Effects of long-term irrigation with reclaimed water on soils of the Northern Adelaide Plains, South Australia

Abstract

On a small scale, reclaimed water (RCW) use has been practised on the Northern Adelaide Plains (NAP) horticultural districts for more than 28 years. The RCW has had approximately 1.7 times the salinity and twice the sodium absorption ratio (SAR) of bore water commonly used for irrigation in the district. Recently, a large-scale reclamation scheme has been commissioned which could eventually supply approximately 30 GL of RCW to over 250 growers on the NAP. This study compared historical water quality and time of use data with physico-chemical properties of soil cores taken from sites where reclaimed (RCW-irrigated) or bore water had been used for irrigation, or sites that had not been irrigated (virgin). The aim was to determine if current farming practices irrigating with RCW could, now or in the future, lead to a decrease in yields through detrimental increases in soil salinity, sodicity, and boron (B) concentrations, and to determine if these changes were significantly different from bore-irrigated or virgin sites. Data suggested that changes in soil salinity and B concentration from RCW use would not decrease yields. However, changes in soil SAR had the potential to restrict drainage and consequently increase salinity; although a more functional critical SAR value for the NAP soils needs to be defined to assess this potential. These findings suggest that farming methods, in the 1967–95 period, did not address the physico-chemical changes associated with the use of more sodic RCW. Considering the future scale of RCW use, the SAR of the irrigation water may need to be decreased and/or appropriate farming methods developed and practised with the use of RCW to protect these soils for future horticultural activities. A low cost soil test, using a simple 1 : 5 soil : water extract was compared with accepted soil extracts (for assessing detrimental physico-chemical soil changes) and is proposed as a grower management tool to assist in monitoring the physico-chemical changes of the NAP soils.