Intensive watering practices are a major concern for producers in areas where irrigation water contains elevated levels of salt.
By Dr. Josh Lofton
Growers throughout the Mid-South have grown accustomed to experiencing hot and dry conditions throughout the months of May and June. These growing conditions, coupled with rapid, early season growth rates experienced by corn, soybean and cotton in the last couple of years, has required many producers to irrigate earlier and more frequently.
While this is a mild concern for some, these intensive watering practices are a major concern for producers with irrigation water containing elevated levels of salt. Furthermore, many may not be aware of their potentially low-quality irrigation water or may be experiencing this threat for the first time. If these trends continue in the coming years, irrigation water with high salt concentrations will become an increasing threat to crop production.
Identifying salt injury
What is the best way to identify crops suffering from salt injury?
Plants will usually resemble drought injury, such as wilting and a reduction in leaf area, even though adequate moisture is present within the soil system. Other visual symptoms include pale green and yellow leaves followed by necrosis.
These symptoms usually begin exhibiting themselves around the leaf margins. Continuous salinity problems will cause these symptoms to spread throughout the plant, and if severe issues are present, the plants will eventually die.
Because these symptoms first appear in the lower leaves, early identification may be difficult to spot until serious conditions exist. An example of this occurred in soybean trials at the Macon Ridge Station, where there appeared to be no signs of damage across the canopy; however, there was clear evidence of salt injury in the under-canopy. Therefore, in-field scouting may be needed to help identify this problem.
How salt affects crops
If salt damage has been identified, how detrimental is this to your current crop?
The answer can be difficult to determine for every situation, not only because crops are affected by salt levels differently, but soil texture and location within the field can influence salt injury. Areas that receive higher rates of low-quality irrigation, such as within the first third of a field under furrow irrigation, usually show higher incidence and intensity of salt injury than areas further through the field.
Lower portions of the field, where irrigation water can accumulate, will show worse salinity problems than higher, well drained areas. Further, sandier soils have greater leaching of salts during rainfall events than soils with higher clay content, which can take nearly five to six inches of rainfall to decrease the salt level of the topsoil by 50 percent.
The crop itself can be highly influential on the severity of the salt injury that occurs. Some crops, such as rice, soybeans and corn, can show a rapid decline in yield compared to cotton and to a lesser degree wheat and grain sorghum.
In addition, within individual crops, varieties/ hybrids can show higher salt tolerance – termed salt excluders – than others, which are termed includers. The LSU AgCenter currently has salt ratings for many commercially available varieties from the 2011 soybean OVTs at the Macon Ridge location for reference and evaluation.
Collect and analyze samples
If a problem field is identified, what is the next step?
Since salt injury can vary in severity and because symptoms can be similar to other deficiencies or toxicities, proper samples need to be collected from the soils and irrigation wells. Samples can be sent to many analytical laboratories, such as the LSU AgCenter Soil Testing and Plant Analysis Laboratory (STPAL) located on the LSU campus in Baton Rouge.
When an analytical laboratory is identified, selecting the appropriate analysis is critical.
Since most laboratories are different and unique, contact your local Extension agent/specialist for proper analysis selection. If your irrigation water/soils are found to be at toxic levels for salts, what steps can be taken within this growing season to minimize the detrimental effects?
Unfortunately, based on the current data available, unless another fresh source of water can be obtained, little can be done inseason. However, at this point, one of the greatest concerns is high salt accumulation in the soil. If this salty irrigation water continues to be applied to the soil, irreparable damage can be done within a very short time and will be detrimental to crops and soils for many years.
The best management practice for the long-term sustainability of the production system would be to limit irrigation events or even completely stop irrigating if water is found to be severely low-quality.
Switching production systems
What steps can be taken during future growing seasons to minimize the impact to production and soil systems?
As mentioned previously, there are varieties within sensitive crops that are more tolerant to salinity than others.
However, if salt levels in the irrigation water are high, switching production systems to a crop that has a lowered irrigation demand, such as grain sorghum or cotton, may be the best alternative.
In these instances, this would change how both producers and landlords determine the crop rotations and land allocations; however, long-term productivity of our valuable resources and being a good steward needs to be a consideration in these situations.
As we continue to see this problem become a greater issue across many areas in the Mid-South, especially northeast Louisiana, everyone within the agricultural community must become more knowledgeable about salinity issues and the damage to our production systems that poor-quality irrigation water can cause.
Dr. Josh Lofton is an agronomist at the LSU AgCenter Macon Ridge Research Station in Winnsboro, La. Contact Lofton at (318) 435- 2157 or JLofton@agcenter.lsu.edu.