Effects of application of compost and inorganic P on microbial activities, nitrogen and phosphorus mineralization in an ultisol
The aim of this study was to assess the effects of separate and combined applications of compost and inorganic P on soil microbial respiration and population, and on N and P mineralization in an UItisol with the view to developing a method for efficient co-application of organic and inorganic fertilizers. The study, which was carried out in the Soil Microbiology Laboratory of the Department of Soil Science and Land Resources Management involved laboratory incubation over 16 weeks. Top soil (0-15 cm) of Iwo series (Ultisol) was collected from a fallow land at the Teaching and Research Farm (T&RF) of the Obafemi Awolowo University, Ile-Ife. The fraction less than 2 mm was used for the incubation study. The study consisted of 500 g of air-dried soil, 5 g kg-1 and 10 g kg-1 compost, Na2HPO4.12H2O as source of P at 60 kg ha-1 mixed with half or the whole soil mixed thoroughly with the half or the whole soil. There were four treatments (5 g kg-1, 10 g kg-1, 5 g kg-1 + inorganic P at 60 kg ha-1 and 10 g kg-1 + inorganic P at 60 kg ha-1, and the control). The treatments were replicated thrice and then arranged in a Completely Randomized Design (CRD). Carbon dioxide evolution, N and P mineralization were determined fortnightly for a period of 16 weeks. Microbial populations were determined at the end of the 8th and 16th weeks. The experiment was repeated in soil culture in the screenhouse using maize (Zea mays L.) as the test crop. There were two consecutive 8-week plantings. Maize agronomic parameters and soil chemical properties were determined at the end of each planting. The data collected were subjected to analysis of variance (ANOVA) and the means separated using Duncan’s New Multiple Range Test (DNMRT) at 95% level of probability. Correlation analysis was also carried out using SAS 2000 package. The mean amount of CO2 evolved from the P-enriched compost applied to half the bulk soil (C1P1h) was significantly (p≤ 0.05) lower than in the remaining treatments. On the other hand, the treatment that had the higher rate of compost (C2P1h) had the highest amount of CO2 evolution. The mean available P in the control (C0P0) was significantly (P≤ 0.05) lower than in the remaining treatments. There was no significant (p≤ 0.05) difference between the available P contents at both rates of compost addition whether applied to the whole bulk or half portion of the bulk soil. The populations of heterotrophic bacteria (THB), actinomycetes (THA) and fungi (THF) at the end of the 8th week of incubation were generally higher than at the end of 16th week. The mean microbial populations showed the following trend: THB > THA > THF. Increase in the application of compost increased plant height, dry matter yield, soil pH, available and tissue P while there was no significant difference in the concentrations of exchangeable cations. It was concluded that inorganic P and compost applied separately to either side of the plant enhanced N and P mineralization and maize agronomic parameters.