A method of evaluation of nutrient release from membrane-coated slow-release fertilizers is presented. The method is based on the gravimetric measurement of water-vapor uptake by individual fertilizer granules placed in a saturated water vapor chamber.
The nutrient release from such fertilizer consists of two stages. In the first stage, water vapor infiltrates into the granule and condenses on the soluble fertilizer salt, leading to the development of pressure within the particle. The elevated pressure leads to swelling of the granule and the outward leakage of the fertilizer solution.
The method yields results characterizing both the average rate of nutrient release and the release rate distribution among the population of fertilizer granules. The results are consistent with measurements of nutrient release rates in soil-fertilizer mixtures.
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