The effects of surface-applied poultry manure on top- and subsoil acidity and selected soil fertility characteristics.
The effects of surface applications of poultry manure on pH, exchangeable AI, exchangeable Al saturation and levels of available macronutrients in the surface and subsurface layers were investigated in some acid soils from the KwaZulu-Natal Midlands. Three field sites with a history of long-term applications of poultry manure were compared with adjacent fields where no manure had been applied. Results generally showed an increase in pHwater, pHKCI, exchangeable Ca, Mg, and K and extractable P and a decrease in exchangeable Al and exchangeable Al saturation in the surface soil to the depth to which the manure had been incorporated. Results also provided evidence for substantial downward movement of Ca and Mg into the subsoil layers (i.e. 30-60 cm) and their accumulation in exchangeable forms. There was a concomitant general increase in pHKcl and pHwater and decrease in exchangeable Al and exchangeable AI saturation in the subsoil layers. These results demonstrate that, under field conditions, surface applications of poultry manure can cause the amelioration of subsoil acidity. This is an extremely important finding since subsoil acidity is characteristically extremely difficult and costly to ameliorate. A leaching column study was subsequently conducted to investigate the mechanisms by which surface applications of poultry manure ameliorate both top- and subsoil acidity. The soil used, originating from one of the field sites, had not previously been treated with poultry manure and had a subsoil pHKCI of 4.25 and an exchangeable acidity concentration of 1.79 cmolJkg. Three forms of poultry manure (layer, broiler and free range) were incorporated into the surface 5 cm of soil columns at rates equivalent to 5, 10 and 30 Mg/ha. Columns were maintained in glasshouse conditions for a period of 108 days and over that period they were leached on four separate occasions (receiving a total of 825 mm of simulated rainfall). At the conclusion of the experiment, the soil columns were cut into sections for chemical analysis. Applications of poultry manure to the surface soil markedly increased pHwater, pHKcl, exchangeable Ca, Mg, K and Na concentrations and decreased exchangeable Al levels in the surface 5 cm layer. They also increased the concentrations of soluble C, soluble salts, total Al and organically-complexed Al in soil solution. These effects were most pronounced with layer manure and greater at the higher rate of application. The manure-induced decrease in exchangeable AI, but increase in total AI, in soil solution was attributed to soluble salts, originating from the manure, displacing exchangeable Al into solution where it was subsequently complexed by soluble organic matter. Analysis of subsoil layers (5-15, 15-25, 25-35 and 35-45 cm) at the conclusion of the experiment showed that surface applications of poultry manure decreased concentrations of exchangeable Al in the subsoil but had no effect on pHKCl and depressed values for pHwater. It was suggested that manure-derived urea leached into the subsoil and was then hydrolysed causing an increase in pH and precipitation of exchangeable Al as insoluble hydroxy-AI oxides. Towards the conclusion of the experiment nitrification began to proceed, causing subsoil pHKCl values to decrease back to their original values. Analysis of the inorganic-N content of leachates and soil layers provided circumstantial evidence for this mechanism. High concentrations of soluble salts in the subsoil layers (caused by leaching from the manure) resulted in displacement of exchangeable A13+ and W into soil solution so that the electrical conductivity and concentrations of total and monomeric Al were elevated and pHwater was depressed in the subsoil (15-45 cm) of poultry manure-treated columns. It was concluded that the results underline the opposing effects that poultry manure applications have on (i) raising soil pH and lowering exchangeable Al but at the same time (ii) greatly increasing soluble salt concentrations and thus displacing At3+ and H+ back into soil solution. They also have suggested the importance of the release of N during manure decomposition in influencing soil pH (through the processes of ammonification, urea hydrolysis and nitrification) and therefore other soil chemical properties. It is, however, clear that the long-term effect of surface applications of poultry manure is generally to ameliorate subsoil acidity by raising subsoil pH and lowering exchangeable Al concentrations.