Submitted to: International Erosion Control Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: January 2, 1996
Publication Date: N/A
Interpretive Summary: Vegetation on or near the soil surface protects the soil from the impact of raindrops or sprinkler drops. On bare surfaces, however, drop impact and wetting can cause soil aggregates, clusters of sand, silt, and clay bound by organic and inorganic constituents, to break apart and form a less permeable seal and, later, a crust on the soil surface. This seal increases runoff and erosion, and the crust can decrease seedling emergence. Polyacrylamide or PAM, an environmentally benign, long- chain, synthetic organic polymer with a slight negative charge, if sprayed on the soil surface, may strengthen soil aggregates so that they won't fracture easily. In a field experiment, we sprayed two economic rates of PAM on a soil with relatively weak aggregates and later irrigated the soil with a lateral-move irrigation system, modified to spray drops at a low and high droplet energy. Before and after the irrigation, we sampled the soil aggregates from the uppermost 5 mm of our plots. We sieved these aggregates we sieved in water to measure their stability. PAM-treated aggregates were more stable than untreated aggregates when droplet energy was low but were only equally as stable when energy was high. If droplet energies are low, PAM sprays may protect soil surfaces. More important, however, is to prevent high energy droplets from impacting aggregates at the soil surface. These soil surfaces could be protected by, for example, adding crop residue or establishing vegetative cover on the soil.
Technical Abstract: Vegetative cover protects soil surfaces and aggregates from erosion caused by the impact of raindrops or sprinkler drops. On bare surfaces, drop impact and wetting causes soil aggregates to slake, releasing aggregate fragments and/or primary particles that can obstruct pores at the soil surface and form a depositional seal. The seal reduces infiltration which, in turn, increases runoff and soil erosion. Polyacrylamide (PAM) applications, known to stabilize surface soil in irrigated furrows, may effectively stabilize soil aggregates as well. This field experiment evaluated the effects of spray-applied PAM and sprinkler droplet energy on surface soil aggregate stability, measured before and after 31 mm of irrigation. A moderate charge-density, anionic PAM at two nominal rates, 10 and 25 kg/ha, was applied as a sprayed solution to replicated plots of a Portneuf silt loam (coarse silty, mixed, mesic Durixerollic Calciorthid) near Kimberly, ID, on 25 July 1995. A lateral-move irrigation system, designed to deliver water to the soil surface at two droplet energy levels, 5 and 15 J/kg, applied a total of 31 mm of water to the plots seven and ten days after treatment. Soil samples (0 to 5 mm depth) were taken before and 72 h after this water was applied. Gravimetric water content and aggregate stability by wet sieving were measured on these samples. Initially, PAM-treated aggregates visually appeared to resist breakdown under sprinkler droplet impact better than non-treated aggregates. PAM applications at economic rates increased aggregate stability when droplet energy was 5 J/kg but had no effect when droplet energy was 15 J/kg.