Changes in the physico-chemical properties of soil under shifting cultivation with special reference to karbi anglong district of Assam

摘要

The effect of shifting cultivation on changes in the physico-chemical properties of soils was studied in the jhum and nearby natural forest areas. Various soil properties investigated were bulk density, texture, pH. hydraulic conductivity, organic carbon, total nitrogen, available phosphorus and exchangeable potassium, calcium and magnesium. Soil properties were compared at five different soil depths: 0-5 cm, 5-25 cm, 25-50 cm, 50-100 cm and 100~+ cm. Lowest and highest soil bulk densities were recorded in land after burning and 1st year fallow, respectively. Natural forest had a moderate soil bulk density. Results indicate that C and N in the soil may be among the most limiting factors for plant growth after a forest is cut and burned. Highest percentage of organic carbon (1.92 percent) and total nitrogen (0.28 percent) were found in pre-burning stages of vegetation and it significantly decrease in early fallow periods due to rejuvenation of vegetation. With the progress in the age of fallow, the percentage of organic carbon and total nitrogen increased significantly due to heavy accumulation of litterfall. A large amount of available phosphorus is released just after burning the forest. The lowest value of available phosphorus found in 1st and5th year fallows indicate that available phosphorus may limit crop production if the fallow periods are too short. Maximum amount of available phosphorus noticed in natural forest is due to heavy accumulation of litterfall. Exchangeable calcium, potassium and magnesium appeared to reach adequate levels in the soil after burning the forest and sharply declined in cropping period and it continuously decreased in early successional fallow. Natural forest exhibited adequate levels of exchangeable calcium,potassium and magnesium, while increase in 15th and 20th years fallow may be due to development of regeneration of tree vegetation. The overall results of this study indicate that the present shifting cultivation cycle of about 5 years is not sufficientto prevent serious soil degradation in the area. A minimum of 10 to 15 years will be required to maintain soil fertility at a reasonable level.