Harry Wakelin, of Dunedin, asks :-

Some manufacturers sell materials called foliar feeders to be sprayed onto the leaves of plants in order to feed them. Can plant leaves extract and process nutrients as the plants root system does?

David Burritt, a botanist at the University of Otago, responded.

Yes, plants can take up nutrients applied as foliar sprays. When nutrients are sprayed onto leaves they move through the stomata, microscopic pores found in the leaves of plants, and are transported down the plant at a rate of about 30 cm an hour. In fact plants are able to absorb nutrients through their leaves 8 to 10 times more efficiently than through their roots.

Dr. H. B. Tukey and his colleagues at Michigan State College proved the value of foliar feeding in the 1950's. They found that plants could use 95 percent of the soluble nutrients applied to leaves, but if similar amounts of nutrients were applied to the soil only 10 percent of the nutrients were available for use by the plant.

They also found that other plant parts such as twigs, the trunk and even flowers could take up nutrients. Many commercial growers of plants are interested in using foliar-fed nutrients, as a small amount of nutrients applied as a foliar spray can replace a much greater amount of nutrients applied to the soil. This can reduce production costs for the grower and is better for the environment.