Organic soil phosphorus considerably contributes to plant nutrition but is neglected by routine soil‐testing methods

Abstract

Abstract In upper layer of arable soils, the percentage of organically bound phosphorus (P) can range from 20% to 80% of total P concentration. Approximately 40% of organic soil P is in the inositol‐P fraction. The objective of our study was to evaluate various soil‐testing methods in measuring phytate soil P as an important source of organic soil P, and to determine the bioavailability of phytate soil P for summer wheat ( Triticum aestivum L.), rape ( Brassica napus L.), sugar beet ( Beta vulgaris L. var. altissima Döll), phacelia ( Phacelia tanacetifolia L.), maize ( Zea mays L.), white lupin ( Lupinus albus L.), Mexican sunflower ( Tithonia diversifolia Hemsley Gray), summer rye ( Secale cereale L.), buckwheat ( Fagopyrum esculentum L.), and pigeon pea ( Cajanus cajan L.). Plants were cultivated in a growth chamber on a subsoil as a substrate with very low organic‐matter content and negligible microbial activity in order to restrict the contribution of mycorrhiza and soil microbial activity for organic–soil P mobilization. Application of organic P (100 mg P [kg soil] –1 as Na‐hexaphytate, C 6 H 6 O 24 P 6 Na 12 ) did not result in a significant increase of extractable soil P measured with routine soil‐testing methods (CAL, DL, NaHCO 3 , EUF, Me‐III) although the applied organic P was available for all tested plant species. The relative P‐uptake efficiency for Na‐hexaphytate compared to Ca(H 2 PO 4 ) 2 was for rape, pigeon pea, and phacelia approximately 100%, for white lupin and maize 80%, for sugar beet, Mexican sunflower, wheat, and buckwheat approximately 60%, and for rye 35%. The results indicate that the bioavailability of organic soil P is not taken into account by routine soil‐testing methods.