Organic and inorganic forms of phosphorus in soils amended with sugar cane filter cake

Studies of phosphorus (P) behaviour in soils receiving sugar cane filter cake are important for determining doses, its contribution to P nutrition and maximizing fertilizer efficiency, especially in soils with low P availability. This study aimed to assess organic and inorganic forms of P in soils treated with sugar cane filter cake, and the relationship between these and plant available P. Soil inorganic (P-i) and organic (P-o) phosphorus forms, organic carbon, pH and resin P were measured in two soils; a Typic Hapludox with clay texture and an Arenic Hapludult with loamy sand texture, subject to three fertilizer treatments (nil P control and 100 mg P dm(-3)as filter cake and triple superphosphate). Inorganic P was fractionated to soluble/loosely bound, Al-bound, Fe-bound, occluded and Ca-bound P forms. Organic P was fractionated into labile, fulvic, humic and residual P(o)forms. Plant dry matter yield and P uptake were also determined. Both fertilizers increased the Al- and Fe-bound P fractions, with the increases caused by triple superphosphate 14.7 and 7.2 mg kg(-1)higher than filter cake, respectively. Filter cake increased the fulvic P(o)by 40.9 mg kg(-1), while triple superphosphate increased it by 28.5 mg kg(-1), in the fine-textured soil. Al-, Fe-bound and labile P(o)fractions in both soils had significant correlations (p <. 01) with plant absorbed P (r = .75, .84 and .54, respectively). Filter cake resulted in lower P(i)fractions and availability, and it has potential to partially replace mineral P fertilizer in fine-textured soils and, entirely, in coarse-textured soils.