There has been a great proliferation of foliar fertilizers on the agricultural chemical market in recent years. Many of these have been useful in correcting nutrient deficiencies in leaves and fruit, but this does not mean that all foliar fertilizers are beneficial.
Remember that plants were not designed to take up nutrients through the leaves and fruit. Only a few nutrients, such as potassium, easily enter the above ground parts of the plant. Most are restricted from entering leaf and fruit surfaces by waxes, oils, hairs and highly charged surfaces. This is why, to be effective, calcium sprays on pears and apples have to be repeated several times a season, with only a small fraction of the applied material entering fruit tissue.
When is foliar fertilization best?
In the case of calcium, transport from roots to fruit is limited, so foliar applications are the best method we know of to get more calcium into fruit tissue to reduce post harvest disorders. The expense of the calcium sprays is more than justified by the potential post harvest losses.
If soil pH limits nutrient availability, and ground applied fertilizers are not taken up, foliar fertilizers may be a valid option. In this case, a soil sample should be taken to determine pH, and a leaf tissue sample taken to determine the need for additional foliar fertilization. In some cases poor root health from compaction, replant disease, crown rot, mouse damage, waterlogging or other problem may warrant foliar feeding of trees. However, the fertilizer in the required amount cannot be phytotoxic as a foliar spray, and foliar uptake must have been demonstrated with the product under consideration.
Zinc uptake deserves special attention. In our soils zinc is largely immobile and it is difficult to supply roots with adequate amounts of available Zn. As a result of limited soil availability, zinc is applied as a foliar spray. Research has shown that only a small amount of Zn can be taken up by leaves, however foliar applications are still more successful than soil applied Zn.
If soil pH is not limiting nutrient availability, root health and growth are not restricted, and transport of the nutrient in the tree is not restricted, soil applications of fertilizers are much more efficient than foliar sprays. Consider spending money on soil and leaf analysis to determine nutrient deficiencies or uptake problems, rather than embarking on a shotgun foliar approach.
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Tree Fruit Research & Extension Center, Washington State University,1100 N Western Ave., Wenatchee, WA, 98801 USA
Wenatchee WA, 20 August 1996