Development and evaluation of rapid tests for the estimation of phosphate and potassium in plant sap

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Abstract
Rapid tests have been developed for estimating concentrations of P and K in plant sap in the field or glasshouse. The tests are based on the Hach Orthophosphate Method (for simple colorimetric determinations in solution) and the Merckoquant K Test Strips (for spot testing on paper). Measurements of phosphate concentration in sap samples (containing 0 to 160 μg ml‐1 P) by the new method were in good agreement with those by a laboratory spectrophotometric procedure, but the method can be readily adapted to estimate sap concentrations of up to 326 μg ml‐1 P. Measurements with K test strips on sap from K‐deficient plants were in broad agreement with corresponding flame photometric determinations over the range 1000 to 2000 μg ml‐1 K, but deviated considerably at lower concentrations. These errors were apparently caused by increases in the concentration of brown pigments and of putrescine and agmatine in the sap of more highly K‐deficient plants which interfered with the test strip measurements. Sap samples with K concentrations greater than 2000 μg ml‐1 K were also determined satisfactorily after dilution. It was concluded that both of the new methods performed satisfactorily over concentration ranges likely to correspond to subclinical deficiencies of P or K for most species and their use could streamline rapid sap testing procedures in the field.