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Accounting for particle movement when assessing the dissolution of slow release fertilizers in field soils

Loganathan, P., Hedley, M.J., Bretherton, M.R., Rowarth, J.S.
Nutrient cycling in agroecosystems 2004 v.70 no.1 pp. 77-84
Fragiaqualfs, earthworms, fertilizer rates, models, nylon, pastures, researchers, rock phosphate, slow-release fertilizers, soil depth, steel, tracer techniques
When fine particulate, slow-release fertilizers are applied to pastures, earthworm activity can physically move the particles both horizontally and vertically in the soil. This physical movement needs to be accounted for when researchers attempt to determine the rate of fertilizer dissolution by measuring the quantity of undissolved fertilizer residue remaining in the soil. A hard igneous phosphate rock (Phalaborwa PR) and an inert chromite ore were used as tracers to follow the physical movement of the reactive PR (RPR), NCPR applied to grazed pasture on a Pallic Soil (Aeric Fragiaqualf) isolated in galvanised steel cylindrical cores (150 mm diameter and 100 mm height) with or without nylon mesh sleeve (63 μm) at the bottom. The soil cores were sampled after 113, 270 and 559 days and analysed for PR and chromite residues. Results from both tracers showed that there was a significant movement of particles laterally out of the cores and vertically below 40 mm soil depth. Phalaborwa PR particle movement to 40-60 mm soil depth was 8% in 112 days and 24% in 270 days. Between 270 and 559 days no significant movement of particles was observed. Particle movement appeared to occur in discrete non-continuous events that were associated with the major end-of-autumn peaks in earthworm activity. When corrected for particle movement, the measured rate of NCPR dissolution satisfactorily fitted the rate of dissolution predicted by two models of PR dissolution.