Availability of organic and inorganic phosphorus fractions to wheat in toposequences of calcareous soils

Information on the availability of different soil phosphorus (P) forms is useful for crop production. Phosphorus contents of 12 Iranian calcareous soils from upper-, mid-, and lower-slope positions of two and and two semiarid toposequences were fractionated to various organic and inorganic pools, and correlations of the P fractions with wheat responses were investigated. Among the inorganic P (IP) fractions, apatite type (Ca-10-P) and dicalcium phosphate equivalents (Ca-2-P) possessed the highest and the lowest amounts of P reserve in the soils, respectively. On average, about 20% of the total P was found in organic form (OP), of which 32% was labile (LOP), 51% was moderately labile (MLOP), and 17% was nonlabile (NLOP). The amounts of the soil P fractions were considerably influenced by the positions of the soils on the landscapes. The maximum contents of soil IP, Ca-2-P, Fe-P (iron-bound P), and Ca-10-P were observed in the lower-slope positions. The amount of soil available [0.5 M sodium bicarbonate (NaHCO3) extractable] P was significantly correlated with Ca2P (r = 0.895), Fe-P (r = 0.760), and Occl-P (iron-occluded P) (r = 0.897). Direct correlation studies, however, showed that wheat shoot dry-matter yield (DMY) was significantly affected by the amounts of Ca-2-P, Fe-P, OP, LOP, and MLOP fractions both at early (4 weeks) and late (10 weeks) stages of growth. All organic and inorganic P fractions, except Al-P (aluminum-bound P), Ca-8-P (octacalcium phosphate equivalents), and NLOP, also showed significant relations to the amount and/or concentration of P in wheat tissues at 4 and 10 weeks after sowing. Among the measured soil properties, the amount of organic carbon was the most affecting factor on the size of the P fractions.