Optimal Conditions for a Non-Shaking Water Extraction Method for Simple on-Farm Testing of Available Soil Phosphorus in Upland Field Soils.

In response to the recent price increase of phosphorus fertilizers, soil testing is being further promoted throughout Japan. We have been working to develop a new water extraction method with acceptable accuracy suitable for on-farm soil testing; we are currently testing a non-shaking water extraction (NSWE) method. Here we present the optimal water/soil ratio (w/s ratio), extraction time, and extraction temperature that allow for stable measurement. Air-dried and sieved samples of gray lowland soil or Andosol collected from fields with high levels of phosphorus were analyzed. Deionized water and soil were gently mixed at w/s ratios ranging from 2.5 to 40 (v/w) at 25 °C and left standing for various periods (2 min to 48 h). The amount of extracted phosphorous from gray lowland soil, for example, reached the apparent maximum within 24 h at w/s ratios less than 10. The influence of temperature was examined at four levels from 5 to 35 °C, and the values relative to those observed at 25 °C were evaluated. For gray lowland soil extracted at a w/s ratio of 2.5 for 6 h, the relative values ranged from 70% to 122%, which was the narrowest range among the three w/s ratios tested. At low w/s ratios, phosphorus extraction from Andosol showed a similar tendency to reach equilibrium in a short period with a small temperature effect on the reaction. These results demonstrate that a w/s ratio of 2.5 is suitable for NSWE because it allows for relatively stable measurements of soil phosphorus in a short period at various temperatures that are likely during on-farm testing.