Runoff and Interrill Erosion in Sodic Soils Treated with Dry PAM and Phosphogypsum

Seal formation at the soil surface during rainstorms reduces rain infiltration and leads to runoff and erosion. An increase in soil sodicity increases soil susceptibility to crusting, runoff, and erosion. Surface application of dissolved polyacrylamide (PAM) mixed with gypsum was found to be very effective in decreasing seal formation, runoff, and erosion. The objective of this study was to investigate the effects of surface application of dry granular PAM (20 kg ha) mixed with phosphogypsum (PG) (2 and 4 Mg ha) and that of PG alone on the infiltration rate (IR), runoff, and wash erosion from four smectitic soil types (ranging in clay content between 10 and 62% and sodicity level between exchangeable sodium percentage [ESP] 2 and 20) exposed to simulated distilled water rainstorms. Increasing ESP from 5 to 20 in the loamy sand decreased final IR from 14 to 2 mm h and increased runoff and wash erosion in the control; similar trends but of different magnitude were noted in the other soil types. Spreading PAM mixed with PG or PG alone was effective in maintaining final IR . 12 mm h, low runoff, and wash erosion levels compared with their control. Use of PAM mixed with PG resulted in higher final IR and lower runoff levels than PG alone in all four soils studied. Conversely, with respect to soil erosion, PAM mixed with PG was more effective than PG alone in reducing wash erosion from the loamy sand and clay and had comparable effects on soil loss in the loam. It was concluded that for rain-fed agriculture, spreading of dry granular PAM mixed with PG was more effective than PG alone in reducing runoff and erosion in soils varying in texture and sodic conditions. SEAL FORMATION at the surface of cultivated soils exposed to the impact of raindrops (i.e., structural seal as opposed to depositional seal formed by translocation of fine particles and their subsequent deposition [Arshad and Mermut, 1988], as often occurs in furrow/ basin irrigation [Kemper et al., 1985]), is a common phenomenon, particularly in arid and semiarid regions (Shainberg and Letey, 1984). Seal formation reduces soil IR (McIntyre, 1958), and increases runoff and erosion (Morin et al., 1981). Seal formation is due to two mechanisms: (i) physical disintegration of surface soil aggregates by rain wetting and drop’s impact and, (ii) a physicochemical dispersion of soil clays, which migrate and clog the pores immediately beneath the surface (McIntyre, 1958; Agassi et al., 1981). Aggregate stability increases with an increase in clay content, and therefore higher wetting rates and impact energies are needed to disintegrate aggregates of clay soils (Shainberg et al., 2003). Physicochemical clay dispersion is enhanced with the increase in soil ESP and the decrease in soil solution electrolyte concentration (Shainberg and Letey, 1984). Interrill soil erosion by rainwater is closely associated with seal formation. Erosion by water involves (i) detachment of soil material from soil mass by raindrop impact and/or runoff shear and (ii) transport of the resulting sediment by raindrop splash and/or flowing runoff. Raindrop detachment is greater than flow shear detachment because kinetic energy of raindrops is much higher than that of surface flow (Hudson, 1971). However, movement of detached soil down slope by rain splash is minimal, and most of the sediments are removed from the interrill area by runoff flow (Young and Wiersma, 1973); this type of erosion is termed “wash erosion.” Furthermore, under dispersive conditions (e.g., sodic soils and distilled water rain), runoff flow may be sufficient for soil detachment (Warrington et al., 1989). Sodic conditions reduce the value and productivity of soils (Sumner and Naidu, 1998). Accumulation of sodium in the soil solution and the exchange phase leads to deterioration of soil physical properties such as structural stability, infiltration rate, runoff, erosion, etc. (Shainberg and Letey, 1984). Many reviews have been published recently on the response of soils to sodicity and salinity (e.g., Sumner and Naidu, 1998; Levy, 1999). These reviews demonstrated that soil texture, clay mineralogy, and the potential of the soil to release electrolytes into the soil solution affect the response of soils to sodic conditions, and should be considered when sodic soils are investigated. Amendments like gypsum (or PG) and PAM have been used to prevent seal formation, runoff, and erosion (Agassi and Ben-Hur, 1992; Bryan, 1992; Ben-Hur et al., 1992a; Cochrane et al., 2005; Flanagan et al. 1997a, 1997b; Fox and Bryan, 1992; Miller, 1987; Shainberg et al., 1990; Shainberg and Levy, 1994; Yu et al., 2003). Gypsum is effective because on dissolution gypsum releases electrolytes into the rainwater (the electrolyte effect) and because dissolved Ca ions displace Na ions from the exchange complex—the reclamation effect (Keren and Shainberg, 1981). Keren and Shainberg (1981) found that PG was more effective than mined gypsum in decreasing clay dispersion and seal formation because of its higher rate of dissolution and the higher concentration of electrolytes in the soil surface solution during rainstorms. Z. Tang and T. Lei, College of Water Conservancy and Civil Engineering, China Agricultural Univ., Qinghua Donglu Rd, Beijing, 100083, PR China; J. Yu, Institute of Water Resources, Huhhot, Inner Mongolia, PR China; I. Shainberg, M. Ben-Hur, and G.J. Levy, Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization (ARO), The Volcani Center, P.O. Box 6, Bet Dagan 50-250, Israel; A.I. Mamedov, National Soil Erosion Research Lab., USDA-ARS-MWA; 275 South Russell St., West Lafayette, IN 47907 USA. Contributions from the Agricultural Research Organization, The Volcani Center, P. O. Box 6, Bet-Dagan 50-250, Israel. No. 622/2004 series. Received 19 Dec. 2004. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:679–690 (2006). Soil & Water Management & Conservation, Soil Physics doi:10.2136/sssaj2004.0395 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ESP, exchangeable sodium percentage; IR, infiltration rate; PAM, polyacrylamide; PG, phosphogypsum. R e p ro d u c e d fr o m S o il S c ie n c e S o c ie ty o f A m e ri c a J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 679 Published online February 27, 2006