No abstract

Impact energy of water drops from overhead sprinkler irrigation can cause seal formation, and an increase in runoff and in soil erosion. The effects of low concentrations (5, 10, and 20 g m−3) of two polymers, an anionic polyacrylamide (PAM) and a cationic polysaccharide (PSD), on soil permeability and erosion from a grumusol (Typic Chromoxerert) and a loess (Typic Haploxeralf), were studied during five consecutive irrigations of 60 mm each. The polymers were added to the irrigation water during the first three consecutive irrigations, and thereafter the soils were subjected to two additional irrigations of water only. During the first three irrigations, the final infiltration rates (FIR) of the soils were significantly higher than those of the untreated samples (control). In the subsequent two irrigations with water only, the FIR values of the treated samples decreased to values similar to those of the control. The low residual effect of the polymers was explained by erosion of the thin treated layer and an insufficient amount of the polymers. A lower concentration of PAM (10 g m−3) was needed for optimal effect on the FIR and cumulative infiltration, compared with PSD (20 g m−3). For the optimal treatments, infiltration parameters were generally higher in the PAM‐ than in the PSD‐treated soils. Soil losses in all the PAM treatments were significantly lower than those in the PSD treatments. Both polymers stabilized soil aggregates, but PAM also cemented aggregates together and increased their resistance to erosion.

The effect of soil sodicity and phosphogypsum treatment on the structure of the crust of two soils (sandy loam and clay soil) was studied using a scanning electron microscope (SEM). The crust of the sandy loam with ESP 1.0, and the two soils at all levels of sodicity when treated with phosphogypsum, consisted of a compacted skin seal attributed to the mechanical impact of raindrops. Little clay movement or development of a “washed in” layer was observed under these conditions. Conversely, when the ESP > 1.6 (in the two soils) the crust consisted of naked sand and silt grains over a dense “washed in” layer. The compacted layer of disintegrated aggregates is not stable when soil and water conditions are favorable for clay dispersion (sodic soils and low electrolyte soil solutions). In soils with no exchangeable sodium, aggregate breakdown and surface compaction by raindrops are the dominant mechanisms for crust formation. In soils with ESP values above 1.5, chemical dispersion of the soil clays (enhanced by the sodicity of the soil, the low electrolyte concentration in the rain, and the stirring effect of the raindrops) and the downward movement of the clays into the “washed in” layer clogging the pores in this layer were the dominant mechanisms in crust formation. Soils with a “washed in” layer had a very low infiltration rate.

The effect of rain kinetic energy and soil amendments on infiltration and erosion from three Israeli soils was studied using a drip-type simulator. The soil samples were from the top layer (0-250 mm depth) of cultivated fields differing in their texture, specific surface area and lime content. Three kinetic energies of raindrops were obtained by varying heights of fall (h = 0.4, 1-0 and 1.6 m) of 3 mm diameter drops. The soil types studied were Typic Chromoxerert, Typic Rhodoxeralf and Calcic Haploxeralf. Soil amendments were phosphogypsum (PG) and a combined application of an anionic polyacrylamide (PAM) with PG. An increase in the impact energy of the raindrops reduced depth of rain before ponding, final infiltration rate (FIR), cumulative infiltration (i.e. infiltration parameters) and increased soil erosion. The addition of PAM+PG to the soil surface significantly increased the infiltration parameters and reduced erosion compared with the PG and control treatments. The Typic Chromoxerert was the least susceptible of the three soils to sealing, probably because of its high smectitic clay content, high specific surface area and high CaCO3 content which stabilize soil structure. The Typic Rhodoxeralf with the lowest specific surface area was the most susceptible to clay dispersion, and seal formation. Relative to the Typic Chromoxerert and the Typic Rhodoxeralf, the Calcic Haploxeralf was intermediate in its susceptibility to seal formation.

The possible use of sludge with high heavy metal concentrations and at high rates in calcareous soil was demonstrated in this study. Mixtures of two sludges were added to soils in various proportions up to 4% sludge content. One sludge was rich in Ni and Cd, while the other was relatively poor in heavy metals. Three soils varying in pH from 7.7 to 5.5 were tested. The concentrations of Cd, Ni, Cu and Zn in the DTPA and saturation extracts of the soil‐sludge mixtures were determined and correlated with their uptake by plants and the yield of Swiss chard (Beta vulgaris L., cv. Ford Hook Giant) grown on these mixtures. The metal‐poor sludge hardly affected the yield of the relatively salt‐resistant Swiss chard. The metal‐rich sludge reduced the yield drastically in noncalcareous soils after a critical amount of that sludge (1.5%) was added to the soils. Yet, even 4% of this metal‐rich sludge increased the yield of Swiss chard, as compared with the sludge‐free control in a calcareous soil.The best fit to yield was obtained by multiple regression with metal content in the soil saturation extract. The solubility in soil solution of Cd, Ni and Zn was strongly affected by the pH. The uptake of Ni and Zn by plants was significantly larger in the acid soil than in the calcareous soil. The difference in the uptake of Cd and Cu between the soils was smaller. Plant uptake of the metals was generally predicted better by the total metal addition or concentration in the DTPA extract than by metal concentration in the soil saturation extract. In noncalcareous soils the total metal addition correlated as well as metal content in the DTPA extracts with the metal concentration in the soil solution, with the uptake by plants and with the yield.It has been known for some time that heavy metals introduced as mineral salts do not exhibit in soil the same behavior as heavy metals released from the sludge itself. The heavy metal contents in the soils were varied, therefore, by mixing the two sludges rather than by adding mineral salts to one sludge. Results of the relatively recent studies in which spiked sludges were used should be interpreted with care.