Nutrient Dynamics in a Coastal Savanna Soil Amended with Neem (Azadirachta Indica) Leaves and Animal Manure

Abstract

Sustainability of agriculture with the ever-increasing trend in population calls for urgent steps to make lands available for agricultural production. On these lands maintenance of soil fertility through the use of environmentally friendly diverse organic materials are imminent. Neem leaves and animal manure, particularly poultry and cow dung manure abound all year round and the potentials of these organic materials are explored in the research study. The study sought to investigate the performance of a soil amendment, the materials of which are various inclusion rates of neem leaves, poultry manure and cow dung applied to a RapJic Acrisol (Benya series), from a coastal savanna ecological zone in Ghana. Field and pot experiments were carried out at the Technology Village of the University of Cape Coast, from September 2002 to October 2003. A proposition that an active ingredient in the neem leaves (azadirachtin A) might have a vital role to play in the amendment was investigated using Gas Chromatography to study its breakdown in the soil. Also, that the neem leaves in combination with animal manure could boost up the nutrient content of the soil as well as influencing the population dynamics of nematodes within the soil were studied. Above all, the efficiency of the soil amendment in achieving the aforementioned properties were tested by monitoring the performance of a test crop (carrot) under controlled conditions using pots. Degradation of azadirachtin A in the soil followed the first order reaction kinetics. Degradation was fastest in the amendment which had 100g neem leaves 1kg soil. The inclusions of 5 g and 109 poultry manure and 109 cow dung to the neem leaves hastened the degradation of azadirachtin A. The interaction between poultry manure and the neem leaves enhanced the release of nutrients in the soil. The peak of release of most of the soil nutrients occurred two weeks after incorporation of the neem leaves and poultry manure. The amount of C02 - C evolved relatively corresponded with the quantity of neem leaves and poultry manure added to the soil. The release of NH4+ -N, available P, exchangeable K, Ca and Mg was a mirror image of CO2 - C evolved and the quantities also corresponded with the quantity of neem leaves and poultry manure incorporated into the soil. The amount of N03- -N released, however, was an exception to the above observed trend, the lower inclusion rates of the neem leaves and poultry manure released more nitrate than the higher levels at a point in time. This exception was assigned to the nitrification inhibitory role played by the neem leaves in the amended soil. The 50 g neem leaves + 5 g poultry manure 1kg soil proved to be the most effective in the control of root-knot nematodes on carrot roots. Generally, with the apparent increase in soil nutrients and the reduced root-knot nematodes of carrot the yield of the crop improved significantly. Neem leaves and poultry manure may be used in place of synthetic compounds to provide soil nutrients and control plant parasitic nematodes, and thus, improve the growth and yield of carrots. Neem leaves may also be used to slow down the release of nitrates in the soil and thus reduce the ultimate pollution caused by nitrate leaching.