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.