Polyacrylamide sprayed on soil surfaces can stabilize soil aggregates

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-1, was applied as a sprayed solution to replicated plots of a Portneuf silt loam (coarse silty, mixed, mesic Durixerollic Calciorthid) near Kimberly, Idaho, on 25 July, 1995. A linear-move irrigation system, designed to deliver water to the soil surface at two droplet energy levels, 5 and 15 J kg-1, applied a total of 31 mm of water to the plots 7 and 10 days after treatment. Soil samples (0 to 5 mm depth) were taken before and 72 hours 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/drop impact better than non-treated aggregates. PAM applications at economic rates increased aggregate stability when droplet energy was 5 J kg-1 but had no effect when droplet energy was 15 J kg-1