A Safe, Simple and Inexpensive Method for On-Farm Soil Testing of Water-Extractable Phosphate in Upland Field Soils.

In response to recent price increases of fertilizers, soil testing is being further promoted throughout Japan. We developed a new nonshaking water extraction (NSWE) method for soil phosphate (P) that is acceptably accurate and suitable for on-farm soil testing. Here we present a new method for measuring the extracted P by using a combination of poison-free chemicals and an inexpensive colorimeter.

Phosphate was extracted by the NSWE method in triplicate from air-dried and sieved soil samples from 68 upland fields. Each extract was analyzed by using the 4-aminoantipyrine colorimetric method with enzymes (enzymatic method) and the reference method (J. Murphy and J.P. Riley, Anal. Chim. Acta, 1962; 27, 31–36). The enzymatic method is easily performed using a commercially available poison-free water-quality test kit, in which premixed powdered chemicals are packed for single use. Because the enzymatic method is a complex system comprised of three successive enzymatic reactions, there was concern about interference from various substances in soil extracts. However, both methods showed similar results. The range that could be quantified by the enzymatic method was at least 4 times greater than that of the reference method when the reaction period was extended from 5 min, as instructed by the manufacturer, to 30 min. Likewise, some modifications were necessary to improve the measurement when using an inexpensive colorimeter, such as changing the process of diluting extracts. This new method should allow for accurate quantification covering nearly the full range of P fertility with a single measurement, without the need to remeasure because of too much color development, for example. The combined use of the NSWE and enzymatic methods and a colorimeter provides a safe, simple, and inexpensive means to measure water-extractable P in upland field soils without any hazardous chemicals or special laboratory equipment.