Water availability in different soils in relation to hydrogel application

A study was conducted on PVC columns in the laboratory to investigate the effect of the application of hydrogel in modifying the hydraulic properties of different soil types. The main treatments consisted of four different soil types, namely sand, alluvial sandy loam, red sandy loam and black clay. The sub-treatments consisted of three levels of gel application: 0.7%, 0.5% and 0% (by weight). A given level of gel was mixed in the top 10 cm-layer of soil. In all soil columns, 5 cm of water was applied, and soil samples were taken with a core auger on the fourth, seventh, 14th and 22nd days after watering (DAW). The undisturbed soil of the rings of the core assembly was used for preparing the soil water retention curve, and the soil of the core was used for determining saturated hydraulic conductivity (Ks), bulk density and gravimetric soil water content. The results of measuring soil water content (SWC) during drying revealed that in sand treated with a 0.7% gel application, the water release rates were relatively uniform throughout the entire drying period, whereas in black clay, water release rates were very high initially (4–7 DAW) but fell appreciably afterwards (less than 0.5% per day). The result of the soil water characteristic curve revealed that water release per unit suction change in the 0–10 kPa range (unavailable to plants) in soil samples not treated with gel was higher compared to soil samples treated with gel, except in alluvial soil, for which rates were similar. However, the water release per unit suction change in the 10–100 kPa range (available to plants) in soil samples not treated with gel was significantly lower compared to that in soil samples treated with gel for all soil types. The above results suggest that gel significantly improved the readily available water capacity (RAWC) of the soils. Furthermore, the exceptionally low value of the water released per unit suction change pattern in all soil types for soil water suction of > 100 kPa indicated that 100 kPa can be considered the critical limit of soil water potential from the water availability standpoint. The efficacy of the gel in improving RAWC was confirmed by the increased midpoint moisture (MPM), reduced median pore diameter (dm) and moisture release index (Im). The time at which a critical SWC (SWC corresponding to 100 kPa) was reached was studied in order to further examine the suitability of gel in improving soil water retention in different soil types. The drying pattern of soils highlighted that under low evaporative condition, the critical SWC with the 0.7% gel treatment reached approximately 7, 14, 22 and 4 DAW in red sandy loam, alluvial sandy loam, sand and black clay soils, respectively. The above-mentioned results thus reaffirmed the suitability of gel for sandy soil because it improved the water availability of the sandy soil for a longer period (nearly 22 days, which corresponds to the irrigation interval of most agricultural crops), while gel was found unsuitable for black soil, in which the critical soil water content was reached early (4–7 days).