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Graphical abstract Highlights • Foliar P nutrition of trees is assessable by using the citric-acid soluble P fraction (P cit) in soil as predictor. • Best predictions result from aggregated P cit stocks down to a soil depth of 10 or 20 cm. • Threshold values for P cit indicate when P deficiency of the trees becomes unlikely. • The determination of P cit is a suitable method for large forest soil inventories. Abstract In the past few decades, high nitrogen emissions have reversed the nitrogen limitation of trees. Meanwhile, phosphorus (P) appears as a primary limiting nutrient of many forests in Europe. However, a simple and economic standard method for the determination of the plant-available phosphorus in forest soils, which drives the P supply of the trees, is not yet available. Therefore, we tested two soil extraction methods [citric-acid extract (P cit), according to Hoffmann et al. (1991) and sodium bicarbonate extract (P HCO3), referring to Olsen et al. (1954)] for analyzing all Bavarian soil samples of the second German National Forest Soil Inventory (NFSI), resulting in a big data set with 342 forest sites and 2232 samples. We combined these results with the standardly gathered soil parameters and tested them as predictors for P nutrition indicated by the foliar P concentrations of the four most important tree species: Norway spruce (Picea abies), European beech (Fagus sylvatica), Scots pine (Pinus sylvestris) and pedunculate/sessile oak (Quercus robur/petraea). During the statistical process, we stepwise reduced the complexity of the prediction models. In our analyses, we achieved the best prediction of foliar P using soil information from the organic layer and mineral soil down to depths of 10 cm and 20 cm. P cit stocks provided by far the best predictions in nearly all cases. Based on this parameter, we derived parametric regression functions to estimate the P nutrition (spruce: R2 = 0.52∗∗∗; beech: R2 = 0.49∗∗∗; pine: R2 = 0.39∗∗∗; oak: R2 = 0.35∗∗∗). Based on logistic regressions, threshold values were deduced for P cit stocks down to 10 cm soil depth (spruce: 33 kg ha−1; beech: 43 kg ha−1; pine: 32 kg ha−1; oak: 41 kg ha−1), above which P deficiency becomes unlikely. Since these models are based on a large number of sites with diverse geologies and soil conditions, they can be used to estimate the P availability of most forests in Bavaria. Furthermore, the applied methods can be extended to other soil inventories. Estimated P availability based on such functions and thresholds can support sustainable nutrient management of temperate forests. [ABSTRACT FROM AUTHOR]