Automated determinations of phosphate content of soils under rubber cultivation

Abstract The phosphate status of rubber‐growing soils in West Malaysia is assessed by measuring the phosphate extracted with acid fluoride and with a mixture of perchloric and sulphuric acids. The extracted phosphate can in each case, be determined automatically on a Technicon Auto‐Analyzer, using the molybdenum blue colorimetric method with ascorbic acid as reductant. For P soluble in acid fluoride, the automated procedure is slightly more reproducible but gives somewhat lower results than the manual stannous chloride procedure, except for very low concentrations where the position is reversed. Ascorbic acid prevents interference by Fe in the determination of perchloric acid‐soluble P; recovery and replicate tests show that the automated procedure is more accurate than the manual phosphovanadate method. Digestion with HCIO4/H2SO4 gives as good a measure of total soil P as HF/HNO3, and although values are slightly lower than with sodium carbonate fusion, the method is more convenient and rapid for routine analysis.