An evaluation of the use of organic amandments to ameliorate aluminium toxicity and phosphorus deficiency in an acid soil.
Mokolobate, Motlogeloa Salathiel.
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The effects of the additions of some commonly-available organic residues to an acid, P-deficient soil (typical of those used by small-scale farmers in KwaZulu-Natal) on soil pH, exchangeable and soil solution AI, P availability and maize yield response was investigated in a number of laboratory and glasshouse experiments. The organic amendments used were ground-up grassveld residues, household compost, filter cake (a waste product from a sugar mill) and layer poultry manure. The soil used was a Hutton form (Farmingham series) (Rhodic Ferrasol, FAO). In an initial laboratory study, addition of all of the organic residues, at rates equivalent to 10 and 20 Mg ha¯¹, raised soil pH significantly and as a result there was a marked reduction in exchangeable AI concentrations. The increase in pH and decrease in exchangeable AI was more pronounced at the higher rate of addition and followed the order: poultry manure> filter cake> household compost> grass residues. The major mechanism responsible for the increase in pH was thought to differ depending upon the type of organic residue being considered. Whilst the relatively high content ofCaC0₃ was probably the main mechanism in the case of poultry manure and filter cake, the proton consuming ability of humic material probably predominated for household compost and decarboxylation of organic acids during decomposition was probably the main mechanism in the case of grass residues. Additions of organic amendments also decreased concentrations of total AI (AIT) in soil solution but the concentration of monomeric AI (AIMono) as estimated by pyrocatechol violet 60 sec. method, was unchanged or even increased. This latter effect was attributed to the high cation content of residues (particularly that of poultry manure) which increased soil salinity and exchangeable AI³⁺ was consequently displaced into soil solution.Additions of amendments also increased the Olsen-extractable P levels in the order: poultry manure> filter cake> household compost> grass residues and their addition also decreased theP adsorption capacity ofsoils. Concentrations of exchangeable Ca, Mg and K, and Na in the case of poultry manure, were increased by additions of organic amendments. In a glasshouse experiment, the four organic residues were applied to soils at a rate equivalent to 20 Mg ha¯¹ with or without the addition of either lime (equivalent to 0, 5 or 10 Mg ha¯¹) or P (equivalent to 0, 10 or 50 kg ha¯¹). Lime applications to the control (unamended) treatment resulted in a marked reduction in exchangeable AI, AIT AIMono and in the proportion of AIT present as AIMono in soil solution. The addition of organic amendments increased soil pH and reduced AIT and AIMono to low concentrations regardless of whether lime was applied or not. There was no yield response in maize to applied lime in any of the amended treatments. There was a yield increase in response to applied P in the control, household compost and grass residue treatments but none for the filter cake and poultry manure treatments. In agreement with this, Olsen-extractable P values in soils followed the order: poultry manure> filter cake> household compost> grass residues > control. It was concluded that the addition of organic amendments to acid soils is a practicable way of liming them and reducing the potential for Al toxicity and that it can also reduce fertilizer P requirements. This research now needs to be extended into the field situation.