Effect of exchangeable sodium percent on surface sealing
Abstract Laboratory studies were conducted to evaluate the effect of exchangeable sodium percent (ESP) on soil surface sealing and of surface sealing on infiltration and evaporation processes. Surface seals with hydraulic conductances 52.20 × 10 −3 , 49.20 × 10 −3 , 39.00 × 10 −3 , 33.00 × 10 −3 , and 17.91 × 10 −3 h −1 were attained on top of a sandy loam soil with ESP of 4 by placing a 5-mm layer of the same soil with ESP's 6, 10, 20, 30, and 60 respectively. Surface sealing increased with increasing ESP and appeared complete at ESP 30. Evaporation remained unaffected by the seals. Infiltration, however, was considerably reduced. Soil water profiles, which largely depended on the amount of infiltrated water, tended to become uniform. Equal moisture contents were also observed under evaporation.
Soil Sealing, Infiltration and Runoff
Arid zones are characterized by vast areas of bare soils, low annual precipitation, but high rainfall intensities with high kinetic energy. Desertification of less arid regions may also be a result of overgrazing and exploitation of vegetation beyond the land’s productive potential. Bare soils exposed to rainfall are subjected to physical and chemical processes which change the properties at the vicinity of the soil surface. When dried, a hard layer is formed in the soil surface which is viewed as a “crust”. This phenomenon, when disregarded, may be harmful to agriculture. It decreases the infiltration rate, reduces the available water at the root zone, increases runoff and soil erosion and affects seedlings and plant growth. Duley (1939) reported that raindrop impacts on the bare soil surface are the cause of such a low permeability crust. Hoogmoed and Stroosnijder (1984) studied the semi-arid region of the Sahel in Central Africa and concluded that it was very sensitive to crust formation and that, on average, 25% of the rain is lost by runoff. They also noted that on untilled soils, the presence of a crust is a permanent feature. The crust formation, which limits the cumulative infiltration of valuable water to the root zone, induces in fact a vicious cycle since it affects vegetation cover, thus leaving the soil surface bare and exposed to rainfall.
Soil crust strength measurement
Abstract Modulus of Rupture measurements on 8 soils treated and untreated with soil conditioner showed that conventional MOR measurements are not appropriate for measuring the strength of rain‐induced soil crusts, because the formation of the crust imposes certain modifications in the method. A modified method for MOR measurements was compared with two other methods of measuring soil crust strength. Results from all three methods correlated well with each other, but gave a correlation of only approximately 0.6 with emergence of turnip seedlings. The disadvantages of the MOR and Fishing Line methods are discussed, and the use of the Shear Vane Penetrometer proposed.