Potassium Availability, Distribution and Categorization of Various Soil Types Under Diverse Rainfed Production Systems of India* Ch.Srinivasarao1, KPR.Vittal1, KN.Tiwari2 and H. Magen3

1 Central Research Institute for Dryland Agriculture, Santoshnagar, Hyderabad, 500059, Andhra Pradesh, India, 2 Potash and Phosphate Institute of Canada, India programme, Gurgaon, 122 016, Haryana, India.3 International Potash Institute, P.O. Box 569, CH-8810 Horgen, Switzerland,

Potassium fertilization is not practiced in rainfed agriculture in India assuming that Indian soils are rich in potassium and corps do not need external K supply. However, under continuous cropping in rainfed regions, huge crop K removals are reported up to 150-200 kg ha-1 depending upon amount and distribution of rainfall and biomass production. These crop removals essentially deplete soil K reserves if cropping continues further for longer period. Therefore, present study was undertaken to evaluate the soil K reserves under diverse rainfed production systems (Rainfed rice, groundnut, soybean, cotton, rabi sorghum, pearl millet, finger millet and maize) and to categorize rainfed soils based on different soil K fractions. Depth-wise sampling was done from 21 locations across different soil types under eight production systems and various fractions of soil K were assessed. In general total K was higher in Inceptisols (1.60-2.28%) followed by Aridisols (1.45-1.84%), Vertisols and Vertic sub-groups (0.24-1.72%) and Alfisols and Oxisols (0.30-1.86%). However, wide variation in total K was observed within each group of soils. Nonexchangeable K reserves also showed similar trend as that of total K in most of the profiles. Many of Inceptisls showed substantial amounts of nonexchangeable K while some of the Vertisols (Akola) and Alfisols (Bangalore) showed exceptionally low. Contrary to nonexchangeable K reserves, Vertisols showed higher exchangeable K than Inceptisols and Alfisols/Oxisols. Nonexchageable K showed significant positive correlation with total K in Inceptisols and Vertisols while it was non-significant in Alfisols/Oxisosls. However, it has significant positive correlation with exchangeable K in all the soil types indicating the dynamic equilibrium between two soil K fractions. Nonexchangeable K reserves were also involved along with exchangeable K in categorizing soils for evolving strategies to manage soil K fertility in rainfed agriculture in India.

* Contribution of Chief Scientists in soil sampling at 21 AICRPDA centres is gratefully acknowledged.