Foliar Fertilization: Throwing Money at Leaves

I often see gardening tips or techniques on the web or on social networks that make me raise my eyebrows. It’s not that I don’t believe in them, but sometimes I have strong doubts, especially when these tricks seem miraculous. And this is even more true when this new method involves the purchase of any product. Please DO NOT put Coca-Cola on your plants!

Ideally, before certifying that something is “true”, there should be a published scientific study on the subject. Then, these studies must be reproduced by others and finally, specialists in the same field must verify the validity of these studies, to ensure that the experiments conducted were done according to the highest standards. It’s never-ending!

And when someone tells me: « It works for me!”, well, that doesn’t work for me!

One problem that we regularly encounter as gardeners is that there’s little research done on residential horticulture. There is a lot of money to be made, and probably more impact, by studying large-scale horticulture and agriculture.

Some techniques that home gardeners use are never studied in depth. Or we try to apply methods that work on an industrial or commercial scale to home gardening when they aren’t necessarily applicable.

I find that foliar fertilization falls into this category.

What’s Foliar Fertilization?

Foliar fertilization is the practice of applying water-based fertilizers directly to the foliage of plants using a hand-held or pump sprayer. It has been known since the 1950s that essential plant nutrients can actually be absorbed through the leaves. For a long time, it was thought that stomata – pores on the surface of leaves that allow them to breathe – were the main entry point, but they are waxy and therefore repel water. Later, micropores, measuring 1 nanometer, were discovered in the epidermis of leaves, and it’s more often through these that fertilizer could enter.

Advantages of Foliar Fertilization

Why fertilize plants through their leaves when their roots are already doing the job very well? It’s true that the root system of plants has evolved to absorb the nutrients they need, not the leaves. The main advantage of foliar fertilization is that it’s faster and works directly on the desired location, in this case the leaves and branches of the plants. It’s also said that a foliar application is more effective because some of the fertilizer applied to the soil can be leached out. Some elements, such as phosphorus, can be fixed to the soil in a form that is inaccessible to plants.

What’s the Problem Then?

At this point, I’m beginning to regret having chosen to write this text, because the mechanisms that make possible the absorption of nutrients by the leaves are very complex. And that’s the problem! Foliar fertilization is presented as a simple solution when it’s anything but simple.

For starters, some elements, once absorbed, travel better than others in a plant. Calcium and iron, among others, have large molecules. Once these molecules have entered the leaf, they tend to stay put without going where they are needed. If you wanted to prevent blossom-end-rot in a tomato by adding calcium, it would have little effect, because the calcium would get stuck in the leaves rather than going to the fruit. Other molecules travel far more easily after foliar application, but never as well as when they are absorbed by the roots.

In general, foliar fertilization works best in deficient soils. One study showed an increase in yield in cotton production with foliar fertilizer, but only when potassium was deficient. Soil type affects the penetration of certain elements as well. Alkaline soil makes it harder for the roots to absorb iron and manganese, so a foliar application would be in order. Or you could try rubbing your plants with a banana. Bananas are full of potassium!

Method and conditions

It’s very important to monitor the method and conditions under which the foliar application is made. Here’s what Gordon Johnson of the University of Delaware suggests: “For foliar fertilizers to be most effective, they should remain in liquid form on the leaves or other targeted plant tissues as long as possible. The urea and ammonium forms of nitrogen, potassium and magnesium are normally absorbed within 12 hours. All other nutrients may require several days of wetting and rewetting before being absorbed. Therefore, it is recommended that foliar fertilizers be applied at dusk or early evening, when dew is on the leaves, with a high water volume and using smaller droplets to cover more of the leaf. Applications should also be made when temperatures are moderate and wind is light.”

The concentration of fertilizer in a foliar application should also be monitored, because when the water evaporates, molecules that have not been absorbed will remain on the leaves and can cause burns or leave spots on the leaves.

(If you found reading that last section painful, imagine what it was like to write it! I need to finish this text, and fast. My poor brain can’t take much more of this).

Leave It to the Pros

In short, foliar fertilization works in certain contexts, at certain times and after establishing, through laboratory tests, that there are specific deficiencies to be addressed. It’s a technique that should be applied by professionals. While studies in this area are promising and important to an agricultural industry struggling with climate change, a faltering economy and the need to green their practices, there’s still no scientific consensus on the widespread effectiveness of foliar application of fertilizers.

Now that you know how complicated foliar fertilization is, you have to admit that you don’t want to do it anymore, right? Unless you’re determined to win a contest for the world’s largest pumpkin or most expensive tomato ever. You might as well throw your money out the window than on your leaves!