Salt Damage to Agricultural Fields in South Louisiana

Linda Benedict  |  8/22/2007 10:22:00 PM

Table 1. Impact of storm surge flooding on rice fields in southwestern Louisiana. Assessments based on salts contained in the surface (0-6”) of soil and the underlying 6-12”.

Table 2. Criteria used for assessing salt impacts following coastal flooding. Salts (ppm) were determined by measuring the electrical conductivity of saturated paste extracts and multiplying by a factor to convert to ‘ppm’, a term more familiar to Louisiana farmers. The sodium adsorption ratio (SAR) in saturated extracts reflects the degree of sodium saturation.

Figure 1. The effects of amending soils with gypsum and lime on the amount of salt remaining in 6” columns of three flooded soils (Baldwin, Jeanerette and Crowley) after leaching with 3 pore volumes of water. 'Initial' refers to soil salinities prior to leaching. Three pore volumes is equivalent to about 8” of rain.

Figure 2. Salinity of floodwater at various times after establishing a 4” flood on 12” columns of three soils. All soils were flooded for several weeks by Hurricane Rita (initial saturate paste salinities shown in legend). Figure 3. Distribution of salts in 12” columns of three soils 21 days after establishing a 4” flood. Soils were dry before flooding.

Vast acreages of agricultural land were flooded with saltwater during the 2005 hurricane season. A key question for many was whether the land was too salty to plant in 2006. Many were asking this question in 2007. (Photo by Gary Breitenbeck)

The behavior of salt varies greatly among soil types. (Photo by Gary Breitenbeck)

Deep sampling 18 months after flooding showed that in this field most salts had accumulated in the hardpan at a depth12-18” below the surface. As the soil dries, some of these salts will move upward; after a rain they move down again. (Photo by Gary Breitenbeck)

Working a salt-impacted field to increase the dissolved salts in floodwater before discharge. (Photo by Gary Breitenbeck)

Gary Breitenbeck, Johnny Saichuk, Howard Cormier and Sonny Viator

When hurricanes Katrina and Rita came ashore in Louisiana in 2005, they were accompanied by storm surges that inundated vast areas in the southern parishes with salt water. Flooding of agricultural land was especially severe in the southwest where the surge from Rita flooded rice and sugarcane fields more than 50 miles inland. In many areas north of the east-west state Highway 14, flood waters receded within a few days. Farther south, water remained for weeks. In some cases, the flood water persisted for several months where the surge was trapped by protective levees. This was sufficient time for much of the water to evaporate and deposit its load of salt in the fields.

LSU AgCenter scientists conducted a preliminary study to assess the extent of salt effects. Of the six sugarcane and nine rice soil types examined within the flooded zone, salinity levels in saturated soil extracts ranged from negligible to more than 5,000 parts per million (ppm). In the surface 6 inches of soil, salinities averaged 3,100 ppm, a level far in excess of established tolerances for optimal yields of rice and sugarcane. Both sugarcane and rice, the predominant crops in the flooded areas, are moderately sensitive to salt. Understandably, farmers with affected fields were deeply concerned that these fields could not be profitably planted in the 2006 growing season.

In the case of sugarcane, this concern seemed to be largely unfounded. Sugar is a perennial crop planted once in a typical four-year production cycle in Louisiana. Many flooded sugarcane fields were in first or second “stubble,” so planting was not required. Selective monitoring and anecdotal evidence suggested that 2006 sugarcane yields from these fields were not substantially reduced as a result of saltwater flooding. Sugarcane is usually grown on raised rows, which may have improved this crop’s ability to withstand elevated soil salt levels. During dry periods, salt tends to wick upward to the tops of rows. Rains can then wash surface salts down into adjacent fur rows. Following heavy rains, the salts accumulating in furrows are subsequently flushed from the field into ditches, canals and bayous.

Rice Fields Hold Salt Water
In contrast, rice is grown on level fields surrounded by levees that control runoff. Most prime rice soils in southwestern Louisiana are shallow because of a well-developed hardpan that inhibits downward flow of water. Elevated salt levels can substantially reduce rice germination, growth and grain filling. To assess the effects of Rita’s storm surge on rice fields, a survey was organized. With the help of parish agents and others, more than 150 fields were systematically sampled. This survey showed that the impact on surface soil was negligible in about 35 percent of the fields (Table 1) located primarily along the northern reaches of the flooded area. Another third showed mild to moderate impact where minor reductions in rice yields were possible. About a third of the fields had been severely affected to the degree that substantial yield reductions were likely to occur. Salt levels in some of the most severely impacted fields suggested that not only catastrophic crop failure was likely, but also that these fields were at risk of becoming permanently unproductive.

The criteria for classifying salt impact (Table 2) are based primarily on measures of salinity and the sodium absorption ratio (SAR). The SAR is a reliable indicator of the impact of sodium on the soil. High sodium not only reduces plant growth, it causes soils to lose structure as well as their ability to absorb and retain water. When the amounts of sodium in the soil solution increase relative to those of calcium and magnesium, sodium can become toxic to even the most tolerant plants. As salts leach from the soil, salinity decreases while the SAR can remain high. Soil pH then rises above pH 8 and micronutrient deficiencies occur. An exceptionally high ratio of sodium causes soil to become impermeable to water. Crops cannot grow even with irrigation because water will not penetrate the soil. Reclaiming soils containing salt is costly because it not only requires large quantities of calcium but also the installation of a drainage system to promote leaching.

