Modeling diffusion and reaction in soils: VI. Ion diffusion and water characteristics in organic manure-amended soil

Knowledge of short-term changes in soil physical properties attributable to manure application is a prerequisite for estimating water and solute movement in manure-amended soils. Ion diffusivity and water transport characteristics were measured after the application of liquid cattle manure in two soils of different texture. Chloride diffusivity apparently decreased in a mixture of a coarse sand and 17% cattle manure compared with diffusivity in the sand without manure application. No difference was seen for a similar mixture using a sandy loam. A large dry matter content in the extracted pore liquid suggested that the coarse sand with low specific surface area only adsorbed small amounts of the added manure dry matter, explaining the increased tortuosity for ion diffusion. Addition of 15-20% manure gave a large increase in water holding capacity (0.10 cm³ cm^-3 at a soil-water potential of -1500 cm H₂O) and an increase of the Campbell soil-water retention parameter, b, by a factor of 3 to 4 for both soils. When manure was applied by direct injection into slits in the soil, the dry matter content of the manure controlled the rapid initial redistribution of water. A model for the relative soil-liquid sorptivity (ratio of sorptivity when applying manure, S, to sorptivity when infiltrating water, S₀) as a function of manure dry matter content is proposed.