Foliar Feeding - Fact or Fiction?

Providing plants with nutrients remains a great mystery to many horticulturists. The many options available (inorganic or organic?, soluble or slow release?, different nutrient combinations or ratios?, soil or foliage applied?) make decision making difficult. In this article I thought I would look at one of the components of this problem, that being the question "Can a plant absorb nutrients through its leaves and is this a viable option for gardeners to use?" Like the answers to many other horticultural queries, the answer to this one is a qualified yes. Plants can absorb nutrients through their foliage but not in great quantity and sometimes not very quickly. Despite those limitations, foliar feeding is sometimes worthwhile to consider.
The usual uses of foliar fertilizer is in trace element nutrition where the plant does not require large quantities of the nutrient in question. Since many trace element problems occur as a result of unfavourable soil conditions (for example high soil pH), foliar applications can sometimes be more effective than applying the fertilizer to the soil. Examples of nutrients which can be applied in this case are zinc, manganese, copper and molybdenum. As an example, an application of molybdenum to cauliflower to overcome whiptail disorder would be 0.25 g ammonium or sodium molybdate per litre of water, applied as a spray early in the growth of the crop. This spray will be held on the foliage more effectively if a wetting agent is included in the spray (because of the waxy nature of the leaves). Given that many trace elements can become toxic if excess is applied, the use of any trace element fertilizer should be based on an accurate diagnosis of a problem rather than an application, "just in case".
Nitrogen is also readily absorbed by leaves and foliar fertilizing can be used to supply nitrogen. Urea is the best source of nitrogen for foliar use and many commercial products use urea as their nitrogen source. Check the fertilizer product label if you are not sure. If you are using pure urea as a foliar fertilizer, make sure it has the impurity biuret at no more than 0.4%. Again, this will be stated on the label. A foliar spray mixture for general use is 10 g urea and 30 g potassium nitrate per litre of water. For sensitive plants, use this at half strength. A wetting agent will be useful if the leaves are waxy. To supply all of a plant’s nitrogen needs through the leaves will require several applications because of the amount that a plant takes up, but foliar feeding can be a way of getting rapid absorption taking place. Should any of the spray fall onto the soil then it will behave as any other fertilizer and ultimately be absorbed by the roots.
One issue which one also has to consider here is that some elements are only absorbed into leaf tissue very slowly. In these cases, foliar fertilizing is unlikely to be of any real use. Two important examples of this are the elements iron and phosphorus. This is something of a pity as both of these elements can become unavailable in soil through unfavourable soil conditions.
So, in summary, foliar fertilizing of plants is possible, but I am inclined to suspect that in most circumstances conventional means of providing nutrients are more logical solutions.