Objectives This study investigated the organic P composition of different organic materials and their P supply capacity as P sources, providing theoretical and data reference for the organic substitutions of P fertilizer.

Methods A wheat pot experiment was carried out, the used organic materials included manures (pig manure, sheep manure), legumes (pea, alfalfa, mung bean), straws (wheat straw, maize straw, rape straw). The total and organic P contents and the C/P ratio in the eight materials were analyzed, and the organic P was fractioned into labile organic P (LOP), medium-labile organic P (MLOP), medium-stable organic P (MROP) and high-stable organic P (HROP) via the Bowman and Cole methods. The wheat P uptake, soil available P content were determined.

Results The total P content in manures, legumes and straws were 5.49–5.52 g/kg, 1.19–2.59 g/kg and 0.57–1.07 g/kg respectively, the proportion of organic P was in range of 31.3%–55.2%. Except mung bean stalks, the organic P contents in all the tested organic materials were lower than inorganic P contents. In the total organic P, the average proportions of LOP, MLOP, MROP, HROP were 8.5%, 45.2%, 41.5% and 4.9%, respectively. The LOP content was in the order of manures (175.5 mg/kg) > legumes (67.03 mg/kg) > straws (25.8 mg/kg). The MLOP contents were relatively high in pig manure and mung bean straw. The MROP content was significantly different and in order of sheep manure > pig manure > alfalfa > mung bean > pea > maize straw > rape straw > wheat straw; The HROP content in manures was significantly higher than that in the other organic materials. The application of legumes and manures significantly increased the P uptake and biological yield of wheat, but application of straws significantly reduced them. The P uptake of wheat was positively correlated with soil available P content, negatively correlated with the C/P ratio of organic materials, and positively correlated with the LOP, MLOP and MROP of organic materials, not correlated with HROP contents. The content of soil available P was negatively correlated with the C/P of organic materials.

Conclusions In all the tested organic materials in this experiment, the total P content is in order of manures > legumes > straws, and the organic P content is lower than inorganic P, except mung bean stalks. Medium liable and medium stable P (MLOP and MROP) are the two main fractions in the organic P. Legumes and manures have relatively high P content and relatively low C/P ratio, their application could increase soil available P, so can be used to replace a certain ratio of chemical P fertilizer. While straws have low P content and high C/P ratio, so the bioavailability of P is low, not suitable for use as part of P fertilizer.