Water quality and surfactant effects on the water repellency of a sandy soil

Differences in irrigation water quality may affect the water repellency of soils treated or untreated with surfactants. Using simulated irrigations, we evaluated water quality and surfactant application rate effects upon the water repellency of a Quincy sand (Xeric Torripsamment). We used a split plot design with two irrigation water qualities, three surfactant application rates, two irrigations, and twelve sampling depths as fixed effects, with four replications. Each water quality x rate x irrigation combination was a main plot and depth was a repeated-measures subplot. A slightly water repellent Quincy soil (average water drop penetration time, WDPT, of 2.5 s) was packed in 25-mm lifts (or layers) to a bulk density of 1.6 Mg/cubic m into 0.15-m-high x 0.105-m-diameter plastic columns. We studied a nonionic surfactant, a blend of an ethylene oxide/propylene oxide block copolymer and an alkyl polyglycoside. We sprayed the surfactant at rates of 0, 9.4, and 46.8 L/ha, diluted with reverse osmosis water (RW) to apply 187 L/ha of solution, onto the soil surface of each packed column. About 24 h after surfactant application, columns were sprinkler irrigated with either RW or well water (WW) at 88 mm/h for 0.25 h without runoff. Well water (pH 7.6) contained 54.90 mg Ca/L, 67.16 mg Na/L, and had an electrical conductivity (EC) of 0.7 dS/m and sodium adsorption ratio (SAR) of 1.7. The RW (pH 5.7) contained 0.882 mg Ca/L, 3.201 mg Na/L, and had an EC of 0.0072 dS/m and SAR of 2.2. After the first irrigation with each water quality, half of the columns were destructively sampled in 12 increments to the 150-mm depth, with water content measured on a subsample and the remainder air-dried. Five days later, sample WDPT was measured. The remaining unsampled columns were oven dried at 30° C for three days to a mean soil water content < 0.04 g/g, cooled at ambient temperatures for 16 h, then irrigated with water of the same quality as before and sampled as previously. After the first irrigation, WDPT at depths from 97 to 117 mm averaged across surfactant rates reached a maximum of 28 s, regardless of irrigation water quality. WDPT was greatest at 117 mm with RW but only at 97 mm with WW. After the second irrigation, maximum WDPT was 1202 s at 139 mm with RW but only 161 s at 117 mm with WW, nearly 7.5 fold less than with RW. WDPT was greatest near the wetting front, irrespective of water quality. We conclude that irrigation water containing modest amounts of electrolytes or salts, in this case mostly salts of Ca and Na, reduces water repellency in the presence or absence of surfactant. Sprinkler irrigating this water-repellent soil translocated repellency-inducing moieties to the wetting front, concentrating them there in either surfactant-treated or untreated soils. Repeated irrigations further concentrated and drove repellency-inducing moieties deeper. When the sprinkler water contained modest amounts of electrolytes, the repellency at the wetting front was reduced nearly an order of magnitude. It appears that irrigating water-repellent soils using water containing electrolytes (probably Ca salts), greatly reduces repellency in the zone where hydrophobic substances accumulate in such soils. In our study, this occurred whether or not the soil was pretreated with surfactant. The results suggest that farmers and managers of turf and other agricultural land may be able to control the soil depth where repellency-inducing substances accumulate. For turf managers, this depth should be well below that of rooting. For farmers, it should be below the depth of rooting or below the depth of annual tillage, whichever is greater. Our experimental results may also help explain erratic surfactant performance under rainfed conditions where neither water quality nor depth of infiltration can be fully controlled.