Impact of long-term mineral and organic fertilizer application on the water stability, wettability and porosity of aggregates obtained from two loamy soils

The impact of fertilizer application on soil aggregate stability is of increasing interest to soil scientists. Aggregate water stability depends primarily on soil organic matter. We studied silty loam and loamy sand aggregates from three long-term fertilizer treatments (control, pig manure and NPK) which significantly altered the quantity of organic matter. A new approach to examining aggregate stability was used: soil aggregates were immersed in methanol-water solutions with methanol at 0, 20, 40 and 60% concentration (C), and non-disrupted aggregates were separated after 30minutes. The aggregate resistance R(C) against each solution was taken as the percentage of stable aggregates. Overall resistance of the aggregates was taken as the R-tot parameter given by the product of four R(C) values. The R(C) values of all aggregates were positively correlated with C. The R(60) values were independent of the applied fertilizer. The R(0) value for silty loam aggregates ranged from 28% (pig manure) to 7% (NPK), while that for loamy sand equalled 7-9% in all cases. The R(20) values were most effective at differentiating the soils and the fertilizer treatments. However, the R-tot value was a better indicator of aggregate stability. Greater differences in stabilities of aggregates were noted in loamy sand. Mineral fertilizer application seemed to decrease aggregate resistance in both soils. Total organic carbon and nitrogen content in all non-disrupted aggregates were negatively correlated with methanol concentration (C) of the solution applied for aggregate separation. The largest decrease was for the pig manure treatment, and the smallest was for the control. Porosity and pore size distributions of the aggregates were derived from micro-tomography and approximated to lognormal pore size distributions. Larger porosities and pores were found in water-stable aggregates than in methanol-stable aggregates. It seems that the dominant mechanisms for aggregate instability during fast wetting were not related only to the pore air compression, but to weakening of attractive forces between aggregate particles by water.