So, here's the scene. It's the early 1950s and we are in the greenhouse laboratories at Michigan State University's Department of Horticulture. Enter Dr. H.B. Tukey, head of the department, and his colleague Dr. S.H. Wittwer.
The lab is meticulously clean. The two men are yielding water misters containing solutions of the radioactive isotopes of known elemental nutrients, such as phosphorus and potassium.
They carefully begin spraying the solutions onto the leaves and stems of some rather unsuspecting plants and then they pause.
Breaking the silent anticipation, Dr. Tukey eventually steps forward. In his hand he holds a Geiger counter and he begins waving it slowly around the plants. The counter is making that static-like clicking noise that resembles fingernails tapping erratically on a counter top.
A smile cautiously forms on his face as he realizes that the radioactive nutrients have been absorbed into the plants and are beginning to move around within. The men quickly start documenting their breakthrough, and history has been made.
Okay, so this might not be exactly how these events took place—and spraying radioactive material with a handheld mister is most likely not safe. However, this is where the widespread interest in foliar feeding began.
When Dr. Tukey and his colleagues discovered that elemental plant nutrients could be absorbed through a different part of the plant, besides just the roots, they in turn sparked new testing, new practices and a new debate. Even now, 60 years later, the conversation continues on the effectiveness and benefits of foliar feeding.
The main point of absorption for elemental plant nutrients is through a plant's roots. But sometimes the nutrients can become locked up with other elements in the soil, rendering them unusable by the plants.
There are many factors that can contribute to nutrients becoming immobile in the soil. If the fertilizer solution you use is imbalanced or if its pH is too high or too low, the nutrients might not be absorbed by the plant.
Poorly managed soils, damaged root zones, excessive watering: all of these situations can lead to lowered rates of absorption of vital plant nutrients. When a nutrient doesn't seem to be working effectively through soil applications, using the foliar feeding method is a possible solution.
The leaves, and sometimes even the stems, of many plants are equipped with tiny, pore-like apparatuses called stomata. The word stomata stems from the Greek word stoma, meaning mouth.
And that is essentially how they work. Regulated by task specific cells, appropriately referred to as guard cells, a plant's stomata will open and close at certain parts of the day. Stomata are essential for two main reasons. The first is to allow oxygen and water vapor to leave the plant (transpiration), which in turn cools the plant down and allows for more water and nutrients to flow from the roots to the leaf cells (translocation).
The other is to provide a point of entry for carbon dioxide from the air to come into the leave and make photosynthesis possible. Stomata can also act as a passage way for getting liquid plant nutrients into a plant. But, as is true with many aspects of life, timing is everything.
The opening and closing of stomata is directly affected by certain environmental conditions. As far as I can tell this is not an exact science yet, but some basic principles seem to be regarded as true.
Stomata are generally open during periods of high light intensity. A reason for this could be that the high level of light is causing a high level of photosynthesis, so the stomata are open to allow more carbon dioxide in.
Stomata also open during times of high humidity, when water is plentiful and plants don't need to worry about conserving. However, stomata remain closed when conditions are exceedingly hot (above 80°F) or very dry.
In these conditions a plant will keep its stomata closed in order to conserve any available water. If you plan on foliar feeding in the hot summer months, it is recommended to feed in the morning or early evening, as the sun is out and the weather is still relatively cool.
Still, understanding when the window of opportunity is for open stomata to occur is only one part of the picture. The next part is figuring out how to get the nutrients in.
Successful foliar feeding is not as simple as just spraying the liquid on the leaves; it is a process that involves careful technique and a little bit of grace. The following is a small list of tips I've compiled to help you along the way:
- Avoid foliar feeding when temperatures are above 80°F. In the summer it's best to spray either in the morning or early evening, when temperatures are lower.
- If possible, foliar feed when the weather is humid.
- Check the pH of your nutrient solution before spraying. The ideal pH is right around 7.0, which is referred to as a neutral pH.
- Mix your solution at a more diluted rate than you would for root feeding. If the directions call for 1 oz. of fertilizer per gallon of water for regular feeding, use 1 tsp per gallon of water when foliar feeding. The smaller the particles are, the more likely they are to enter into the stomata.
- When spraying the solution, use a sprayer that creates the finest mist possible. This will ensure a better and more even spread of the solution on the leaf.
- Use a wetting agent or surfactant. Water has a high surface tension rate, which causes it to bead up when sprayed. Adding a wetting agent will lower the water's surface tension, allowing it to thin and spread out.
- On most plants the stomata are on the underside of a leaf, but at times they reside on the top. So, just in case, spray both the tops and bottoms of the leaf until they are completely covered and excess solution runs off.
Possibilities and realities of foliar feeding your plants
A trend that has been occurring in the liquid fertilizer industry for some time now is to market foliar feeding as a simple fix for what could be a major problem. Many companies include language like "maximizes plant health" or "increases yields" in there literature regarding foliar feeding.
I remember reading an advertisement that said, in so many words, that foliar feeding is effectively the best way to battle bad soils. At that point I took a step back and thought to myself, "Is it really?" My feeling is that bad soils need to be carefully amended in order to obtain maximum plant growth.
However, it is true that foliar feeding can achieve much higher nutrient absorption percentages than root feeding. But it is also true that nutrients absorbed through the stomata do not travel throughout the plant as extensively as nutrients absorbed through the roots do.
Also, it is impossible to get significantly large amounts of nutrients through the stomata. Essential elements such as nitrogen and phosphorus are needed by plants in high quantities, levels that are only achievable through root entry.
Minor elements, such as iron and magnesium, are needed in smaller amounts that can be obtained through foliar feeding. In the case of iron, when your plant shows signs of iron deficiency, cut a leaf of the plant and dip half of it in the nutrient solution you plan on using. If, after a few hours, the symptoms begin to subside, go ahead and use the solution on the whole plant.
Another element that can become immobile in the soil and can be of benefit in foliar feeding is calcium. Using calcium in a foliar treatment can help battle blossom end rot in tomatoes and peppers.
So, foliar feeding can effective as a way to supply a plant with micronutrients and as a short term solution to many different nutrient deficiencies. However, if you are experiencing the same nutrient deficiencies on a consistent basis, foliar feeding might not be the answer.
Foliar feeding is usually more of a temporary fix instead of a solution to a problem; a fix that can be labor intensive and, at times, can become rather expensive, especially when used on a large scale.
I've always believed that healthy plants come from healthy soils. Properly amending the soil in your garden should be your first step.
Perhaps get your soil tested to see what it is lacking or what there might be too much of. If minor nutrient issues arise along the way or if you just want to give your plants a little boost, foliar feeding (when done correctly) can be an effective addition to your gardening repertoire.