Potassium-Supplying Capacity of Six Upland Mollisols from Puerto Rico

Abstract

The capacity of the most important upland Mollisols of Puerto Rico to supply K to plants was determined in pots that were cropped intensively with pangolagrass for 4 years. Exchangeable K was a reliable predictor of the long term K-supplying capacity of the udic Mollisols, whereas HN0₃-extractable K was a reliable index of the long term K-supplying capacity of the ustic Mollisols. Uptake of K from Mollisols formed under an udic-moisture regime averaged 90, 75 and 55 kg/ha/yr during the last 3 cropping years, respectively. Release patterns indicated that 50 kg of K/ha/yr probably represents the long term capacity of this group of soils to supply K to grasses. During the 4-year cropping period, 80% of the total "available" K determined in udic Mollisols at the beginning of the experiment was removed by the grass. Uptake of K from Mollisols formed under an ustic moisture regime averaged 315, 270, and 175 kg/ha/yr during cropping years, 2, 3 and 4, respectively. Release patterns indicate that the long-term K-supplying capacity of this group of soils is about 140 kg of K/ha/yr. During the 4-year cropping period, 70% of the total "available" K determined in the ustic Mollisols at the beginning of the experiment was removed by the grass.