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Not AvailableAlthough the soils of the Indo-Gangetic Alluvial Plains (IGP), endowed with fine-grained micas, are natural suppliers of K in sufficient amounts to plants, yet the crop response to K fertilizer treatment in long term fertilizer experiments (LTFE) on rice-wheat (R-W) cropping systems has been anomalous. After almost 25 years of rice-rice-wheat (R-R-W) cropping, systems responded to K fertilizer in a LTFE conducted since 1978 in soils of the IGP, Bhairahawa, Nepal. This is in contrast to a LTFE in soils of the IGP, India conducted since 1975 at the Central Soil Salinity Research Institute (CSSRI), Karnal, India. At the CSSRI, even after 28 years of R-W cropping, the system has not responded to K fertilizer treatments. In order to gain comprehensive information on the crop response to K fertilizer, soils and their size fractions (silt and clay) of LTFE and original plot at Bhairahawa were subject to repeated batch type Ba-K exchange to study the K release. The rate of K release is primarily dependent on the amount of biotite mica only while muscovite remains as an inert source of K in soils. The richness of biotite in soil size fractions can be ascertained by a higher 001/002 ratio (2–4) of basal reflection of the mica peak. The rate and amount of release from soils and their size fractions are quite favourable but are unable to meet the demand for K required by R-R-W cropping systems under sub-humid climatic conditions due to very low content of silt biotite unlike in soils of LTFE under semi-arid climatic conditions of the CSSRI, Karnal. The present study alongwith the information available with the LTFE at Bhairahawa suggests that the rate and amount of K release from silt and clay micas may be enough for early rice but inadequate for R-R-W cropping systems, and the present high dose of K (90 kg K2O ha−1) may be replaced with the incorporation of straw with a small dose of K (10–20 kg K2O ha−1). Such management intervention may not only slow down the rapid vermiculitization of biotite mica but also help in reducing the consumption of a costly input like K fertilizer. Additionally, incorporation of straw may help further sequestration of soil organic carbon and nitrogen that may in turn reduce the adverse effect of bicarbonate ions added through irrigation from tube well.Not Availabl