The role of Ca-organic interactions in soil aggregate stability .III. Mechanisms and models

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Abstract
Serial additions of glucose after drying-wetting cycles led to substantial aggregation of the surface soil from a red-brown earth. Addition of glucose and Ca compounds resulted in up to 80% of the soil bound in particles >2000µm with only a small portion of the soil in particles 4 0 µm. Treatment of the soils with Na4P2O7 caused dispersion of most of the clay but did not disrupt larger aggregates. More disaggregation was caused by treatments with HCI (0.02 M and 0.1 M), indicating the presence of pyrophosphate-resistant binding mechanisms which were presumed to be due to Ca bridging. Addition of Ca to the soil reduced the efficiency of extraction of organic materials by NaOH. This effect was eliminated by a pretreatment with 1 M HCl, indicating a 'bridging' effect of Ca between polycarboxylic macromolecules (humic acids) and clays. The role of Ca in clay and organic matter flocculation, the bridging of clay aggregates to organic materials and interactions of these colloidal aggregates with the biologic cycle is illustrated in some pictorial models.