Polyphosphates as sources of phosphorus for plants

Abstract

Polyphosphates vary in their rates of hydrolysis and degree of sorption by soil constituents, which could affect the efficiency of P recovery from these compounds by plants. In this study, four linear oligophosphates (P₂, P₃, P₁₅, and P₄₅) and one cyclic polyphosphate (trimetaphosphate) were tested in greenhouse experiments for their ability to supply P to plants, compared with that of the conventional P source, orthophosphate (P₁). Annual ryegrass (Lolium multiflorum Lam.) and corn (Zea mays) were used as the indicator plants. Ryegrass was grown on four Iowa soils (Clarion, Webster, Primghar, and Ida), treated at the rates of 0, 20, 40, and 80 mg P kg⁻¹ soil (1.8 kg of soil pot⁻¹) and harvested four times at 30-day intervals. The soils of the control pots were recovered, repotted (150 g pot⁻¹), and seeded with corn after treatment with polyphosphates at the same rates as given. The corn plants were harvested after 35 d. With all P sources, dry matter yield and P yield increased with increasing rates of P application for both ryegrass and corn. Statistical analysis (Duncan's Multiple Range Test) of P yields of ryegrass and corn grown in soils amended with the various P sources indicated that there were no P compounds that consistently showed significantly greater amounts of P recovery in plant tissue. Considering all the P sources and rates of P application, dry matter yield was significantly correlated (exponentially) with P uptake by ryegrass from all four soils. The dry matter yield of corn was significantly correlated (linearly) with rates of P application with all P sources, indicating that the effectiveness of a unit of P taken up by plants in increasing the dry matter yield was similar among the polyphosphates and the conventional P fertilizer, orthophosphate. Measurements of extractable P (Bray-Kurtz I on the acid Clarion, Webster, and Primghar soils and Olsen's test on the alkaline Ida soil) upon completion of each experiment indicated that the residual P effects were similar among the P compounds after the short cropping period with corn (35 days) and the long cropping period with ryegrass (120 days).