A survey of the cation content of piggery effluents and some consequences of their use to irrigate soils

Abstract

Piggery effluent contains high concentrations of potassium, and its repeated irrigation raises soil exchangeable potassium to levels, relative to divalent cations, that may degrade soil structure. We surveyed 6 big piggeries extending from south-eastern Queensland on a self-mulching Vertosol, to an Arenic Rudosol in south-eastern South Australia. We sampled effluent used for irrigation and also soil profiles to permit 'fenceline' comparisons between soils that had and had not been irrigated. The major water-soluble cations sodium (Na+), potassium (K+), calcium (Ca2+), and magnesium (Mg2+) were measured in the effluent and the soil saturation extracts, and also their exchangeable forms on air-dried soil samples. Ammonium-nitrogen (NH4+-N) was also assayed. The effluents were similar, with pH values between 7.5 and 8 together with very high water-soluble NH4-N, lower values for K+ and Na+, and quite low concentrations of Ca2+ and Mg2+. Cation concentrations varied across effluents; sodium and potassium adsorption ratios (SAR and KAR) were relatively constant but smaller than an ammonium adsorption ratio (Am-AR), which we conceive to estimate the influence of NH4+-N relative to the divalent cations in the effluent. Exchangeable K+ ratios in all profiles that had been irrigated were greater than their non-irrigated partners, as were the KAR values in their saturation extracts. Despite high concentrations of NH4+-N and high values of Am-AR in the effluents, there was no evidence of exchangeable NH4+ in the soils when sampled, which, we presume, is rapidly taken up by plants or oxidised. We present data that support a useful relationship between total cation content and effluent and the soil saturation extract electrical conductivity (EC), We also observed a modest increase in the EC of the saturation extract of irrigated soils. Farm records were insufficient to permit material balance calculations.