Cumulative and Residual Effects of Organic Fertilizer Application on Selected Soil Properties, Water Soluble P, Olsen-p and P Sorption Index

Long-term application of fertilizers containing P, especially organic fertilizers, usually increases the water soluble and available P of soil and at the same time may result in P accumulation in soil. Organic fertilizers may also increase movement of P in the soil profile that could result in surface and ground water pollution. An experiment was conducted on field plots that received three applications (۲۵, ۵۰ and ۱۰۰ Mg ha-۱) of solid dairy manure, sewage sludge, or urban solid waste compost, and one application of chemical fertilizers (۲۵۰ kg ha-۱ urea plus ۲۵۰ kg ha-۱ mono-ammonium phosphate) for ۱, ۲, ۳, ۴ or ۵ consecutive years in order to investigate the residual and cumulative effects of organic and chemical fertilizers on selected soil properties, water soluble and bioavailable P as well as the P sorption index of the soil. Average organic matter content in the soil increased as a result of organic fertilizer applications. The increase was proportional to the rate of application and was highest for dairy manure and lowest for urban solid compost. Both the water extractable and bioavailable P contents of soil increased with the rate of application, the nature of organic fertilizer and the number (years) of applications, with the rate being the most effective and the nature of fertilizer the least effective. Dairy manure had the largest and solid waste compost the smallest positive impact on both the water soluble and available P content of soil. Effect of sewage sludge application on available P content of the soil was greater than its effect on the water extractable P. The phosphorus sorption index was independent of the source of P (organic or chemical), the nature and the rate of organic fertilizers and was only significantly dependent on the number (years) of fertilizer applications.