Potassium and magnesium in some Kenyan soils: their mineral sources and release to Ca‐resin

SUMMARY Five soil types from Kenya were examined for their ability to release K and Mg to crops by extracting them with a calcium‐saturated cation exchange resin, and for the mineral sources of K and Mg. Amounts and rates of release of exchangeable, short‐ and long‐term reserves of K and Mg were determined. This was done for K before and after one, two and 10 wetting and drying (WD) cycles, designed to mimic the effect of the local climate on K release. The order of decreasing K and Mg contents of the soils was Muguga > > Katumani > Kampi‐ya‐Mawe (KYM) ∼ Ithookwe ∼ Kwale. In the case of K, wetting and drying increased the exchangeable fraction in all of the soils. The Muguga, Ithookwe and KYM soils have the capacity to release a great deal of K in the long‐term, but at very different rates; the Kwale soil has few reserves and fixes K on WD. The K and Mg contents of the soils were closely related to the mica contents of the clay + silt fractions. Exchangeable and non‐exchangeable K was released from soil micas primarily by ion exchange, with some contribution to non‐exchangeable K from dissolution. Release of exchangeable Mg was primarily by ion exchange, whilst that of non‐exchangeable matrix Mg was by dissolution of trioctahedral mica in the clay + silt and the sand fractions of the soils. Response would be expected only to K fertilizers and only with the Kwale soil. However, all the other soils should, in the interests of good husbandry, receive maintenance dressings of K and Mg at convenient times in the crop rotation.