The importance of calcium, as it relates to physiological disorders such as tip burn in lettuce, is fairly well recognized.
But recent research by University of California Cooperative Extension suggests that the disorder may have more to do with environmental factors than the amount of calcium in the soil or applied through irrigation water.
“Most soils in California test high for calcium,” says Tim Hartz, UC vegetable crops specialist. “Even with high soil calcium levels, it is common in California for growers to apply additional calcium through the irrigation water.
“So, if the calcium is there in the soil, how available is it to the plant, what is the best method of testing for it, and why aren't we seeing more of an impact on preventing tip burn?”
To answer those questions, Hartz and other UC researchers began by evaluating three methods used to test for soil calcium to determine which would best predict plant calcium uptake.
Those methods included ammonium acetate extraction, saturated paste extraction, and extraction of soil solution by centrifugation to characterize soil calcium status.
“The ammonium acetate extraction method, which is commonly used, actually tells you very little about calcium availability,” Hartz says. “The saturated paste test did a pretty good job, while the soil solution test did the best job.
“The soil solution test is not practical on large scale basis, but it is important to note that if you are basing decisions on the ammonium acetate extraction method, you're not making very informed decisions.”
With that part of the equation out of the way, Hartz and his team turned their attention to evaluating the impact of applied calcium on tip burn. They evaluated two field trials in 2005 and one in 2006, where three forms of calcium (calcium nitrate, calcium thiosulfate, and calcium chloride) were applied through drip irrigation water during the final two weeks of growth. These treatments were compared to an untreated check.
“The results showed that these applications had minimal effects on tissue calcium concentration and tip burn severity,” Hartz says. “So, if tip burn is not appreciably reduced if large amounts of calcium are available in the soil, and it is only marginally affected by applied calcium, what is driving the incidence and severity of tip burn?”
Hartz looked at the dilemma from the standpoint of evapotranspiration data and discovered an interesting correlation.
By comparing evapotranspiration trends during the worst incidences of tip burn to a 10-year average, he found that severe tip burn occurred only under conditions of low reference evapotranspiration (ET) and moderate temperature.
The telling story was evapotranspiration in 2005, compared to a 10-year average, according to Hartz. About July 17, 2005, ET dropped precipitously for about four days, then rebounded rapidly.
“That scenario was where the worst cases of tip burn were later recorded,” he says. “Under these conditions, a transient calcium deficiency was apparently induced in rapidly growing leaves by the restricted volume of transpirational flow. It's not the amount of calcium in the soil — it's the amount of calcium in the transpirational flow, or the transpiration per unit of growth potential. It's not a soil issue — it's an environmental issue.”
That doesn't mean that all calcium applications are wasted, says Hartz.
“I think calcium applications will most likely be helpful in coarse-textured soils with low soil solution calcium, in soils with low calcium in saturated paste extract tests, and in low pH soils.”
Based on these trials and observations, growers may find themselves wondering what, if anything, they can do about tip burn. The first thing they can do, Hartz says, is be aware of the conditions that are conducive to the problem.
“If you get fog rolling in, the temperatures don't necessarily change that much, but ET falls off the cliff,” he says. “Then if it rebounds quickly, the plant starts growing faster, and it's difficult for the plant to supply those developing leaves with enough calcium.
“If you see that type of weather pattern, you might want to consider harvesting earlier than you normally would in order to avoid problems. Even if you're sacrificing poundage, you're saving marketability.”