Effects of surfactant treatments on the wettability of a water repellent grass-covered dune sand

The objective of this study was to evaluate the effectiveness of the surfactant formulation Primer®604 for amelioration and management of soil water repellency in grass-covered dune sand. The soil is severely to extremely water repellent to a depth of >0.50 m during dry periods. Primer®604 was applied 12 times between 22 April and 23 November 1999. During that period, soil samples were taken in the untreated and treated plot—8 times in transects and 2 times in soil blocks. A total of 4950 samples were collected for assessment of the actual water repellency and for the spatial and temporal variability of the water content of the soil. Resistance to wetting was determined by measuring the wetting rate of field-moist samples. Measurements of water repellency revealed that applications of Primer®604 resulted in less persistent water repellency in the surface layer to a depth of 0.05 m. No effects were observed deeper in the soil profile, likely due to adsorption of the surfactant material in the surface layer. In the surface layer (0–0.025 and 0.025–0.05 m), the critical soil water content below which the soil is actually water repellent in the field was lowered distinctly by the application of Primer®604, potentially due to coating of water repellent particle surfaces by the surfactant. This suggests that the surface layer (0–0.05 m) in the Primer®604-treated soil can dry to lower water contents than in the untreated soil before water repellency is induced. The thatch layer (0–0.025 m) of the treated soil was often found to have slightly higher water contents than of the untreated soil. The surfactant did not equalise the uneven moisture distribution in the soil below the surface layer (0–0.05 m). Primer®604 applications increased the wetting rate of field-moist samples from the thatch layer. This may result in a more effective wetting of the root-zone during rain events or irrigation, and a reduction in runoff.