Can soil crusting be reduced through application of gypsum, organic waste, and phosphoric acid?

Soil crusting is a form of land degradation in which the breakdown of aggregates results in the formation of a thin impermeable layer on the soil surface. An earlier pilot trial indicated that application of paper waste, gypsum, phosphoric acid, and covering the soil surface with wire mesh showed potential for reducing soil crusting. This study was established to evaluate the use of products for reducing the severity of soil crusting, while also testing different approaches for measuring the severity and likelihood of soil crust formation. Gypsum was applied at 0.25 and 0.50 kg m^–2 (low rate [LG] and high rate [HG], respectively), paper waste was applied at 1, 2.5, and 7.5 kg m^–2 (low [LPW], moderate [MPW], and high [HPW] rates, respectively), and phosphoric acid was applied at 80 and 160 mL m^–2 (low rate [LP] and high rate [HP], respectively). Combinations of these products were made including (1) wire mesh (WM) and 0.50 kg m^–2 gypsum (WM + HG); (2) 0.50 kg m^–2 gypsum and 80 mL m^–2 phosphoric acid (HG + LP); (3) 2.5 kg m^–2 paper waste, 0.50 kg m^–2 gypsum, and 80 mL m^–2 phosphoric acid (MPW + HG + LP); and (4) 7.5 kg m^–2 paper waste and 160 mL m^–2 phosphoric acid (HPW + HP). The likelihood of crust formation was inferred from aggregate stability determined by rainfall simulation and wet sieving, while the severity of soil crusting was inferred from crust density, hydraulic conductivity, and penetration resistance. The four measures of crust severity/likelihood were highly correlated with each other (R² = 0.57 to 0.80). The HPW + HP, MPW + HG + LP, and MPW treatments increased hydraulic conductivity by 72%, 66%, and 45%, respectively; increased aggregate stability determined by rainfall simulation by 28%, 37%, and 39%, respectively; reduced surface density by 10%, 7%, and 6%, respectively; and reduced penetration resistance by 33%, 37%, and 34% as average at all five sampling dates (days 8, 14, 28, 71, and 197). Moreover, the high rate of gypsum significantly reduced bulk density by 7% and penetration resistance by 26%, yet had no effect on any other measure of crusting. Phosphoric acid (HP) significantly increased aggregate stability determined by rainfall simulation by 29% (days 8, 14, 28, and 71), reduced bulk density by 6% (days 8 and 14), and increased hydraulic conductivity at day 8 by 110%. Reduced severity and or likelihood of crust formation following application of gypsum and paper waste were attributed to the increase in calcium cations (Ca⁺²) and soil organic carbon (C). The paper waste and gypsum were the most effective amendments over the duration of the trial, while phosphoric acid reduced the severity of crust formation in the 14 days after application. Recommendations are provided on the efficiency of different approaches for measuring soil crusting, in which penetration resistance is preferable because of its high correlation with other measurements and being the least time consuming.