A Study on Tensiometer Measurements in Salt Laden Soil Used for Irrigation Scheduling

Tensiometer is one of the most popular and simple instrument for in situ and laboratory measurement of negative pressure or suction in soils. Such measurements in the top soil or root zone are useful for automated optimized irrigation scheduling for different vegetation. The top soil accommodates various salts/contaminants whose main source is storm water runoff, irrigation water and application of crop fertilizers. These salts accumulate in the soil due to evaporation. It is an established fact that tensiometer measures only soil matric suction with the help of a ceramic interface. It is believed that osmotic effect of salts present in the soil does not influence tensiometer measurements. However, there is no systematic experimental verification in the literature to understand the influence of salt laden soil on tensiometer measurements. It needs to be ascertained that the pore size of the tensiometer ceramic will not exhibit semi-permeable membrane characteristics leading to osmotic effect. Such verification is important as it would influence the irrigation scheduling of crops. It is also of interest to know whether the osmotic suction (due to the salts) would increase the water retention characteristics and hence the irrigation requirement. If so, then the tensiometer based irrigation scheduling would be an underestimate of the actual irrigation requirement of crops. To investigate these issues, the present study has performed controlled laboratory suction measurements using tensiometer in a locally available soil, contaminated with known concentration of inorganic salt solution. The total suction of the same salt laden soil has been measured using the WP4 dew point potentiameter technique. The results indicate negligible influence of salts present in the soil on tensiometer measurements. The study demonstrates the incapability of tensiometer to assess the osmotic component of salt present in the soil. This would result in the under prediction of water retention and hence irrigation requirement of the soil.