Analytical methods and quality assurance

Abstract

Field testing of fertilisers is expensive and slow. Laboratory testing procedures are preferred, at least for initial comparisons of materials. However, equilibrium solubility measurements can give misleading results, especially when the fertiliser contains both soluble and partly soluble components. Methods that measure nutrient release rates are better for evaluation of fertilisers, as they more closely simulate the field situation. We have developed a laboratory leaching procedure for comparing fertiliser nutrient release rates. The materials are continuously leached with water or a buffer solution using a peristaltic pump. Leachates are passed through ion exchange columns to capture the nutrient of interest. Periodically the capture columns are changed and the leached ion determined by routine analytical methods. A curve showing the cumulative nutrient release over time is constructed and likely field behaviour of new materials estimated by comparison with known fertilisers. Use of a multi‐channel peristaltic pump enables several fertilisers to be investigated simultaneously. This method can be applied to a wide range of materials by suitable choice of ion exchange resin to capture the leached nutrient. Nutrient elements we have investigated include Ca, Mg, Co, Cu, Mo, B and P. Where more than one nutrient in a fertiliser is being investigated these can often be captured by the same ion exchange resin (e.g., Co and Cu) or by two columns in sequence (e.g., Ca, Mg and P leached from phosphate rock). This paper gives examples provided by this method for trace element fertilisers, phosphate rocks and limestones. Use of this method to investigate P leaching from soil is also demonstrated.