By Dr. Josh Lofton, Agronomist – Macon Ridge Research Station
Throughout the month of May and the first week of June the majority of the state experienced hot and dry conditions. Coupled with the rapid growth rates experienced by corn, soybeans and cotton, this situation has required some producers to irrigate for almost two months. While this is a mild concern for some, this intensive irrigation season is a major concern for producers with irrigation water containing elevated levels of salt. Many may not be aware of their potentially low quality irrigation water or may be experiencing this threat for the first time this year, and if current trends continue, irrigation water with high salt concentrations will become an increasing threat to crop production in coming years.
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 (Figure 1). 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 (Figure 2); however, there was clear evidence of salt injury in the under-canopy (Figure 3). Therefore, in-field scouting may be needed to help identify this problem.
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 also 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. However, it takes approximately 5 to 6 inches of rainfall to decrease the salt level of the topsoil by 50%. 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 (Table 1). In addition, soybean varieties can show higher salt tolerance, termed salt excluders, than others, termed includers. Damage ratings are currently available for soybean varieties through the 2011 soybean OVTs at the Macon Ridge location.
If a problem field is identified, what is the next step? Since salt injury can vary in severity and 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 the LSU AgCenter Soil Testing and Plant Analysis Laboratory (STPAL) located on the LSU campus in Baton Rouge. A soluble salts test, at $5 per sample, can be used to determine the amount of salts in your soil that could potentially be affecting your crops. Additionally, irrigation samples can be submitted to the STPAL with a “quick water analysis” for $6 per sample, determining not only total salts in the irrigation water but also electrical conductivity (E.C., estimate of soluble salts), sodium, and chloride concentration.
If your 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 in-season. 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.
What steps can be taken during future growth 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 of Louisiana, everyone within the agricultural community must become more knowledgeable about salinity issues and the damage to our production systems that this issue can cause.