Modification of hydraulic properties of a semiarid soil in relation to seasonal applications of sewage sludge and electrolyte-producing compounds

A field experiment was carried out to study the effects of sewage sludge and three chemical amendments (gypsum, cement dust, and ground phosphate rock (P.rock)) on volumes of runoff generated from 15 treatment plots, each of 2 × 2 m² (three randomized complete blocks each with one control and four treatment plots). The experiment was carried out in the Muaq'qar Experiment Station (Jordan) for five successive winter seasons (87/88 to 91/92). The surface soil (fine, mixed, thermic Typic Calciorthid) was annually mixed to 5 cm depth with sewage sludge (20 tons/ha) or the respective chemical amendment (500 kg/ha) before the beginning of each winter season. The volume of runoff water generated from each plot was measured following rainstorm events of known depth, intensity and duration. Results of multiple linear regression analysis indicated that runoff volume depended mainly on rainfall depth and to a lesser extent on rainfall intensity. Regression coefficient of the runoff-rainfall relation increased and, consequently, structural stability of the soil surface decreased in the following order: sewage sludge > gypsum > cement dust > P.rock > control. In addition, infiltration rate (IR) of the treatment plots decreased in a similar manner. The final IR for the sewage sludge- and gypsum-amended plots was about 10 and 5 times greater than that of the control plots (3.97 ± 0.68 mm h⁻), respectively. Decreasing structural stability of the chemically-amended plots conformed to the decreasing solubility of the chemical amendments and, thus, showed the role of electrolyte concentration in preventing surface crust formation. Structural improvement of the sludge-amended plots was due to the increase in both soil salinity and organic matter content.