Potassium Supplying Power of the Major Ultisols and Oxisols of Puerto Rico

Abstract

The K supplying power of 17 Ultisols and 6 Oxisols typical of vast areas in the tropics was determined by cropping 181 pots, each filled with 11.4 kg (25 lb) of soil from as many sites, with Pangola grass for 4 consecutive years. During the first year of cropping the soil groups released the following amounts of K: Oxisols, 234 kg/ha (209 lb/acre); Ultisols of the uplands, 260 kg/ha (232 lb/acre); clay Ultisols of the coastal plains, 230 kg/ha (206 lb/acre); and sandy Ultisols of the coastal plains, 90 kg/ha (81 lb/acre). After the first year, K removal dropped off sharply averaging about 50 kg/ha (45 lb/acre) yearly for the Oxisols and clay Ultisols of the coastal plains, 35 kg/ha (31 lb/acre) yearly for the sandy Ultisols of the coastal plains and 90 kg/ha (80 lb/acre) for the Ultisols of the uplands. There was close agreement between various soil K values determined at the beginning and at the end of the experiment and K removal by Pangola grass over different periods. The close relationship between exchangeable K at start of the experiment and K re moved by Pangola grass over the first year of cropping shows that exchangeable K values are a good criterion for evaluating the capacity of these soils to supply K to grasses.