Chapter 5: Discussion
5.1 Salts
The relatively high concentration of dissolved salts in BE is a concern when using BE in irrigated crop production systems. The electrical conductivity (EC), sodium absorption ratio (SAR) and sodium concentration all increased in all the soils irrigated with BE. This resulted in a decreased osmotic potential between the soil water and root plasma, meaning that plants spent more energy on obtaining water and less on growth (Munns & Termaat 1986, Jacoby 1994). Therefore the BE treatment process that resulted in the lowest EC should be
151 the most suitable for crop irrigation. Post-AD and post-PFP BE were measured with the lowest EC values, and the lowest concentrations of sodium, which identified them as being the most suitable for irrigated crop production systems. Evapotranspiration during the HRAP and CW treatments processes increase the sodium concentration of BE, thus decreasing its suitability for irrigated crop production.
Sodium enters root cells passively through K+ transporters in the root plasma membrane via an electro-chemical gradient (Blumwald et al. 2000, Apse & Blumwald 2007). However the efflux of sodium out of the cell is an active process (Blumwald et al. 2000, Apse & Blumwald 2007). This means that plants irrigated with BE had to spend more energy on
osmoregulation and sodium efflux, which in turn compromised plant growth. Sodium extrusion is mediated by the plasma membranes H+- ATPase which pump out H+ generating an electrochemical gradient H+ gradient which allows the Na+/H+ antiporters to couple the movement of H+ into the cell with the extrusion of Na+ (Hassidim et al. 1990, Blumwald et al.
2000). An increase in extracellular H+ has been shown to increase the efflux of sodium through the plasma membrane (Nass et al. 1997, Blumwald et al. 2000). The pH adjustment of BE decreased the sodium leaf content of cabbages, saltbush and lucerne. This could have been due to two reasons; firstly the addition of H+ would decrease the ratio of Na/positively charged ions thus, decreasing the sodium electrochemical gradient between the root
plasma and extracellular water; and secondly, the addition of H+ may enhance sodium efflux. Future research should investigate this to determine how pH affects the sodium tolerance and sodium efflux of plants.
Sodium has also been shown to effect the fertility of the soil (Agassi et al. 1981, Qadir &
Schubert 2002). The addition of sodium ions to the soil increases the Na/plant nutrient
152 cation ratios which has been shown to decrease the uptake of certain plant nutrient cations such as K+ (Maas & Grieve 1987, Alam et al. 1989, Ehret et al. 1990). Plants grown in a high saline environment often display nutrient cation ion deficiencies, of elements such as Ca, K and Mg, (Maas & Grieve 1987, Alam et al. 1989, Ehret et al. 1990). Post-AD and post-PFP irrigated plants did not show signs of nutrient deficiencies but after prolonged irrigation the sodium may build up in the soil to levels which cause sodium induced nutrient deficiencies.
Sodium also affects the dispersion and flocculation of soil aggregates (Agassi et al. 1981, Qadir & Schubert 2002). Irrigation with waters with a SAR above 9.0 cause soil dispersion, which is accompanied by a deterioration in the hydro-physical properties of the soil (Buckman & Brady 1967, Khouri et al. 1994, Miller & Donahue 1995). The physical properties of soils irrigated with post-AD and post-PFP BE did not show signs of deterioration. However, the sodium concentration of the soils increased, and after
prolonged use of BE irrigation, the build-up of sodium in the soil will be accompanied by a reduction in the stability, rainfall infiltration rate, and porosity of the soil, as observed by numerous authors (Agassi et al. 1981,Abu Sharar et al. 1987, Muyen et al. 2011, Kumar &
Chopra 2012, Dakoure et al. 2013).
The accumulation of sodium in the soil from irrigation with treated wastewaters is the major limitation to using wastewater in agriculture and practices need to be developed in reduce the accumulation of sodium in soils (Qadir et al. 2003, Muyen et al. 2011). The key to the successful use of saline irrigation waters depends on the following: cultivation of salt and sodium tolerant crops; adequate leaching of sodium while avoiding deterioration of the soils physical profile; and the integrated use of saline and non-saline water (Qadir & Oster 2004).
Certain crop species have been shown to aid in the removal of sodium from the soil (Qadir
153 et al. 2001, Qadir et al. 2005).It is therefore important that crop species are identified that can reduce the build-up of sodium in the soil.
None of the crops grown in the crop suitability experiment (Chapter 3) were able to stop the build-up of sodium in the soil. They were able to reduce the build-up when compared to soils irrigated with No Crops. The sodium content in the soil was not related to the sodium content and growth of the crops. This suggests that there are probably other plant-soil interactions that aided in the removal of sodium from the soil, such as plant assisted leaching (Chaudhri et al. 1964, Gritsenko & Gritsenko 1999, Owens 2001). Sodium is removed from the soil through two processes; firstly sodium is assimilated into the plant tissue thus reducing the salt content of the soil (Chaudhri et al. 1964, Gritsenko & Gritsenko 1999, Owens 2001). Secondly, “the ability of plant roots to increase the dissolution rate of calcite, resulting in enhanced levels of Ca2+ in soil solution to replace Na+ from the cation exchange complex” (Qadir et al. 2005). Qadir et al. (2003) found that soil sodium removal by lucerne plant tissue accounted for less than five percent of the total sodium removed from the soil. The majority of the sodium was removed from the soil through leaching (Qadir et al. 2003). The leaching of sodium through the soil was higher in lucerne planted soil compared to unplanted soil (Qadir et al. 2003). Respiration from roots and associated microorganisms released CO2 into the soil water which forms carbonic acid. The generation of H+ from carbonic acid dissociation can then react with soil calcium carbonate to produce Ca2+ and HCO3-. The Ca2+ can now exchange with sodium at the soils cation exchange sites resulting in sodium ions being leached out in the soils percolating water which in turn reduces the soils sodium content and SAR (Qadir et al. 2003, Qadir et al. 2005). The addition of H+ in pH adjusted irrigation treatments increases the dissolution rate of calcite in the soil and thus increased the levels of Ca2+ in soil solution that can replace Na+ from the cation
154 exchange complex. Acidic irrigation waters should increase sodium leaching through the soil due to the addition of H+ which can react with calcium carbonate. Future research should be done to determine the effect of acidic irrigation waters on the leaching rate of sodium from soils as this may aid in the reclamation of salt affected soils.
Saline water can successfully be used for crop irrigation provided proper management strategies are put in place to reduce the negative impacts it has on the soil (Murtaza et al.
2006). These include the cyclic use of non-saline water and saline water in crop production systems, cultivation of crops that aid in the assimilation and leaching of soil water sodium and the addition of gypsum to the soil or irrigation water (Qadir et al. 2001, Murtaza et al.
2006). Future research needs to investigate the use of BE in soil crop production systems where these management practices are optimised to ensure sodium does not build in in the soil to levels where it becomes detrimental to the soil’s fertility and impacts upon crop yields.