Excessive soil salinity has become a severe problem for growers in several parts of the U.S. including Florida, Texas High Plains/Rio Grande and California Central Valley. Successive years of drought have forced growers to rely on irrigation water, which can contain salts. If the salts are not leached out of the soil by rainwater or other clean water source, then the salts accumulate, and salinity increases.
Soil salinity can affect crops in three ways; the salty soil draws water from the plants through osmosis, making it difficult for plants to uptake water, increased levels of sodium in particular can alter the structure of the soil, causing the soil surface to transform from flocculated to dispersed, forming a barrier to water infusion, and last there can be specific ion toxicity with certain crop plants and certain soils.
Common salts in soil
Elements and compounds that commonly exist in soil as ions or mineral salts include sodium, calcium, magnesium, potassium, sulfate, chloride, carbonate and bicarbonate. Sodium can be a particularly difficult issue, as an ion it is not soluble, and will not leach out with application of clean water. When a soil has too much sodium it is sodic soil.
Some crops are less susceptible to soil salinity issues, notably barley and cotton. Others such as fruits and nuts can be very sensitive. That said, when salinity gets too high, all crops experience some level of yield loss. Without remediation, the soil may become too salty to farm. These fields go fallow, or in some cases are reverted back to grassland.
Managing sodium in soil
With careful management, soil salinity can be remediated to the point where some crops may be planted. For sodium, this can include the injection of gypsum, which degrades to calcium. Calcium displaces sodium, and initially increases salinity, but calcium also encourages soil flocculation, which helps water added to penetrate. Where it is known that the soil contains lime, sulfur may be added, which forms sulfuric acid that breaks down the lime, freeing calcium to displace the sodium. Remediation strategies require good knowledge of soil chemistry, and are often managed by professionals who assist growers with particular issues. An under-developed plan for solving a soil salinity issue can create another. Sodic soils that have formed a dispersed “crust” near the surface may require tilling to break up the soil for further remediation.
Cover-cropping for managing soil salinity
Cover cropping can be helpful in managing soil salinity. Where the water table is high, they can draw water from the soil which can lower the water table, carrying salt away from the surface. Some crops will do better with lower salt just after germination/emergence, later in the growing cycle they can tolerate more salinity. Cover crop roots can create channels in dispersed soil, allowing water to percolate downward. And cover crops add nutrients to the soil as an additional benefit.
Soil leaching remediation methods
All soil salinity remediation plans ultimately involve leaching mineral salts down past the root zone through the addition of clear water from the surface. A good guide calls for 6 inches of clear water percolated down through about 30 inches of soil. While the concept is clear, it can be difficult for the grower to know that the additional water has really penetrated the prescribed 30 inches. The grower can be tempted to add more, potentially wasting vital water.
Nitrate leaching risks
One risk of leaching to remove salt is that soluble nitrates, originally added as chemical fertilizer, can also leach down into the water table after harvest when there is no crop to absorb it. Nitrates pose a health risk in drinking water, and the introduction of excess nitrate into a river or lake can cause eutrophication, the result of a harmful algae bloom where oxygen is removed from the water, resulting in a decrease in the population of fish and invertebrates, possibly even their demise. To control nitrogen, its use should be limited to what is necessary, and a legume cover crop such as field pea can be effective at scavenging nitrogen from the soil.
Increasing soil salinity is a challenge for many growers. Effective management of soil chemistry, clear water leeching and cover cropping to control excess nitrates can keep fields productive. AquaSpy Crophesy is an excellent tool for monitoring soil moisture and water uptake by growing roots, as well as EC for tracking salinity.
Soil monitoring technology from AquaSpy
Soil monitoring technology from AquaSpy can track moisture every 4 inches down to 48 inches and provide soil salinity data every four inches by measuring electrical conductivity (EC). With AquaSpy Crophesy™ wireless probes, the grower can accurately track the effect of an irrigation or rain event to ensure that the leaching exercise completed successfully.