A comparison beween short-term chemical and biological methods to measure phosphorus availability in sludge-amended sesquioxic soils

Abstract

Due to the transformations which sludge undergoes after addition to soil, single measurements of P availability are of little value in soils which have recently received additions of sewage sludge. This study assessed six procedures on two soils—a clay of the Griffin form (Typic Haplorthox) and a sandy clay loam of the Clovelly form (Tropeptic Haplorthox). Five chemical tests based on laboratory incubation and chemical analyses were compared to a short-term bioassay procedure. Soil—sludge and soil—inorganic P mixtures (supplying 375, 751 and 1503 mg P kg⁻¹ soil) were incubated in the laboratory for up to 92 days, and available P assessed by extraction with Bray 1, Bray 2, Olsen, ISFEI and CaCl₂ solutions. The bioassay was based on the short-term uptake of P by P-deficient Italian ryegrass (Lolium multiflorum var. Midmar) from soil—sludge or soil—inorganic P mixtures. Ryegrass cumulative P uptake and amounts of plant-available P extracted by the various reagents was regressed against amount of P applied for each soil x P—source x time combination. The ratio of the slopes of the (linear) regression lines for the two P sources was then used as a measure of the relative efficiency (E value) of sludge as a P fertilizer compared with inorganic P source. All the chemical tests greatly underestimated the effectiveness of sludge P as a source of fertilizer P. %E values derived by laboratory incubation and chemical extraction were approximately 50–70% initially for Bray, Olsen and ISFEI methods, and tended to remain constant or decrease with time. %E values from incubation and extraction with CaCl₂ were generally less than 10%. In contrast initial %E values from the bioassay were 44 and 64% for the Clovelly and Griffin soils respectively, and these values increased to 82 and 118% after 97 days of root—P source contact. Thus laboratory incubations of soil—sludge mixtures and extraction of various P forms cannot be reliably used as an indication of the P fertilizer value of the sludge.