Combatting soil salinity: Strategies and solutions for healthier soils

High salinity in soil can be caused by many factors - some more complicated than others.

Primary Salinity: Refers to naturally occurring salt deposits built up over time through geological processes such as the weathering of rocks. Over long periods, salt-water intrusion can occur in coastal areas from sea-level rise and rainfall (precipitation) deposits (Manchanda and Garge 2008).

Secondary Salinity: This is largely a result of human activities and occurs due to land clearing, excessive use of synthetic fertilisers, poor land management and inefficient irrigation practices (Mukhopadhyay et al. 2021).

Factors such as land topology, location and rainfall distribution are out of farmers' control, but can cause increased soil salinity. On the other hand, salinity can accumulate from poor farming practices. Often, significant salinity issues are the result of a combination of both.

Dry-land salinity: Land clearing for agriculture, involving the removal of native plants and their replacement with shallow-rooted crops for farming, leads to reduced water absorption by vegetation. This excess water drains through the soil into groundwater, raising the water table. The movement allows soluble salt to rise through soil horizons including the root zone (stunting crop growth) and up to the surface layer. When water evaporates into clouds, salt is then left behind in the soil (Salt and Water Quality, n.d).

Irrigation salinity: Similarly, poor irrigation systems can create a water surplus which drains into groundwater, raising the water table. This occurs because crops typically can't absorb the complete supply of irrigated water, and the resulting water surplus creates the same chain reaction seen in dry-land salinity (Salinity and Water Quality, n.d).

Although salt tolerance differs across plant species, the result is always the same: osmotic stress and the production of reactive oxygen species (ROS) (physiological byproducts of cells) (Manchanda and Garg 2008). This condition lowers nutrient absorption and increases the risk of ion toxicity which impacts plant growth and reduces crop yield (Shrivastava and Kumar 2015).

Additionally, because of the effects of high-salinity, areas where plants have died (bare soil) are vulnerable and subject to erosion. Over-watered (poorly irrigated) soils also increase the risk of runoff. This can cause significant environmental damage in coastal regions where fertiliser runoff can end up in river systems or flow into the ocean, impacting reef ecosystems.

Finally, salinity can cause smaller nuisances, such as corroded machinery, which takes time to resolve and places unnecessary financial burdens on farmers (Salinity and Water Quality, n.d).

There are many strategies used to combat soil salinity and they vary globally depending on a region's social, environmental and climatic factors. For example, management strategies will differ between dry-land and irrigated farmland. The following management strategies include a range of sustainable, best-practice farming techniques that can be trialled in different regions worldwide.

1. Efficient irrigation and drainage systems - Sustainable irrigation systems utilising clean water are considered the best solution for tackling soil salinity. From an environmental perspective, irrigation systems that reduce water consumption are pivotal in relieving water scarcity and breaking the cycle of soil salinity.

2. Chemical amendments and conditions - High-salinity soils create an imbalance in plant mineral absorption due to the overbearing presence of some minerals (Manchanda and Garg 2008). Zeolites and gypsum are commonly used in agriculture to alleviate this problem, as they have high ion exchange rates and absorb salts in high-salinity soils (Mukhopadhyay et al. 2021).

3. Crop rotation - Substituting saline-sensitive plants with more saline-tolerant species on a seasonal crop rotation basis is one possible solution for tackling soil salinity. Additionally, some plant species may help tackle this issue; while legumes are not particularly salt-loving plants, many can reduce soil salinity and pH while increasing nitrogen uptake.

4. Alternative composting - Using alternative compost has proven highly effective in reducing soil salinity in different regions worldwide. Lashari et al. (2013) demonstrated this positive change using biochar and poultry manure in combination with pyroligneous solution (acid created from the thermochemical breakdown of organic matter, typically wood). Soil salinity and pH decreased over the two-year experiment, and crop yield significantly increased (Lashari et al. 2013).

5. Agroforestry and vegetation restoration - As mentioned previously, secondary salinity can result from land clearing and substituting deep-rooted native plants with shallow-rooted crops. Re-introducing native plants to salinity-affected areas is a simple and highly effective way of mitigating salt accumulation on surface soil (Salinity and Water Quality, n.d).

Drip-Irrigation Practices Coupled with Crop Rotation and Bed Planting

The above explanations provide clues as to why traditional flood/furrow irrigation systems produce lower yields over time. In South Asia, many countries rely on rice as their main food source. In India, for example, roughly 70% of the water used for agriculture is used for cultivating rice (Mukhopadhyay et al. 2021). Traditional rice growing techniques are notorious for the demands placed on surface and groundwater, energy, fertilisers and labour. In an attempt to be more water efficient, Sandhu et al. (2019) converted a flat-planting, rice-wheat flood irrigation system to a bed-planting, maize-wheat drip irrigation system. Their aim was to enhance crop yield, nitrogen fixation and water efficiency over a two-year trial period in North-West India. During maize rotation, water productivity increased by 66% (88mm) and during wheat rotations, it increased by 259% (168mm) (Sandhu et al. 2019). Crop yield also increased for maize (13.7%) and wheat (23.1%) (Sandhu et al. 2019). Residue retention on permanent beds was also successful in increasing nitrogen uptake.

Sandhu et al.'s study was the first of its kind in South Asia and had groundbreaking results for an issue that confronts farmers globally. And while these results are not strictly related to reductions in salinity, this improved irrigation system subsequently reduces the rate and concentration of salinity reaching the root zone and surface soils.

Alternatively, Shrivastava and Kumar (2015) recommend using plant growth-promoting bacteria to enhance the resilience of crops under salinity stress. Crops grown on saline soils suffer due to high osmotic stress, nutritional disorders and toxicities and poor soil physical conditions.

'Crops grown on saline soils suffer due to high osmotic stress, nutritional disorders and toxicities and poor soil physical conditions.'

Shrivastava and Kumar (2015) explain that plants have naturally occurring endocellular and intracellular microorganisms such as rhizoplane, rhizosphere, endophytic bacteria and symbiotic fungi that support plant health. Microorganisms are essential because they initiate osmotic response, promote growth hormones and provide vital nutrients to plants. These functions ultimately increase crop growth and yield. Supplying plants with specific growth-promoting bacteria and fungi has the potential to alleviate salinity-induced stress as they replenish fixed nitrogen, phytohormones, iron and soluble phosphate and increasing the overall health and resilience of the plant (Shrivastava and Kumar 2015).

Soil salinity presents a significant challenge to agriculture, and the cumulative effects of conventional farming practices, climate change and food security only compound the pressure on farmers. But, an increasing number of practical and successful management strategies are available for combatting soil salinity. Though each agricultural region is unique, a rapidly increasing portfolio of solutions is available worldwide for reducing soil salinity and creating healthier soils for plants to thrive.