the compost seems to be faster than release from the CaCO3. Thus, the CaCO3 effect was detected in our experimental system primarily with low compost levels. However, the effect of the enhanced solubility of CaCO3 is most likely the dominant one under realistic levels of compost application. It is postulated that the compost particles may either release soluble or colloidal chelating agents or contain chelating or other cation-binding sites on the surfaces. It is also possible that the compost colloidal particles lead to raised dissolution of CaCO3, bind the Ca, and transfer the dissolved Ca ions onto the cation exchanger where they replace Na+. Application of MSW compost to sodic soils may raise the solubility of CaCO3 typically found in such soils and thus contribute to the reclamation of such soils.