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Soil Salinity and Field Flooding
Soil salinity refers to the total salt concentration in the soil solution, the soil water consisting of soluble and readily dissolvable salts. All soils contain some salts through naturally occurring processes such as weathering of rocks, seawater intrusion, and atmospheric deposition.
Soil salinization is the accumulation of soil salinity and is a consequence of various natural and anthropogenic (human-related) processes. In irrigated fields, the primary sources of salts are irrigation water, chemical fertilizers, and animal wastes.
The most common driver of the salinization mechanism in these soils is water loss through evapotranspiration (i.e., combined processes of evaporation from the soil surface and plant transpiration) selectively removes soil water, leaving salts behind.
Soil salinity can reduce plant growth, lower yields, cause crop failure, reduce soil health, permeability, and water holding capacity, and increase water runoff and soil erosion. Globally, around 62 million ha, which is 20 % of irrigated soils, are affected by salinity (Qadir et al., 2014). The issue of salinity is an expensive one, costing around US$ 27.3 billion for 2013 (Qadir et al., 2014).
Salinity affects almost all aspects of plant development, including germination, vegetative growth, and reproductive development (Shrivastava and Kumar, 2014). Soil salinity imposes ion toxicity, osmotic stress, nutrient (N, Ca, K, P, Fe, Zn) deficiency, and oxidative stress on plants. Soil salinity significantly reduces plant phosphorus (P) uptake (Bano and Fatima, 2009).
Some elements, such as sodium, chlorine, and boron, have specific toxic effects on plants. Excessive sodium accumulation in cell walls can rapidly lead to osmotic stress and cell death (Munns, 2002). Salinity also affects photosynthesis mainly through a reduction in leaf area, chlorophyll content, and stomatal conductance (Netondo et al., 2004).
Flood irrigation practices use large amounts of water, which bring many salts into the fields even if salt concentration is low in the water (e.g., surface water).
Also, there is a considerable time interval between irrigation events in which soil dries, concentrating the salts and adversely affecting crop yields. When combined with shallow groundwater, the field can become barren, drinking water quality can deteriorate, and soil erosion increased and might create a “dust bowl.”
Overall, it is essential to note that increased salinity has direct effects on growers in the form of:
- Reduced field productivity
- Reduced farm income
- Limited options for crop production
- A decrease in water quality
- A decrease in land value
- Reduced livestock health
By adding to the list the range of impacts on the environment, such as the decline of vegetation, loss of nesting sites, drinking water quality decline, and soil erosion, one can see an immediate need to replace flood irrigation practices with more sustainable ones.
Therefore, the adoption of smarter irrigation methods and appropriate water and soil management practices must be implemented. N-Drip irrigation system reduces irrigation amounts by over 50%, reducing the number of salts introduced into the field by the same value. Frequent irrigation using the N-Drip system lowers the oscillation in water content and reduces the concentration effect of evaporation in the salts. More efficient fertigation can reduce fertilizer input into the field, lowering even further salts input.
Written by Sharon Dabach, Ph.D. Soil & Water Sciences.
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