MSc

A field incubation study with four treatments and five replications in RED was carried out in the typical red loam soils (Alfisol) of Vellayani, Kerala to determine the extent of nutrient immobilization and mineralization by soil microorganisms under the influence of various treatments like glucose, paddy straw and inorganic fertilizers during the rainy, winter and summer seasons in 1988-1989. Representative soil samples were collected from the experimental plots before and immediately after the application of treatments and after 21 days of incubation under field conditions. These soil samples were analysed for the status of organic carbon, available N and P and exchangeable K, Ca and Mg.
A portion of the soil samples collected after 21 days of incubation under field condition was subjected to chloroform fumigation for 72 hours and after reinoculation with fresh garden soil was incubated for another 21 days. After the specified period, the soil samples were analysed for the status of nutrients mentioned earlier. From the analytical data the amount of nutrients assimilated or immobilized and subsequently released through mineralization was derived and the data subjected to statistical analysis.
The study has shown that there is considerable variation between treatments in the extent of immobilization and mineralization of nutrients during the three seasons. Appreciable difference between the treatments was obtained for the contents of organic carbon and available nitrogen during the rainy season, available phosphorus during winter and organic carbon and exchangeable K, Ca and Mg during the summer season. The influence of treatments on the transformation of other nutrients during the different seasons was not appreciable eventhough a variation during the three seasons was noticed between treatments.
The extent of organic carbon immobilization and mineralization was maximum during rainy season in the glucose amended plots registering a net assimilation of 18.0 – 23.9% and net mineralization of 14.1 – 17.3% respectively of the total organic carbon content of the soil. Amendment of the soil with glucose, paddy straw and fertilizers did not cause any appreciable difference in the mineralization of organic carbon. The fertilizer treated plots registered the highest rate of nitrogen mineralization equivalent to 26.2 – 34.0 kg N ha-1 during the rainy season. The immobilization and mineralization of available phosphorus was highest during summer with a net immobilization of 4.0 – 16.5 kg P ha-1 and mineralization of 3.0 – 12.5 kg P ha-1. In the case of exchangeable potassium, the net immobilization and mineralization was maximum during rainy season in the fertilizer treated plots. The rates of immobilization and mineralization of calcium and magnesium showed a random trend.
The influence of different treatments on the microbial population of the soil has resulted in the highest population of bacteria, fungi and actinomycete during the rainy season in the straw amended plots. The favourable effect of a suitable temperature and moisture during the rainy season has presumably lead to a greater microbial activity and a correspondingly higher rate of immobilization and mineralization of the various nutrient elements.