AbstractCapacity of the nine most extensive upland Inceptisols of Puerto Rico to supply potassium (K) was determined through intensive cropping in pots with Pangola grass during 4 consecutive years. In addition, field experiments were conducted on two of these soils, Mucara and Pandura, for comparison. Seven of the lnceptisols tested formed under a udic moisture regime. They supplied an average of 279/kg of K/ha to Pangola grass during the first year, and then dropped to 110, 114, and 93 kg/ha for the second, third and fourth years, respectively. The average of 106 kg/ha for the last 3 years probably represents the long term capacity of this group to supply K to plants. The amount of K released by Mucara and Pandura soils (both udic lnceptisols) in the field experiments agreed fairly well with values obtained in the pot experiments. Two of the lnceptisols tested formed under a ustic moisture regime released and average of 507 kg of K/ha during the first year, and then 254, 233, and 140 kg/ha the last 3 years. The average K release per year for the last 3 years was 209 kg/ha. However, the long range K supplying capacity of these soils is probably lower than this value. The exchangeable K values obtained at the beginning of the pot experiment were significantly correlated with the amount of K extracted by Pangola grass during the first year of cropping in pots, whereas the HN03-soluble K values were correlated with the average release of K during the last 3 cropping years.
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