Polymer effects on surface mechanical strength of a crusting loessial soil

Abstract

Addition of synthetic polymers to crusting soils generally improves seedling emergence and this is thought to be caused by lower mechanical strength of the crusted soil surfaces. The objective of the present study was to evaluate the mechanism by which polymer addition contributes to the reduction in crust mechanical strength. Three polymers were used, 2 anionic polyacrylamides differing in their molecular weight, and a cationic polysaccharide. Samples of a silty loam calcareous loess (Calcic Haploxeralf) were packed to a depth of 78 mm in round pots having an internal diameter of 120 mm at the top. Polymer was added to the soil surface, at a rate of 20 or 40 kg/ha, by spraying a concentrated polymer solution. The pots were then exposed to 40 mm of simulated rainfall comprising either distilled water or tap water (electrical conductivity 0·97 dS/m). In an additional study, polymer was added to the tap water to form a dilute (20 or 40 g/m3) polymer solution which was then used in the 40-mm rain event. After the exposure to rain, the pots were transferred to a temperature-controlled room (30°C). Weight of the pots, moisture content of the upper 10 mm of the soil, and crust strength (impedance) determined with a 3·24-mm diameter probe were measured periodically up to 95% water loss in the soil. In most of the polymer treatments, moisture content immediately after the rain was in the range of 32–39%, which was significantly higher than that in the control (27–29%). In general, for a given moisture content, crust strength did not differ significantly between the control and the polymer treatments. However, moisture content for a given cumulative drying time was higher in the polymer treatments than in the control, indicating that polymer application delayed crust drying and maintained a crust with a lower mechanical strength for a longer period of time. Polyacrylamide with a low molecular weight, applied at a rate of 40 kg/ha, emerged as the most effective treatment for maintaining high moisture content in the upper soil layer, and hence a crust with a low mechanical strength.