At-Risk Fields
It is important that we identify fields at risk of permanent damage from salt. Applying calcium as lime to acid soils or as gypsum or slag to neutral soils can prevent salt damage. To determine the status of a salt-affected soil, soil samples can be submitted to the LSU AgCenter’s Soil and Plant Testing laboratory on the LSU campus in Baton Rouge and request the “Storm Surge” analysis. Results will include both a measure of salinity, the SAR and other data. As a rule, we should be seriously concerned if the SAR is greater than 13. We have sampled a few fields flooded by Rita that now have SAR values greater than 25. A SAR of less than 4 is ideal.

Most flooded fields are not at risk of permanent damage even though salinity values remain high enough to threaten yields. Resampling rice fields that tested high for salts in the initial survey more than a year after the initial flooding showed that salinity values remain substantially greater than 1,000 ppm. Monitoring the change in salt concentrations over time is complicated by vertical movement of the salts. In general, as soils dry, the salts accumulate near the soil surface. After a rain, they rapidly move down into the soil profile. Extensive monitoring of one field showed that salts also move laterally, accumulating near levees and in depressions where water evaporates. Even so, the finding that high salts remained for more than a year despite near normal rainfall suggests that natural processes cannot be relied upon for remediation of rice fields with heavy salt loads. The most common methods for removing salts include either leaching them deep into the soil or flushing them from the field in runoff. Several laboratory studies were conducted to better understand the behavior of salts in the silt loam soils typical of rice fields in southwestern Louisiana. We tested the efficacy of adding gypsum or lime to accelerate leaching salts from three soil types collected from fields that had been flooded with several feet of saltwater (Figure 1).

These studies showed that passing one “pore volume” of water removed most of the free salts. Pore volume is the air space between soil particles in a given volume of soil. The rate of water infiltration varied greatly, and none of the soil columns in the laboratory contained a thick hardpan that slows leaching in most rice fields.

After passing three pore volumes through the soils, the salinities of all three were low. With each leaching, however, the soils became less permeable. After the second leaching, the Commerce soil became extremely compacted, and infiltration stopped entirely because of the high ratio of sodium that remained in that soil. Adding lime or gypsum did not significantly improve either the rate of salt leaching or the rate of infiltration. Because it contains sulfate, too much gypsum could cause sulfide toxicity problems once a field returns to rice production. The benefits do not seem to justify the costs and risks unless excessive sodium threatens to destroy the soil structure.

Flushing Fields
An alternative approach lies in flushing the fields to remove excess salts. Laboratory studies using columns of three soils showed that salt concentrations of water in a simulated irrigation flood continue to increase for about two weeks and then stabilize (Figure 2). This suggests that closing levees before a heavy rain and opening the levees after two weeks may be a practical means of reducing salts. We found, however, that the rate that salts diffuse into this floodwater varies greatly among soil types. Crowley silt loam, a common soil in the affected area, gave up only a small portion of its salt to the floodwater (Figure 3). Most of the salt moved downward following flooding and remained in the soil.

Additional studies suggested mechanically mixing the soil after flooding and then retaining the floodwater for 3-5 days, offered a practical means of rapidly removing salts. This technique was field-tested in Vermilion Parish. This field consisted of a sequence of three levee cuts, each draining into the next until water was discharged from the lower cut into an adjacent bayou. Each field was worked dry, flooded by about 5 inches of rain over a two-week period, then worked again with standing water and allowed to settle for five days.

Comparison of the dissolved salts before and after working the flooded soil indicated that working the soil increased salts dissolved in floodwater by 60 percent to 190 percent before discharge. The levees were then opened and the field drained. The soil retains about an inch of water in each 2 inches after draining, so it is not possible to remove all of the soluble salts in a single flushing. Nevertheless, we calculated that this procedure resulted in the removal of between 480 to 993 pounds of salt per acre. After flushing, the SAR and pH were sufficiently low so that the soil was not at risk of collapsing, though the addition of lime or slag would provide added insurance.

Some farmers with highly saline soils have temporarily converted their rice fields to hay production. Both common and coastal Bermudagrass are highly salt-tolerant and can accumulate substantial amounts of salt in their tissue. Harvesting the hay will eventually reduce salts to a level where those fields can return to rice production. Fertilization, especially with nitrogen, will accelerate plant growth and salt removal rates. Potash is a high-salt fertilizer and should be applied only where soil tests show that it is absolutely essential.

The results of these studies can ultimately be used to remediate highly saline soils should storm surge happen again in South Louisiana.

Gary Breitenbeck, Professor, School of Plant, Environmental & Soil Sciences, LSU AgCenter, Baton Rouge, La.; Johnny Saichuk, Professor, Rice Research Station, Crowley, La.; Howard Cormier, County Agent, Vermilion Parish, Abbevile, La.; Sonny Viator, Professor and Coordinator, Iberia Research Station, Jeanerette, La.

(This article was published in the summer 2007 issue of Louisiana Agriculture.)

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