Edward Twidwell | 10/14/2005 12:25:51 AM
There is definitely concern among cattlemen about saltwater intrusion into pastures and hayfields in several south Louisiana parishes.
The capacity of plants to grow satisfactorily in salty conditions depends on a number of interrelated factors. The physiological condition of the plant, its stage of growth and its rooting habits certainly are among them. All soils contain some water-soluble salts. Plants absorb essential plant nutrients in the form of soluble salts, but excessive accumulation of soluble salts, called soil salinity, suppresses plant growth. When soil salinity exceeds a plant’s tolerance, growth reductions occur. As salt concentration increases, water becomes increasingly difficult for the plant to absorb. A plant can actually die from water stress (lack of available water) in a moist soil if the salt concentration becomes high enough.
Symptoms of salt injury in plants resemble drought. Both conditions are characterized by water stress (wilting) and reduced growth. Severe injury caused by prolonged exposure or high salinity results in stunted plants and tissue death. Reduced growth caused by salinity is a progressive condition that increases as salinity increases above a plant’s tolerance threshold. Plants vary in their response to soil salinity. Salt-tolerant plants (plants less affected by salinity) are better able to adjust internally to the effects of high salt concentrations than salt-sensitive plants. Salt-tolerant plants are more able to absorb water from saline soils. Salt-sensitive plants have a limited ability to adjust and are injured at relatively low salt concentrations.
Forage crops commonly grown in south Louisiana vary in their tolerance to salty conditions. Bermudagrass is generally considered to have a high tolerance to salt. Some references indicate that bermudagrass can tolerate salt concentrations of up to 5,000 parts per million (ppm) for extended periods. Other perennial forages species such as bahiagrass and carpetgrass aren’t as salt-tolerant as bermudagrass, but, in relative terms, they are still considered to be tolerant of salts. Most cool-season forage species such as ryegrass, wheat and barley are considered to be moderately tolerant to salty conditions. Most clover species, including white, red, arrow leaf and crimson, are considered to be sensitive to salty conditions.
With regard to bermudagrass pastures and hayfields, most of the resource information indicates these areas should recover well from saltwater intrusion. Even though some amount of salt may have accumulated in these fields, it is anticipated that leaching by winter rains may remove a great deal of this salt. One source indicates that 5 inches of rain will remove about 50% of the salts in the soil.
The cause for greatest concern this fall is attempting to plant ryegrass into areas that received some level of saltwater intrusion. In general, well-established plants will usually be more tolerant of salty conditions than young plants. In reviewing the literature, it is obvious that ryegrass seedlings are much more susceptible to salt injury than are mature plants. A review committee within the LSU Agricultural Center has established the salinity threshold value for seedling ryegrass as being 1,800 ppm. The probability of problems with ryegrass germination caused by saltwater intrusion is reduced with each successive rain.
Producers can determine the salt content of their soils by taking samples and sending them to the LSU Agricultural Center’s Soil Testing Laboratory for analysis. If there is a salt accumulation, it will not be uniform. “Hot” spots will be in the least well-drained areas of the fields. For each sample, collect soil from 10 or more places in each sampling area so as to get a representative sample. Mix thoroughly in a plastic bucket before filling a container to be mailed to the lab. Samples should be taken from the top 3 inches of soil. On the soil test request form, write at the top of the page “Soil Test for Storm Impact.” Enclose a check for $7.00 made out to Agronomy STPAL. Producers should receive results within four to five days after the lab receives the sample. Results will include pH, salts and a sodium absorption ratio. Producers can check with their local Extension agents if they have questions about this procedure.
Producers can also perform a simple experiment (bioassay) on their own with a small amount of soil from affected areas before planting large acreages of ryegrass. The bioassay can help predict potential crop injury. A bioassay does not measure the amount of salt residue present in the soil, but it may indicate whether or not enough residue is present to injure ryegrass seedlings.
The procedure is as follows:
Injury symptoms should become apparent within seven to 10 days after planting. Possible symptoms include:
It is a good idea to compare the germination and growth of the seedlings in the salt-affected containers to those in the “check” containers. While this is not a precise experiment, it should provide producers with an idea of how ryegrass may germinate and grow in areas affected by saltwater intrusion.
Also, electrical conductivity meters are available to measure the salt content of soil directly in the field. These meters cost in the range of $100-$200 to purchase. If you have questions about any aspect of your forage program, contact your local Extension agent.