Polyacrylamide and Water Quality Effects on Infiltration in Sandy Loam Soils

Slow infiltration rates constrain effective and economical irrigation in some sandy loam soils in California. Polyacrylamide (PAM) has increased soil infiltration in some areas, especially in soils high in clay or silt. Field trials near Fresno, CA, with PAM failed to show improved infiltration. Laboratory experiments were conducted to investigate PAM effect on infiltration of various quality waters in sandy loam soils. Two formulations of a highmolecular weight PAM, a liquid emulsion and a granular, were evaluated on a Hanford sandy loam soil (coarse-loamy, mixed, superactive, nonacid, thermic Typic Xerorthents) in packed soil column experiments. Applying PAM continuously in the infiltration water always decreased infiltration for all PAM concentrations tested (5–20 mg PAM L). Final infiltration rates of 5 mg PAM L relative to infiltration rate of deionized water were 65% for emulsion PAM and 36% for granular PAM and these ratios decreased with increasing PAM concentration. Reduction of infiltration rates when PAMwas applied with water containing Ca (applied as gypsum) was less than with PAM solution containing Na. Permeability tests of PAM solutions through uniform sands showed a decrease of permeability with increased concentrations, due to an apparent increase in effective viscosity of the solution. The decrease in infiltration rates in this study was likely due to this increase in viscosity when PAM is added to water. This research concluded that PAM applied in irrigation water will reduce infiltration unless the material improves surface soil aggregate structure and sustains pores sufficient to mask the effect of solution viscosity. SLOW INFILTRATION rates can make irrigation more difficult and expensive (Trout et al., 1990). Slow infiltration rates on sloping surface-irrigated fields can result in high runoff, requiring longer irrigation durations and greater frequency of irrigation to meet irrigation demands. Slow infiltration on level surface-irrigated fields can result in crop damage due to standing water or inadequate aeration in the root zone, and can even result in algae growth on the soil surface that further slows infiltration. Infiltration rates slower than sprinkler or drip emitter application rates result in water ponding and reduced application uniformity. Water standing on the soil surface can increase evaporation losses. Wet surface soil increases weed growth, changes the weed species mix, and delays access to the field. Slow infiltration is commonly associated with finetextured soils. However, slow infiltration can develop in sandy loam soils with low organic matter content and is a major problem for crop production in some areas of California’s San Joaquin Valley (Singer and Oster, 1984). Slow infiltration in medium and coarse-textured soils in California can be caused by restrictive layers at the surface such as crusts or seals, or below the surface such as compacted layers, pans, lenses, fine-textured strata, or cemented layers (Oster et al., 1992). It can also result from dispersion of the fine particles due to sodicity, or lack of sufficient divalent cations such as calcium, or from swelling of smectite clays with wetting. Various types of PAM have been used for over 50 yr to improve soil structure and permeability. Experiments conducted with high molecular weight PAMs at low concentrations in furrow irrigation water have been very successful in reducing irrigation-induced erosion (Lentz et al., 1992; Lentz and Sojka, 1994; Sojka et al., 1998a, 1998b). Several studies have noted increased infiltration rates when PAM is added to furrow irrigation water (Trout et al., 1995; Sojka et al., 1998a, 1998b), to sprinkler irrigation water (Levy et al., 1991; Bjorneberg and Aase, 2000; Bjorneberg et al., 2003), or sprayed on the soil surface (Flanagan et al., 1997; Green et al., 2000; Zhang and Miller, 1996). McElhiney and Osterli (1996) showed that PAM, applied to a fine-textured soil in the San Joaquin Valley, resulted in a 10 to 40% increase in infiltration rate. Because erosion is reduced and aggregates are more stable with PAM, deposition of low permeability surface layers is reduced. When PAM is used, the more porous structure of the surface soil is often visible. Although PAM does not increase soil permeability, it reduces the decline in permeability due to aggregate breakdown and seal formation (McElhiney and Osterli, 1996). Most of the PAM studies have been performed on clay loam or silt loam soils with low aggregate stability and on moderate to steep slopes where soil erosion is evident. The sandy loam soils in the San Joaquin Valley are generally on level or very low slopes and erosion is not evident, although surface seal formation has been proposed as a reason for low infiltration. Trout and Ajwa (2001) performed a series of field tests near Fresno, CA, on Hanford sandy loam soils to determine whether emulsion PAM addition to furrow irrigation water resulted in increased infiltration rates. Two series of field-scale trials in a furrow-irrigated vineyard and two series of recirculating infiltrometer trials in a fallow field using water of three ion concentrations (EC 5 0.03, 0.3, and 1.2 dS m) failed to show any increase in infiltration with PAM. In fact, the results from several trials (15 of 18 comparisons) indicated that PAM addition to irrigation water slightly decreased infiltration. Although the effect of PAM on the structure of the furrow perimeter was sometimes visible, very little erosion or H.A. Ajwa, Univ. of California-Davis, 1636 E. Alisal St., Salinas, CA 93905; T.J. Trout, USDA-ARS, 9611 S. Riverbend Ave., Parlier, CA 93648. Received 15 Mar. 2005. *Corresponding author (haajwa@