Can salt water be used for irrigation?

Saline water is water that contains a high concentration of dissolved salts. The salinity of a sample can be measured by electrical conductivity and is given in the units, deciSiemens per metre (dS m^-1).

In most countries across the globe, freshwater is used for agricultural purposes. This has led to the overexploitation of groundwater resources in some countries, resulting in the intrusion of saline water into freshwater aquifers. This makes the water unsuitable for human consumption thus, placing restraints on the amount of fresh water available for domestic use.

Saline water often has negative impacts on the yield of crops due to the osmotic potential being lower in the soil solution. This prevents water uptake by plant root cells, therefore inhibiting maximum growth. Salinity is often a major issue in hot, dry regions such as the Sahel Region, where salt accumulates as a result of evaporation. An arid climate paired with a lack of rainfall will lead to the build-up of salts, known as secondary salinisation [Karlberg, 2005]

Despite negative impacts, research has been undertaken to investigate the possibilities of using saline water for irrigation in Africa. To use saline water, investment into specific technologies and management procedures must be made. According to Karlberg, 2005 and Selim et al, 2019, drip-irrigation is the most effective method of irrigation in comparison to furrow irrigation and sprinkler irrigation. The benefits of drip irrigation include high water application efficiency and high frequency of water delivery [Karlberg et al, 2007]. Karlberg states that drip-irrigation is 80%-90% efficient in water application compared to surface irrigation schemes which only have an efficiency of 50%. In addition to this, the high frequency of water delivery prevents the dehydration of soils thus, preventing fluctuations and peaks in the soil salt concentration.

Research conducted by Karlberg et al, 2007 on the use of saline water drip-irrigation in southern Africa, shows that the yield of crops can be increased with the use of saline water. Three different levels of saline water (0 dS m^-1, 3 dS m^-1,6 dS m^-1) were used to test the effect on the growth of tomatoes. The results showed that even with  the most saline water sample (6 dS m^-1), the yield was significantly greater than the average marketable yield for South Africa of 31.4 Mg ha^-1. Furthermore, the addition of plastic mulch to soils prevented evaporation of water from the soil, increasing yields further to an average of 75 Mg ha^-1.

Other research shows that optimal yields can be obtained when the salinity level is 11 dS m^-1. This means that drip-irrigation technologies can be utilised in countries such as Tunisia where groundwater salinity levels are below 3 dS m^-1 [Selim et al, 2019].

In conclusion, with the correct technologies, saline irrigation is an ideal method of irrigation as not only does it increase crop yields, it increases the availability of fresh water for domestic consumption. This is especially beneficial in Africa where 85% of its inhabitants rely on agriculture to make a living and freshwater is scarce [You et al, 2010]. Nevertheless, various crops respond to and uptake saline water differently, meaning irrigation mechanisms must be suited to the crop planted. As a result of this, initial set-up costs may be high, causing farmers to produce cash crops to make a profit. Small-holder farmers may, however, be able to access simple drip-irrigation kits that are affordable for small garden size plots.