Linda Benedict, Gauthier, Stuart, Saichuk, John K. | 11/30/2011 11:10:04 PM
Johnny Saichuk and Stuart Gauthier
Saltwater intrusion into rice production has been a cause for concern in southwest Louisiana for decades. But storm surges from recent hurricanes coupled with recent droughts have brought this problem to the forefront.
Rice is considered a salt-sensitive crop, yet rice varieties have not been selected for salt tolerance. Today’s varieties are similar in salt tolerance to the varieties of nearly 50 years ago.
Rice farmers have learned to measure the amount of salt in water before they use it for irrigation to avoid short-term crop injury and long-term soil contamination. The most commonly used tool to indicate the degree of salt content of water is a conductivity meter. The more salt the water contains, the easier it is for electricity to flow through it.
Rice at all growth stages can tolerate water containing 600 parts per million (ppm) of sodium. As the plants grow, their tolerance also increases. Irrigation water containing 1,300 ppm may be harmful to seedlings but is usually tolerable at later growth stages. Levels above 1,300 ppm begin to harm rice in varying degrees until they reach a fully toxic level of around 5,000 ppm. These figures assume a single application of water at this level. Repeated application has a cumulative effect, both on the plants in the short term and on the soil in the long term.
Following Hurricane Rita, the LSU AgCenter conducted extensive soil testing to determine how much salt remained in the soil once the tidal surge had subsided. In many cases, salt levels in the soil were too high for normal rice growth, and some farmers were unable to plant a crop the following summer. Research conducted at the Rice Research Station in Crowley resulted in the recommendation to avoid planting rice where salt levels were above 750 ppm.
After Hurricane Ike in 2008, a select number of sites were visited again to determine if there had been a cumulative effect of the salt deposition from Hurricane Rita combined with Ike. Each site was sampled from October 2008 until November 2009. One site was a control where inundation did not occur. Approximately 65 inches of rain fell on the area during the test period.
When sampled in October 2008, seven of nine sites that had been flooded showed salt levels above 2,000 ppm (Figure 1). At one site located near the Vermilion River, salt levels dropped quickly from a high around 2,000 ppm to 500 ppm a month later. This was attributed to the already saturated soil at the time of flooding, the short time the flood remained on the field, and ample supplies of fresh water that helped to flush the salt out of the system. The most heavily flooded areas required several months of rainfall to achieve the same decline in salt levels in the soil.
One phenomenon observed during the sampling period was the rise and fall of salt levels in the soil. The changing levels indicated salt migrated with the water in the soil profile. In saturated soils, salt is fairly evenly distributed, causing dilution and lower readings. As moisture evaporates from the soil, salt tends to accumulate near the surface. If sampled at that time, the reading would likely be high. If rainfall occurs, especially if it is heavy over a short time, however, accumulated salts can be flushed from the field. Slow, steady rain resulting in little runoff from the field will simply move the salt back down into the soil profile.
Another characteristic of salt is that it is randomly distributed in a field with some areas showing high levels and others much lower levels. The uneven distribution is often seen in seedling rice planted into affected fields where odd patterns of seedling injury can be observed.
Reclamation of soils in these situations is lengthy and often expensive. Studies of different techniques revealed the most effective method is to work the field in a flooded state (on silt loam soils that have a subsurface hard pan), allow the field to stand for a couple of weeks, then release the water. This procedure can be repeated to hasten the recovery. Some farmers simply let Mother Nature take care of things. Rainfall is the best treatment for the problem because water draining from the fields carries dissolved salt back to its source.
Johnny Saichuk, Rice Extension Specialist, Rice Research Station, Crowley, La.; and Stuart Gauthier, Extension Agent, Vermilion Parish
(This article was published in the fall 2011 issue of Louisiana Agriculture magazine.)