Citrus and salt just don’t go together. That’s why it makes sense to use a potassium source that won’t add chloride to the soil where salinity is a problem in citrus groves. In south Florida, where water tends to be high in salt content, many citrus producers are choosing to go with sulfate of potash.
“Sulfate of potash is not toxic to the trees. Chlorides can build up salt content, so growers have to be more careful with those,” says Bob Rouse, citrus horticulturist at the University of Florida’s Southwest Florida Research & Education Center in Immokalee.
“A lot of the salinity problem occurs because of high-saline irrigation water. If you have high salinity water and build that salinity up, that makes the problem worse. Salt is toxic to the leaves. It turns leaves brown around the edges, then they fall off the tree. That salt is then reabsorbed by the roots.”
Saline level in water from some south Florida wells runs more than 1,000 ppm. For good citrus production, that level should be no more than 200 to 300 ppm, Rouse says. Increasing the salt problem with chlorides just makes a bad problem worse.
“If you have salinity problems, you don’t want to add to that,” says Mongi Zekri, who works southwest Florida as a multi-county Extension citrus agent stationed in LaBelle.
“In areas where we’re getting seawater intrusion in wells, growers are tending to apply sulfate of potash. They may not apply it any differently from potassium chloride. I don’t think the motivation is the nutrient value of the sulfate. There are others sources of sulfate. The motivation is to avoid chloride,” says Arnold Schumann, soil and water specialist at the Citrus Research & Education Center, Lake Alfred, Fla.
Citrus trees need potassium in order to reach full potential production.
“Potassium is a very important element in citrus production. The quality of the fruit, including sugars, is affected by potassium. If you have good amounts of potassium, you get good fruit that’s good-sized. Growers put out a lot of potassium in the spring. It really boosts fruit size,” Zekri says.
Zekri normally recommends using the same amount of potassium as nitrogen.
“Sometimes if there are problems with high pH, we recommend higher rates of potassium because the plants struggle to take it up. With high pH, there’s too much calcium in the soil. In those cases, we may recommend 25 percent more potassium,” Zekri says.
In southwest Florida’s groves, sulfate of potash is blended with nitrogen and phosphorus and applied to the soil three to four times a year.
“It depends on the age of the tree. Young trees may get four applications or more a year. Older trees most likely get three fertilizer applications. It is applied with a fertilizer spreader. We make no foliar fertilizer applications because growers are afraid it will burn leaves. The fertilizer is a soluble material. It is taken up by the roots and moves through the tree,” Zekri says.
“A lot of people are using sulfate of potash and are happy with it. Sulfate of potash is in the middle ground for price and it does not have a high salinity index,” Zekri says.
“Sulfate is a good middle ground, one that’s safe, considering the price and risk,” Rouse says.
Some California citrus producers also face high-salinity problems but, due to production techniques, their situation differs quite a bit from the Floridians’.
“In general, Kern County soils are fairly high in potassium, probably because of relatively low rainfall conditions. In some areas, especially the Edison area, we do have considerable citrus acreage planted on sandy soils which tend to be naturally low in most nutrients,” says Craig Kallsen, University of California farm advisor in Bakersfield, Calif.
“Most citrus acreage is on low-volume irrigations systems, and potassium sulfate is not very soluble in water and thus is difficult to apply through the irrigation systems. Some growers use the very finely ground potassium sulfate that is able to go through the low-volume emitters,” Kallsen says.
Kern County growers tend to plant cotton and pistachios where salt content is high in the soil. “However, some growers are irrigating citrus with wells that are a little on the high side with salts and alkalinity. Generally, sulfur and gypsum–with leaching– or other organic amendments are the cure when salinity and alkalinity start to go up. In general, I do believe that potassium sulfate is always a valid option where soil and tissue analysis suggests potassium is low,” Kallsen says.
“We do have a potassium deficiency in the foothills area,” says Joe Connell, Butte County, Calif., farm advisor. “I suggest citrus growers there use sulfate of potash at the rate of 10 to 15 pounds per tree, soil applied in bands on either side of the tree row. This citrus is grown on shallow foothills soils, and I recommend sulfate of potash because of chloride's toxicity.
“We have soils there that are high in magnesium and shrinking and swelling clays that tie up potash. We have to overcome that, so that's why I recommend the high rate of sulfate of potash,” he says.
Gary Bender, San Diego County farm advisor located in San Marcos, Calif., says citrus growers there have not yet faced problems with soil salinity. Avocado growers have, however.
“Avocados show tip burn easily when growers put chloride through the irrigation system. I recommend potassium sulfate for those situations,” Bender says.