Growth of Phaseolus vulgaris on brackish ground water using sub-surface membrane irrigation under variable climatic conditions

Abstract

A subsurface membrane irrigation was used to germinate and cultivate seedlings of Phaseolus vulgaris (common beans) on brackish water under variable climatic conditions. Changes in the air vapour pressure deficit (VPD), representative of increased accumulative evaporative demand between autumn and summer, from 14.8 to 21.5 kPa over a 15 day growth period, resulted in an increase average daily water uptake of 1.4 to 2.5 mm/d. Increasing irrigation water salinity from 0.2 dS/m to 1.4 dS/m and 1.4 dS/m to 3.1 dS/m reduced initial water uptake by a factor of 1.8 and 3.1 respectively indicating an imposed osmotic resistance due to salt retention. Plant water uptake after the establishment of a root network at 15 days after sowing was 36% higher for irrigation systems configured from the reverse osmosis (RO) membrane (flux 1.5 mm/d) compared to the forward osmosis (FO) at a salinity of 0.2 dS/m. The observed differences in flux can be attributed to distinct resistances imposed by the two membrane structures on water transport, although scanning electron microscopy (SEM) images also suggest an effect of membrane type on root structural development. More importantly, the use of both FO and RO resulted in a reduction of soil salt accumulation by 40% at the highest salinity of 3.1 dS/m, which reduced the projected decline in bean yield of 22.2% per dS/m to 5.6% per dS/m. This corresponds to an average increase in yield potential of 40% for cultivation of beans on irrigation water with a salinity of 3.1 dS/m. The paper demonstrates that subsurface membrane irrigation has the potential to accommodate the use of brackish water in a warmer and drier climate without compromising yield and exacerbating soil salinization.