The study assessed the effect of heating vermiculites on extractability of phosphorus, iron, zinc and manganese with respect to their potential agricultural use. Of these elements, phosphorus was from apatite and monazite that occur as accessory minerals in vermiculites. Vermiculites were heated at 15–800°C and digested by acetic acid for extracting phosphorus and diethylene triamine pentaacetic acid (DTPA) for extracting zinc, iron and manganese. Phosphorus in the extract was analysed by a flow injection method while zinc, iron and manganese were measured by atomic absorption spectrometry. The results showed that heating vermiculites to 400°C enhanced extractability of phosphorus from apatite and monazite to a level of 335 mg kg–1. Further heating to 800°C reduced extractable phosphorus to less than 75 mg kg–1. Maximum extractable zinc, iron and manganese found were 2.7, 19.1 and 22.9 mg kg–1, respectively, values that are beneficial and tolerable by most plants. Thus, it was concluded that heating vermiculites to ⩽ 400°C optimizes extractability of phosphorus from incorporated apatite and monazite and some essential plant micronutrients in vermiculites.

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