Resin capsules for on-site assessment of soil nutrient supply in lowland rice fields

Using ion-exchange resin capsules for assessing nutrient supply in lowland rice (Oryza sativa L.) fields requires strategies for estimating field means of resin adsorption quantities (RAQs). To develop recommendations for on-site use of capsules, we conducted two experiments in rice fields on Typic Tropaquepts and Tropaqualfs in the Philippines. Resin capsules were placed in situ (ISI) or inserted into composite soil samples for on-site incubation (OSI). Capsules were retrieved after 1, 2, 4, 7, and 14 d and RAQs of NH4-N, P, K, Ca, Mg, Na, Fe, and Mn were measured. Ln ISI or OSI of single soil cores, coefficients of variation in RAQs of N, P, and Fe ranged from 17 to 71%, whereas those for K and Ca ranged from 10 to 20%. Between one and five capsules per 0.25-ha field were required to estimate the average RAQ of P, K, and Ca after 14 d with a precision of +/-10%, compared with four to 10 capsules per field for estimating the RAQ of NH4-N and Fe after 14 d. Soil core sampling and OSI of composite soil samples reduced microvariability in RAQs. About eight to 10 soil cores (0-15-cm depth) were required per field for resin capsule OSI. Because of soil physical changes, degassing, and oxidation during soil mixing, RAQs of P, Fe, K, and Ca were lower for OSI than for ISI. The ISI method is recommended for routine soil testing in lowland rice fields. The OSI requires fewer capsules but more labor. Its reliability depends on the extent to which mechanical disturbance, degassing, and exposure to air can be minimized.