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Post-hurricane salinity drops in Louisiana sugarcane belt

The tidal surges accompanying hurricanes Katrina and Rita in August and September 2005 deposited a considerable amount of salt throughout the coastal parishes of Louisiana. Sugarcane and rice were particularly hard hit.

A survey, conducted shortly after the water subsided in the areas planted to sugarcane, revealed soil salinity levels in several areas of six to eight times the published damage threshold level of 1,100 ppm.

Out of concern that the 2006 crop was in jeopardy, three methods were used to evaluate the effects of salinity on sugarcane production.

The first was to monitor changes in salinity and to measure yield at the original sites selected for sampling in October 2005.

Second, microbial products, designed to reduce soil salinity, were evaluated at the two highest salinity sites identified in the survey.

The third method was to sample partially flooded fields to compare soil salinity and yield levels between the submerged and the unflooded areas of the fields.

Averaged over all the sites in the initial survey, salinity decreased 69 percent from the time of the survey to the harvest season, a period of approximately one year. Sugar yield averaged 10,512 pounds per acre, based on hand-harvested samples.

All three sites that had initial salinity levels above the damage threshold had excellent yields. The one site with high salinity at harvest had the lowest yield.

Microbial products designed to reverse the negative impact of soil salinity were evaluated at the two highest soil salinity sites. While the products appeared somewhat effective in reducing salinity in the sugarcane rooting zone (both products reduced salinity more than the control), they were ineffectual in producing yields higher than that of the control plots.

The final approach was to compare sugarcane production on flooded and non-flooded areas of partially flooded fields. Fields were chosen by aerial maps, with articulated flood surge lines, and by growers' eyewitness accounts.

Although the unflooded areas produced numerically higher yields, which suggests flooding and salinity adversely affected production, the data were highly variable and treatment mean differences for all variables measured were not statistically significant.

While high soil salinity levels existed at the time of the initial survey in October 2005, rainfall was sufficiently heavy to leach much of the salt from the root zone by harvest of 2006. Average salinity was considerably below the damage threshold by the fall.

Nevertheless, when factoring in all sites across the five-parish area in which samples were taken, there was a moderate but statistically significant decrease in plant cane tonnage as salinity increased.

The association between soil salinity and yield in the stubble phase of the production cycle was not as strong.

Evaluating the effects of soil salinity on sugarcane yield was made difficult because conventional unsalted control plots were not available for comparison. Also, different rainfall amounts and the direct effects of the flood waters had confounding effects across the many locations from which data were collected.

While several high-salinity sites produced acceptable yields, there was statistical evidence that the overall trend was for soil salinity and flood waters to moderately reduce yield. It is believed, however, that the adverse effects of soil salinity were minimized by Louisiana's high rainfall environment.

At harvest, over 90 percent of the plots across all sites had soil salinity levels well below the damage threshold. The outlook for future sugarcane production in the affected areas is encouraging.

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