Effect of sample disturbance on soil water repellency determination in sandy soils

Effective irrigation of managed lands suffering from soil water repellency can benefit from understanding the spatial distribution of repellency in the surface soil layer at actual field conditions. We examined whether the common procedure of mixing and homogenizing samples from the soil surface layer gives representative values of field repellency. Water drop penetration time (WDPT) measurements were made for more than 300 pairs of undisturbed and disturbed samples collected across different surface soil layer (0–5 cm) transects in a citrus orchard. Most of the samples were obtained from a plot rendered repellent due to irrigation with treated sewage effluent. Results show that WDPT measurements on disturbed soil samples are not representative of measurements on undisturbed soil samples. The extent of deviation in absolute WDPT values is very large and unpredictable for a given sample. Repellency class designations, by definition less sensitive than absolute WDPT values, are also subject to significant distortion as a result of sample disturbance, with up to 77% of soil samples changing by at least one repellency class or more, and up to 54% of samples changing by 2 classes or more. Statistically, sample disturbance tended to decrease repellency class designation, although up to 40% of the disturbed samples were classified as more repellent. There was no relationship between repellency change and soil moisture content or organic matter content. The extent of difference between repellency measurements on undisturbed and disturbed soils is as great as those differences reported between field moist and air-dried samples, air-dried and oven-dried samples, and air-dried samples equilibrated at different relative humidities. The major potential reasons that repellency of disturbed and undisturbed samples may differ are concluded to be: (i) differences in surface roughness, pore size distribution, pore connectivity, and soil bulk density; and (ii) a change in the distribution and orientation of materials responsible for repellency (e.g., organic matter, fungal filaments, biofilms, leaf litter).