J.D. Widick and his colleagues at Arkansas State University have been working on the problem of chloride toxicity in soybeans for the last 12 years. Now, with more instances of salt in soybeans being reported, the professor's work is more important than ever before.
What causes toxicity? “If we look at soybean characteristics, the roots of most varieties take up chloride at the same rate,” says Widick, who spoke at the 2003 Arkansas Soybean Research Conference held in Brinkley, Ark. “There aren't any great differences amongst varieties.
“Some varieties we've, perhaps improperly, referred to as ‘chloride excluders.’ They keep most the chloride from the leaves and stem of the plant instead storing the chloride in the roots.”
Other varieties are known as chloride “includers.” These varieties take in chloride and rapidly move it to the top of the plant. If there is enough chloride available, includers can concentrate the offending substance to the point of killing the plant.
Foliar symptoms of toxicity occur in chloride includers almost exclusively, says Widick. It's rare that an excluder will show foliar symptoms although yield and growth may be reduced.
“Mild symptoms include faint chlorosis between leaf veins. If symptoms increase, the chlorosis will become more severe and will show severely at leaf margins. In the most severe cases, leaf tissue will die and drop.”
Widick says about a decade ago, the second field identified as suffering from chloride toxicity harvested 4 bushels per acre. “That is a terrible yield loss. Chloride excluder varieties normally yield around 35 bushels.”
Chloride toxicity most likely will occur where elevated levels of chloride have been used in irrigation over an extended period. Widick shows a picture of a field with areas of scorched-looking plants.
“The field where this picture was taken had a well that was pumping over 50 pounds per acre inch of just chloride. That was equivalent to 100 pounds of table salt per acre inch of water. That's quite a bit to expect a soybean field to overcome. But it's especially interesting to note that, in most cases, the problems are seen in fields in a soybean/rice rotation.”
Widick next shows a map put together by Bill Baker (who used to run the soil lab at Cotton Branch) that shows where Arkansas chloride hot-spots are. The hottest, which show up on the map in red, contain 211 to 290 parts per million of chloride. There is one such spot in Chicot County. There are also some scattered problems in Desha County where, Widick says, “we've recently heard some horror stories. The point is, at some level, this problem affects almost all the principal soybean-growing areas of the state.”
Widick says he's often asked by farmers what chloride problems will mean specifically: Yield troubles? Physiological changes in the plants?
“It's rather difficult to predict what a producer can expect to see from a chloride problem. Some years it may not even be visible. Weather, obviously, enters into what symptoms are seen. If rainfall is adequate and chloride-affected irrigation water isn't used, symptoms are less obvious. However, when chloride-containing irrigation water is used, yields can be cut to less than 10 bushels per acre.”
Other problems can be associated with predicting chloride accumulation in a field. Typically, among the first couple of questions Widick encounters from the newly afflicted are: Can we soil test for it? How much is too much?
Unfortunately, soil testing isn't much help in working with chloride. Chloride ions move rapidly in the soil as water is added or removed. There can be large differences in chloride levels between sites that are just a few feet apart. You can also find large differences in chloride content of the soil in samples taken from the same site 24 hours apart.
“So, really, other than to prove chloride is in the soil, a soil sample won't help much,” say Widick.
“We began screening for chloride reaction in the state variety tests around 1991. We began by studying all the state variety entries in the field. The results were erratic, so we moved to a greenhouse screening program.”
With the program still running, the screening results are now available to the Extension Service. That data, in turn, is available to growers and soybean industry representatives.
“We begin this process by receiving seed from Don Dombek (University of Arkansas variety test coordinator) and various seed companies. These seed are planted in a potting mix at an ASU greenhouse. When unifoliate leaves are expanded, we transplant five plants of each variety into a Styrofoam sheet. We then suspend the sheet over a complete nutrient solution.”
When three fully expanded trifoliate leaves, the researchers add a chloride treatment to the nutrient solution. The chloride is added in three installments of sodium and calcium chloride, made in 48-hour intervals. This reduces the possibility of shocking the plants.
Following the third treatment, the researchers harvest the leaflets of the two uppermost trifoliate leaves. The leaves are dried in the oven at 40 degrees Celsius, then ground up using a mesh screen.
“To do the chloride analysis, we extract 0.10 gram of plant material and mix it with 50 milliliters of distilled water. This mix is then shaken for 20 minutes and the leaf material is then removed by filtration.”
After being put into sampling tubes, the samples are put into a digital chloridometer — a machine normally used to test chloride levels in human blood. After running these tests, researchers are able to identify excluders and includers.
Widick says the excluder characteristic is controlled by one dominant gene: Ncl. Inclusion occurs in plants that carry the recessive form of the gene in the homozygous condition: nclncl.
“Ncl has been traced back to CNS, one of the parents of the old variety Lee, which served as the foundation of Southern soybean breeding programs.
Today, many Southern varieties have the excluder gene present. Exclusion of chloride is relatively rare in the northern varieties.”
There is a lack of selection for exclusion, says Widick. Few breeding programs have been actively screening for chloride reaction among their experimental strains.
“Thus, varieties developed from a cross of one excluder parent and one includer parent may have some plants that are includers and others that are excluders.”
Finding excluder seeds in such a parentage case is a game of probability. For example, the chances of picking five excluder (or includer, for that matter) seeds from a seed lot containing equal numbers of includers and excluders: 1/32.
“Every year, we hear from two or three farmers with wells pumping chloride,” says Widick “If you've got a well with high chloride content, I suggest you avoid varieties with a high incidence of includer seed.”