The use of biochar and charcoal as soil amendments to improve allelochemical-laden soils in the landscape

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The landscape and turf industries are based on beauty. Some trees in the landscape suppress the growth of any other plant species beneath them. This is reported to be caused by the presence of allelochemicals which are released into the soil by the plants, a mechanism known as allelopathy. Soil amendment is therefore needed to curb the effects of these allelochemicals and make the nutrients in the soil available to other plant species that may be planted beneath the allelopathic trees. There is evidence from thousands of years of traditional use of charcoal as amendment in the terra preta soils of Brazil. Biochar, a pyrolised biomass, is a fine-grained, highly porous charcoal substance that is used as a soil amendment. The study was conducted to find out the ameliorative effect of biochar and charcoal in allelochemical-laden soils to improve on the physicochemical properties of the soil. Charcoal produced from Tectona grandis tree, biochar produced from four different types of sawdust (Tectona grandis, Celtis mildbraedii, Entandrophragma cylindricum and Khaya senegalensis) and absolute control were the treatments used. The study was carried out beneath three trees suspected to be allelopathic (Tectona grandis, Eucalyptus grandis and Bambusa sp.). St. Augustine’s grass was used for the study because it prefers shaded growing environmental conditions. The experimental design employed was Randomized Complete Block Design (RCBD) and the experiment was replicated three times. Data collected over a period of twelve weeks included presence of allelochemicals in the soil and in the tree species, rate of spread and rate of growth of grass, weed count, soil nutrient analysis, soil water-holding capacity, presence of soil microorganisms and soil pH. The results of the study indicated that the three landscape trees are allelopathic. The biochar and charcoal were able to ameliorate the effects of the allelochemicals and hence allowed the grass to grow well under the trees. Significant differences were observed in the rate of spread of grass as well as the rate of growth. Available phosphorous and soil potassium were increased in the biochar amended plots whereas total nitrogen was reduced due to adsorption of NH 3-and NH 4+ from the soil solution onto the biochar surface. Soil organic carbon was reduced due to the priming effect of biochar. Water-holding capacity was increased tremendously in all amended plots due to the porous nature of the charcoal and biochar. Both amendments were able to improve soil pH to an optimum range for most plants. It was recommended that other feedstock types for biochar and charcoal production should be considered in further studies to find out which feedstock type can also improve the soil where allelochemicals have been found.
A thesis submitted to the school of research and graduate Studies, Kwame Nkrumah University of Science and Technology, in partial fulfillment of the requirements for the award of Master of philosophy (Mphil landscape Studies) degree,