Absorption of artificial piggery effluent by soil: A laboratory study

Abstract

Urine, faeces, and waste feed flushed from pens of intensive piggeries produce large volumes of nutrient-rich effluent, which is often most productively used for irrigation. These effluents tend to be similar, with pH values in the range 7.6–8.0 together with high water-soluble ammonium and potassium but lower concentrations of calcium, sodium, and magnesium. There is little experimental information on the behaviour of these mixed ion systems in soils, so the effects of artificial but chemically similar effluents were examined in absorption experiments on columns of ‘natural’ soils. In one series of experiments, NH4+ was excluded from the artificial effluent to assess the consequences of its presence or absence. Water content and solute concentration data scaled according to distance divided by the square root of time in experiments terminated at different times. This showed that basic flow and reaction equations were valid and that water-soluble and exchangeable cations appeared to be in equilibrium in the ‘time-scale’ of absorption. Systematic patterns of behaviour emerged: chloride, the dominant anion in the absorbed solution, moved with the water and its front coincided with the piston front that would exist if the absorbed water completely displaced the original soil solution; cation exchange was restricted to the inflow end of the columns in an environment of constant Cl– concentration and was retarded relative to the moving water. High concentrations of NH4+ appeared not materially to affect the exchange isotherms of other cation pairs, although they reduced retardation of other exchangeable cation fronts. The method provides a useful way to define exchange isotherms in an appropriate environment of competing cations.