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Abstract
Experiments have been conducted on measuring the combined effects of (1) time after sludge application, (2) temperature of equilibration, and (3) soil properties (principally pH and texture) on P availability and chemical fractions. Plant-available P (measured by iron-oxide strip extraction) was highest in the coarse textured soil. Plant-available P decreased from 20 to 60 d after application, probably due to combined effects of chemical fixation and biological immobilization, but increased thereafter. Plant-available P in all soils was lower at 37°C than at 25°C over the entire incubation period. This difference might reflect greater immobilization of P by microorganisms at 37°C. Sludge P was predominantly found in the non-occluded P (Fe and Al-bound P) fraction which is biologically-available, and thus may contribute to eutrophication if eroded into surface waters.
A parallel study was carried out on the contribution of 4 different manures to soil P fractions and their potential for leaching into the soil. Beef feedlot, sheep feedlot, turkey house, and composted paunch manures were applied to a central Nebraska soil in alternate years from 1991 to 1997 in amounts approximating replacement of P removed by irrigated corn. Highest available P (Bray and Kurtz-1) was in turkey manure-amended plots. In all cases, P accumulated in the 0–15 cm tillage zone. All inorganic P fractions increased with manure application, but Ca-P decreased as a percentage of total inorganic P. Based on depth of increase of various P fractions, non-occluded P is the form most likely to move downward from the zone of application. Phosphorus movement was most apparent in compost-amended soil.
