In experiments in West Virginia, a kaolin particle film was tested as an alternative to a plastic mulch for promoting red color, with surprising results.
Scientists who experimented with a sprayable reflective film on the orchard floor, as an alternative to a Mylar film for improving fruit color, were surprised to discover that the sprayable film improves fruit size.
Growers commonly lay reflective plastic films beneath or between orchard tree rows to reflect light back up into the trees and enhance fruit color.
Dr. Michael Glenn, soil scientist at the U.S. Department of Agriculture’s Appalachian Fruit Research Station in Kearneysville, West Virginia, began experimenting with a sprayable kaolin particle film because of the labor involved in laying a plastic film between the tree rows and the problems of disposing of it after use.
In an Empire orchard at the research station, he used an herbicide sprayer to apply a kaolin material on the grass in orchard alleys at a rate of 20 pounds per acre, starting two weeks after petal fall. Because the grass grew during the season, the material was reapplied every three weeks until harvest to maintain a reflective covering. The grass was not mowed. For comparison, a plastic film was laid in other orchard rows two weeks after petal fall and maintained throughout the season.
During the multiyear study, Glenn found that the plastic film consistently increased the red color of the fruit, whereas the sprayable film improved color some seasons but not others. However, the sprayable film consistently improved fruit size, which the plastic film did not do.
Glenn said the light reflected from a plastic film is different from the light reflected from the particle-film–covered grass in both quality and quantity. Most of the light that hits the very reflective plastic films bounces up into the tree, and it is similar to sunlight.
However, when light hits the particle film on the grass, much of the red light that comes from the sun is absorbed by the grass to use in photosynthesis, and the light reflected into the trees has a lower ratio of red to far-red radiation. This deficit of red light stimulates biochemical processes in the tree and changes the partitioning of dry matter so that more carbon goes into the fruit. The result is larger fruit.
He concludes that the quality of light being reflected is as important as the quantity of light. "We can manipulate to some degree the quantity of light that’s reflected, but it may be there’s some value in manipulating the quality of light."
Glenn attributes the inconsistent effects of the sprayed material on fruit color to the fact that in some seasons the weather was cloudier than others and the sprayed film is less reflective than the plastic film. However, in each year of the experiment, fruit weight was greater with the sprayed material than with the plastic film or no film at all.
Glenn said he’s not aware of commercial growers applying a kaolin product between the rows. For his experiments, he used an experimental rain-fast material from Engelhard Corporation, which was similar to the company’s Surround product. Surround has since been purchased by Tessenderlo Kerley, Inc., and Glenn said he hopes that company will commercialize a rain-fast kaolin formulation. "It really would be the most appropriate kind of formulation to use for this," he said.
Even in arid growing regions, the grass in the alleyways needs to be irrigated to keep it alive, he said. Experiments have shown that when a particle film is applied to bare ground, it tends to disappear as the soil particles shift, and if it were applied to dead grass, the grass would not absorb the red light as living grass does.