Subcritical water repellency of aggregates from a range of soil management practices

Subcritical water repellency is a poorly acknowledged physical property of soil. It refers to soil where water uptake appears to occur readily, yet is impeded to some extent by the presence of hydrophobic surface films. It was only after the recent development of a sensitive testing technique that subcritical water repellency was shown to be a common feature of many soils. It is a fundamental physical property of soil and has implications for the resistance of soil structure against disruption by wetting, bypass flow, and surface runoff. Using a technique adapted by Hallett and Young (1999), we assessed a water repellency index, R, of individual soil aggregates from a range of cultivation practices with different fertilizer inputs and depths. The parameter R is extremely powerful since it is directly proportional to the decrease in water sorptivity caused by repellency. The hypotheses tested are (i) that soil disturbance reduces R and (ii) that high levels of plant nutrients (fertilizer) will enhance R. Cultivation was found to cause a twofold decrease in R for all soils tested except one pasture treatment. Pasture soil from another site had an R value that was three times higher to a depth of 60 cm than an adjacent plowed soil. Soil aggregates were more repellent from no-till than plowed treatments. Higher levels of N added to field soil did not affect R.