Improved method for assessing rice soil suitability to restrict recharge

Groundwater recharge below ponded rice fields is a significant contributor to rising groundwater levels and the threat of soil and water salinisation. Reduced groundwater recharge is essential for sustainable rice based, irrigated farming systems in southern Australia. Currently a soil is deemed as suitable for rice production if 1 soil profile per 4 ha contains 2 m or more of continuous medium or heavy clay textured material. However, this study showed clay content to be a poor predictor of groundwater recharge. Electromagnetic induction surveys of rice fields can show variation in soil physico-chemical properties across rice fields. This allows the delineation of distinctly different areas of a field based on EM readings and allows accurate targeting of soil sampling and measurements. This study used soil chemical and physical properties and groundwater recharge data to assess the adequacy of the current rice soil suitability classification method. A 3-stage classification scheme of rice land suitability is proposed including EM31 soil mapping and measurement of soil sodicity as key components. The proposed scheme is as follows: (i) include the soil as suitable if the EM31v measurement is equal to or greater than 150 mS/m; (ii) if EM31v <150, include the soil if the ESP of the top 60 cm of the soil is greater than 6, or the ESP of the depth interval between 60 and 150 cm is greater than 12; (iii) all other soils are excluded from ricegrowing. The proposed scheme's ability to correctly identify low permeability soil in rice fields as suitable for rice growing was confirmed by the recharge values measured in these fields.