Andosols are widely distributed in upland fields in Japan and are characterized by a high phosphorus (P) retention capacity. Consequently, the efficiency of P fertilizers is low in Andosols, requiring a high rate of P fertilizer application. However, P resources are becoming depleted globally, and excess P accumulated in soil inhibits trace element uptake by plants, pollutes the environment, and triggers outbreaks of soil-borne diseases. Available P in upland fields in Japan is usually estimated using the Truog method, but the results do not correlate with plant P uptake in Andosols. Soil organic P is also not usually evaluated as available, but remains an important P source for plants. The P in microbes (biomass P) is particularly important, because P from dead microbes is released directly into the soil or easily decomposed. We found a significant correlation between biomass P and P uptake by kidney beans. Therefore, biomass P may serve as a reliable indicator of P availability in Andosols.