Use of organic amendments as ameliorants for soil acidity in laboratory and field experiments.
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Laboratory studies and field trials were carried out to investigate the effect of addition of some organic residues to acid soils on soil pH, exchangeable and soluble AI, nutrient status, microbiological and biochemical indices and maize response. The organic wastes used in the first laboratory study included plant materials (maize. sorghum, kikuyu grass, soybean, red clover residues and acacia prunings), animal manures (kraal, pasture-fed and feedlot cattle manure, layer and broiler poultry manure and pig manure), household compost, sewage sludge, and filter cake. The poultry manure, pig manure and leguminous plant residues had the highest content of basic cations while sewage sludge had the highest N content. Poultry manure had very high values for proton consumption capacity, CaC03 content and ash alkalinity. Proton consumption capacity, ash alkalinity, total basic cation content and CaC03 content were closely correlated with one· another. Soil pH was increased and exchangeable AI and total (AIT) and monomeric (AIMono) AI in solution were decreased by addition of all the organic wastes: the effect was greater at the higher rate of application. Strong correlations were recorded between the rise in soil pH and proton consumption capacity, ash alkalinity, CaC03content and basic cation content of the residues. The major mechanisms responsible for the elevations in pH were suggested to be the substantial CaC03content of poultry and pig manures, and filter cake, the proton consumption capacity of humic material present in household compost and manures and decarboxylation of organic acid anions during the decomposition of plant residues and manures. It was proposed that ash alkalinity is a suitable laboratory test for predicting the potential Iiming effect of organic residues since it is strongly correlated with the rise in pH that occurs, it is relatively simple to measure and the values reflect the initial content of organic acid anions, humic materials and CaC03in the residues. A preliminary field experiment was set up to investigate the effectiveness of kraal manure as a Iiming material in an acid soil (pHwater =4.1) at a site close to a Zulu village. The experiment consisted of two rates of lime (L1 = 2.5 and L2 = 5.0 t ha1) and two rates of kraal manure (K1 = 10 and K2 = 20 t ha-1 ) which were banded and incorporated in a 30 cm wide strip down the plant rows. Treatments were arranged in a randomized block design with three replicates. A commercial maize cultivar PAN 6710 and a traditional variety EMBO, used by the farmers in the locality, were grown. Soils in the plant row were sampled at tasselling and at harvest. The addition of kraal manure significantly raised soil pH and reduced concentrations of exchangeable AI and those of both total and monomeric AI in soil solution. Lime raised pH and the pH continued to increase between tasselling and harvest. Maize yields for control, kraal manure (K1 and K2) and lime (L1 and L2) for PAN 6710 were 2.5, 3.7, 5.1, 5.3 and 6.3 t ha-1 ; respectively and for EMBO they were 3.0, 5.4, 5.8, 5.9and 8.2 t ha-1 , respectively. These results demonstrate the high yield potential of the traditional maize variety under small scale farming conditions, and show that large yield increases can be obtained by applying kraal manure. The long-term effects (24 weeks) of incubation of organic wastes (soybean residues, poultry, pig and kraal manures and sewage sludge) with an acid soil were investigated in a laboratory study. After incubation for six weeks incubation, soil pH was raised and exchangeable AI and Air and AIMono in soil solution were decreased . by addition of the wastes. Soil pH generally declined and exchangeable and soluble AI increased over the remainder of the incubation period. The decline in pH was attributed mainly to nitrification of NH4+ originating from mineralization of wastes-derived organic N. Addition of organic materials generally resulted in a decrease in the proportion of solution Air present as AIMono. That is, the effects of addition of organic materials was two-fold; an increase in pH in the short term and complexation of AI by organic matter. Since these effects occur simultaneously, it would be desirable to separate them. For this reason, short-term equilibration experiments (3 days) were conducted to study the solubility of AI in aqueous solution or in an Oxisol when in equilibrium , with 3 manures (kraal, pig and poultry) at pH values of 4.0, 4.5, 5.0, 5.5, 6.0 and 6.5. Addition of manures tended to reduce the concentrations of total AI in solution (AIT) in the lower pH range (Le. pH 4.0 and 4.5) but increased AIT concentrations compared to the control, at higher pH values (Le. at pH 5.5 and above). This was explained in terms of the complexing ability of both the solid and solution phases. At lower pH, where AI is highly soluble, complexation by added solid phase manure-organic matter results in a reduction of AI solubility. However, at high pH, where AI solubility is limited, the most important mechanism is complexation of AI by soluble organic matter and this increased AI solubility. Additions of manure reduced the proportion of Air present in monomeric form (AIMono). This effect was more pronounced in aqueous solution but was also clearly evident above pH 5.0 in the Oxisol. This reflects the fact that a large concentration of soluble C in solution can maintain relatively high concentrations of complexed AI in solution but at the same time maintain low concentrations of AIMono. It was concluded that formation of AI-organic matter complexes caused by additions of organic manures can alter the solubility of AI and reduce the amount of phytotoxic AIMono present in soil solution. A second field trial was conducted to compare the effects of additions of kraal manure, grass residues, lime and fertilizer (N-P-K) under field conditions, on soil pH, AI solubility and maize response and, at the same time follow concomitant changes in the size and activity of the soil microbial biomass and enzyme activity. The greatest effects of kraal manure in increasing soil pH and decreasing AI toxicity were recorded six weeks after planting whereas those of lime and grass residues were recorded at harvest. Kraal manure and fertilizer increased significantly AMBIC extractable P and exchangeable K and Zn. In addition, Kraal manure, and to a lesser extent lime significantly increased exchangeable Ca and Mg.. Soils in the plant row in the grass residue treatments had the highest microbial biomass C and microbial quotient,followed by kraal manure, lime and controls. Basal respiration rates and arginine ammonification, protease, aryl sulphatase, and acid phosphatase activity rates were significantly increased by addition of all treatments and these increases tended to be accentuated by fertilizer. Low values for metabolic quotient in the grass residue treatments were associated with high values for microbial biomass C in these treatments. The addition of all treatments tended to increase maize yields and, in general, these yields were greater for the high rate of application of each amendment. Yields for unfertilized kraal manure were markedly greater than those for the unfertilized grass residue and lime treatments. This was attributed to the ability of kraal manure to both increase pH and add nutrients to the soil.