Aggregate strength in calcareous soil fertilized with pig slurries
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Effects of applications of pig slurry on soil aggregate stability are not well understood in dryland agriculture. This research aims to (i) identify aggregate stability tests that give a reliable description of the soil's behaviour when pig slurry (PS) is applied to calcareous soil and (ii) interpret them in terms of chemical, biological, morphological and physical soil properties for soil quality assessments. Soil samples from eight fertilizer treatments (mineral fertilizers and PS), applied over seven growing seasons were analysed. We applied five methodologies to examine different mechanisms of aggregate breakdown. Porosity was characterized by image analyses. There was minimum resistance to the mechanical breakdown of aggregates when slurries were applied 12 months before analysis. Recent applications of slurry (3 months before the analysis) improved resistance to implosion caused by the penetration of water into dry aggregates (slaking), although the opposite result can occur if the method of evaluation is not chosen properly. Recent applications of PS also enhanced soil respiration and increased soil porosity in the 25-100 μm size range (packing pores between aggregates) and in the 100-400 μm size range (interaggregate or faunal pores). In dryland systems and in the winter cereal cropping season, the resistance of dry aggregates to slaking is improved temporarily if PS is applied at N rates equivalent to around 1.7 Mg OM ha-1 year-1.