Foliar Feeding of Plant Nutrients & Bonsai

Plants of all types are capable of absorbing nutrients and moisture through the stomata or pores within their foliage. This is a proven fact. However for many years, horticulturists have debated the merit of foliar feeding of plant nutrients. A commonly held opinion is that foliar nutrient feeding is best employed only where a specific minor element deficiency may exist. Dramatic and fast correction of such nutrient deficiencies are generally always seen from such foliar applications.

Dr. H.B. Tukey, renowned plant researcher and Head of Michigan State University's Department of Horticulture back in the 1950's, working with research colleague S.H. Wittwer at MSU, first proved conclusively that foliar feeding of plant nutrients really works. Researching possible peaceful uses of atomic energy in agriculture, they used radio-active phosphorous and radio-potassium to spray plants, then measured with a Geiger counter, the absorption, movement and utilization of these and many other nutrients within plants. They found plant nutrients moved at the rate of about one foot per hour to all parts of the plants. Comparing efficiency of plant use of foliar-fed nutrients versus soil-applied nutrients near roots, they found foliar feeding provided about 95 percent efficiency of use compared to about 10 percent of use from soil applications! Likewise, speed of absorption and use by foliar applications was immediate, whereas from soil applications absorption and plant use were both very slow, thus proving a major benefit of foliar feeding where a specific plant nutrient deficiency may exist, be it major or minor plant nutrient.

The reason that many plants suffer nutrient deficiencies is almost always due to growing media problems. The cation exchange capacity (CEC) is a value given on a soil analysis report to indicate the capacity of the soil to hold cation nutrients. It is a value that indicates a condition or possibly a restriction that must be considered when working with a particular soil. Unfortunately CEC is not a packaged product, it is not something that is easily adjusted. The two main colloidal particles in the soil are clay and humus.

The CEC of the soil is determined by the amount of clay and/or humus that is present. These two colloidal substances are essentially the cation warehouse or reservoir of the soil and are very important because they improve the nutrient and water holding capacity of the soil. Sandy soils with very little organic matter (OM) have a low CEC, but heavy clay soils with high levels of OM would have a much greater capacity to hold cations.

The disadvantages of a low CEC obviously include the limited availability of mineral nutrients to the plant and the soil’s inefficient ability to hold applied nutrients. Plants can exhaust a fair amount of energy (that might otherwise have been used for growth, flowering, seed production or root development) scrounging the soil for mineral nutrients. Soluble minerals or fertilisers applied in large doses to soil with a low CEC cannot be held efficiently because the cation warehouse or reservoir is too small.

In bonsai cultivation it is a fairly simple task, upon re-potting, to improve our soil characteristics and raise the CEC of the soil in order to help the plant to thrive. On the down side of course we are growing a plant in a very limited volume of soil that will be very quickly stripped of any inherent plant nutrients, hence our relative pre-occupation with "feeding" our trees. Foliar feeding can be a valuable aid to maintaining healthy bonsai, not just a 'pick me up' for a sickly plant.

Armed with the knowledge they dug out of the research journals, commercial agricultural chemists began developing foliar feeding formulations. Their continuous product improvement and research has resulted in products containing not only specific plant nutrients, but also natural plant sugars that aid rapid entry and movement into and through plants, plus cytokinins extracted from seaweed, now stabilized for several years of shelf life.

Cytokinins (a class of plant growth substances or plant hormones active in promoting cell division, and growth) together with nutrients aid natural plant defence mechanisms to resist many plant diseases and insect pests. We know that healthier plants, like humans, are better able to resist many pests compared to those in stressed, poor condition. Also we can see that the weakest plants are the ones most often attacked by insects, disease and mite pests. So a relatively small amount of plant nutrients, foliar-applied can result in a greater volume entering into the plant than is possible by root feeding alone. This fact can be exploited by the bonsai grower to great advantage because it enables us to get nutrients into a plant who's root system has been recently disturbed following re-potting or collection from the wild…..Thus aiding recovery.

The best time to foliar feed is late evening to early morning. These are the times when the stomata (the small pores on the leaves) are open. Avoid foliar spraying when the temperature is above 80F and when the weather is hot and dry. Foliar spray when the temperature is 72F or below, when the cells of the leaf are full of water ie. the plant is not wilting and when the air temperature is cooler than soil surface temperature. Most stomata are located on the underside of the leaves so spray from beneath as well as above. As with all fertilizer applications only apply during active growth.

Graham Potter.
© Kaizen Bonsai Ltd 05/2007


References cited for further reading:

1. Tukey, H.B. and Wittwer, S.H., 1956. The entry of nutrients into plants through stem, leaf and fruit, as indicated by radioactive isotopes. Progress in Nuclear Energy Biological Sciences Scries Six, pp. 106-114. McGraw-Hill. New York and Permagon Press, London.

2. Tukey, H.B., Wittwer, S.H., Teubner, F.G., and Long, W.G., 1956. Utilization of radioactive isotopes in resolving the effectiveness of foliar absorption of plant nutrients. International Conference on the Peaceful Uses of Atomic Energy, Vol. 12: 138-148. United Nations, N.Y.

3. Witter, S.H., Teubner, F.G. and McCall, W.W. 1956. Comparative absorption and utilization by beans and tomatoes of phosphorus applied to the soil and foliage. Proceedings, American Society for Horticultural Science. (needs vol and pp numbers from Barden).