Water Journal : Water Journal April 2011
refereed paper horticultural use water APRIL 2011 155 The effectiveness of different wetting agents in improving soil wettability has been demonstrated in several studies (for example, Miyamoto, 1985, Throssell, 2005 and Karnok, 2006). However, other observations have demonstrated that the coating of sand by anionic and non-ionic surfactants resulted in enhanced water repellency (Wiel-Shafran et al., 2005). The authors suggested that even at the low concentration of 10mg/kg of anionic and non-ionic surfactants significant water repellency was observed. Although not directly tested, it was suggested that in contrast to the theoretical mode of action (Figure 3), the hydrophilic head is attached to the sand surface, leaving the hydrophobic tail facing the aqueous phase. In other words, the surfactant molecule behaves just like natural hydrophobic molecules once absorbed on the sand, thus enhancing its hydrophobicity. Therefore, the aim of this current study was to test the potential effect of commercial surfactant-based wetting agents on soil water-repellency. Materials and Methods The efficiency of five commercial wetting agents (two granular and three liquid-based products) was tested by studying their effect on capillary rise and infiltration of water into typical native, partly hydrophobic, sand that is used for gardening in Perth, WA. About 1m3 sand was collected from a garden area at Murdoch University. The soil was sieved using a 2.0mm sieve and re-mixed. A representative subsample was dried (1050C) and characterised. The sand properties are summarised in Table 1. Note the low content of clay and silt. Capillary Rise Experiments The effect of surfactants and wetting agents on capillary rise was determined on a subsample by laboratory experiments according to a procedure suggested by Wiel-Shafran et al. (2006). A number of polypropylene tubes (length 295mm, internal diameter 38mm) were used and their bases covered with a fine mesh net. The columns were then packed with the native sand (control), native sand that was pre-coated with product D (one of the commercial wetting agents), native sand that was pre-coated with anionic surfactant, Linear Alkylbenzene Sulfonate (LAS), and native sand that was burned in a muffle furnace for four hours at 450oC to remove the organic matter. Each experiment was replicated four times. Coating the sand with wetting agent was done by mixing at a volumetric ratio of 2:1 sand to wetting agent solution that was prepared according to the manufacturer's instructions. The sand was then dried in 105oC overnight, and packed into the columns. Similarly, sand was pre-coated with 100 mg/L LAS surfactant solution. The columns were attached to a stand and placed on a balance, as illustrated in Figure 4. An open reservoir containing tap water was then raised beneath the column until the water surface touched the bottom of the column. As a measure for water repellency, capillary rise was assessed as the weight of water rising in the column. The weight change was recorded with a data logger once every five seconds. Figure 4: Illustration of the capillary rise experimental set-up. Measurement of Infiltration Rate using a Double-Ring Infiltrometer Experimental procedure This procedure was conducted to mimic common irrigation practice of large pots. The mixed, sieved sand (50L into each barrel) was introduced into 18 plastic barrels (length 650mm, inner diameter 420mm, height 470mm). Barrels were shaken after each sand load was introduced to settle the sand. The infiltration rates were measured by the double ring infiltrometer technique, as commonly used to evaluate the saturated infiltration rate in soils (Lai and Ren, 2007) and illustrated in Figure 5. Two 22 cm high plastic rings were driven concentrically 10 cm deep into the soil with minimum soil disturbance. The outer and inner ring diameters used were 170mm and 83mm respectively. Figure 5: Illustration of the double-ring infiltrometer experimental set-up. The outer ring was filled with water, after which the inner cylinder was filled to a level equivalent to an initial 70mm- 80mm head. The time taken for the water level in the inner cylinder to drop to 20mm was recorded using a timer. Thereafter, a measured volume of water equivalent to 20mm in depth in the ring was filled successively and the time taken to infiltrate this amount was recorded. When the amount of water entering into the soil was fairly constant over time for five consecutive measurements, steady-state saturated flow was assumed and the average saturated infiltration rate was calculated (based on these last five measurements). In order to mimic more closely pot irrigation, the "initial" infiltration rate was determined by recording the time taken for the water level in the inner cylinder to drop 20mm for the first time. The water level in the outer ring was maintained at a level approximately the same as the water level in the inner ring. Initially all 18 barrels were conditioned by wetting with water; overall 1830ml of water was added in the inner ring, which was 130% of the void volume of the sand bed underneath this ring. The barrels were left for five days before the study was commenced. Five commercial wetting agents designated as Products A, B, C, D and E were studied (Table 2). Each wetting agent was applied into three barrels and Table 1: Physical and textural characteristics of sandy soil used in the study. Org C (%) CEC* (meq/100) Water content (%) Sand fraction (%) Sand (%) Silt (%) Clay (%) Fine (20-212μm) Coarse (212-2000 μm) 0.4 + 0.02 4.6 + 0.02 0.4 + 0.02 49.21 + 0.21 47.37 + 0.13 96.6+ 0.3 0.6 + 0.02 2.2 + 0.05 Cation Exchange Capacity (CEC) is Na+ + K+ +Ca2+ +Mg2+ (meq/100g).
Water Journal March 2011
Water Journal May 2011