The Connecticut Christmas Tree Growers Association has enlisted the help of gifted Connecticut Agricultural Experiment Station researchers Richard Cowles and Thomas Rathier to study some serious problems plaguing their Fraser, balsam and Canaan fir plantings.
One field studied had poor interior shoot color — so poor that in the past the only solution would have been to cut down the fir trees and replant, which is an expensive proposition.
“The outside shoots had great growth and color,” explained Cowles, “but if we went slightly toward the inside of the tree, the shoots were off-color. Some were even white or brown. Growers of Fraser firs will be familiar with this problem,” which has been associated with various soil conditions and pests. “Although the mineral nutrition in the soils may be adequate, if the trees’ roots aren’t functioning well, the trees respond by sending all their ‘goodies’ to the outside shoots. As a result there aren’t enough minerals for the inside shoots, leading to what I call ‘Poor Root Function Syndrome.’
“We first tried to correct the problem with foliar nutrients, assuming it to be the result of nutrient imbalance.” The foliar nutrients had a significant but barely measurable effect on the discolored inner shoots, but not an economic effect. The trees were still unsalable.
Researchers next took soil samples around individual trees in the field, recording which trees looked poor and which trees looked acceptable, and tested the soil samples. They also sent soil samples to a nematologist. “The most significant finding was that the lower the pH in the soil around a given tree, the better the tree’s appearance!”
That made sense to Cowles. “As a general rule of thumb, the lower the pH, the fewer the organisms able to survive in that soil, including some root-damaging water molds like phytophthora, which thrive at a higher pH. Also, the natural habitat of these firs is thin mountain soil with a naturally low pH.
“Our intent in acidifying the soil to a pH of four was to test the hypothesis that Fraser and Canaan firs are naturally adapted to very acid soils, whereas the Phytophthora organisms — there are several species involved — may not be. Some of the worst trees I’ve seen have been planted in former corn fields, which are routinely limed to a pH of six. Therefore, we might be able to give the trees an edge against their pathogens by lowering soil pH. Further nematode sampling is on my wish list.”
Phytophthora infection, he noted, is caused by the growth of the hyphae and the spread of zoospores through waterlogged soil, but is enhanced by other root stressors, possibly including plant parasitic nematodes. Lance nematodes were identified in the original soil and root samples. The wounds to roots caused by their feeding “could open a ‘court’ of infection.”
Both Cowles and soil scientist Thomas Rathier emphasize that their results are still preliminary. Their studies have been carried out on only sandy loam soil and only in central Connecticut. The researchers caution interested Christmas tree growers in other areas, should they wish to experiment with lowering pH at this point, they should try it only on a few trees to avoid risking damage to a whole field.
Ideally, for the pH in the entire soil profile to lower quickly, sulfur should be applied and tilled in prior to planting. Using pellets increases ease and safe handling. However, in the field afflicted by interior shoot discoloration, in which the trees were already several years old, tilling in the sulfur was not possible. So the grower applied the pelleted sulfur to the surface of the ground around each tree.
The results were stunning. In about a year after application all the trees, including those which would have previously been considered untreatable, “were gorgeous and salable.” Sulfur affects the soil pH once it is converted to sulfuric acid through the action of microbes. “The hydrogen and sulfate ions are very mobile in the soil, affecting the entire soil profile to a much greater extent than an application of lime.”
In a second Christmas tree field, widespread phytophthora infection had resulted in the deaths of all Fraser firs within three years of planting. But even the Canaans, which are somewhat tolerant of phytophthora, were showing signs of infection. The grower was at his wits end.
“In this field, because we were plowing and replanting, we were able to incorporate the sulfur with a rototiller and did so six months prior to planting. When we planted the next spring, we could still see bits of yellow sulfur in the soil, indicating the sulfur had not completely broken down over the winter.” By September of that year, one year after sulfur application and incorporation, the pH had dropped from 6.2 to four. “I’ve tested the pH of the soil in August/September of the two years since incorporation and the pH has remained constant, indicating that the full effect had been achieved one year after incorporation at that site.”
Now, two years after planting the Fraser and Canaan seedlings in that previously deadly field, the trees are still alive and healthy in the acidified plots. In addition, the growth of the trees in the second year was twice that of the trees in the higher pH soil, possibly indicating better initial root development and overall tree health in acid soil conditions. At this site, regular measurements on every tree are being recorded throughout the entire study. The preliminary results, albeit limited to one soil type and one climate, are impressive.
Cowles has not tried planting Turkish, Nordmann, Noble, Grand, Douglas, or concolor firs in a highly acid soil and conjectures they may be less benefitted by this treatment.
“We have several hypotheses for why lowering the pH is beneficial to Fraser and Canaan firs, and these hypotheses are not mutually exclusive. We think we have ruled out the hypothesis that sulfur itself is working as a kind of fungicide-biocide, as well as the role of high concentrations of calcium in the soil.”
One hypothesis that is fairly high in terms of Cowles interest right now involves the availability of manganese “which has important implications for plant resistance to disease. Lowering the soil pH to about a pH of four makes manganese highly available to the plant, with the result that the plant is better able to defend itself. Manganese availability is not easily affected other than by lowering pH from a practical point of view.”
Another hypothesis is that the lowered pH is also changing the microorganisms around the roots. “Does the lower pH make organisms more antagonistic to water mold growth and infection, and also to root-damaging nematodes, more numerous? Or might the low pH increase populations of root-protective organisms?
“We know from laboratory studies that a very low pH does reduce the growth of hyphae of water molds, including phytophthora. We will be looking at the survival of zoospores, the infective agents of phytophthora, which swim through waterlogged soil pores to spread the infection to new trees. I suspect the zoospore could be the stage we’re most interested in interrupting, since it is the infectious stage in water molds.
“We’ll be working on several hypotheses for the next four years to try to get a fuller understanding why the lowered pH appears to be of significant benefit to certain fir tree species.” Stay tuned!