Foliar feeding is becoming more popular among gardeners, in part because it seems to be an easy way to fix plant nutrient issues. When your plant is showing low levels of iron, spray some chelated iron onto the leaves and you have solved the problem. The second reason foliar feeding is more popular is that there are more products on the market. These include specialty foliar feed products as well as regular fertilizer which is being recommended as a foliar spray.
There is also great interest in using home brews like compost tea and comfrey tea . Use of compost tea as a soil drench has not really shown any benefits, so maybe it works better as a foilar spray?
It’s time for me to have a close look at foliar feeding and determine if gardeners should be using these products.
What is Foliar Feeding?
Plants normally absorb their nutrients through the roots, but in the 1950’s some research showed that plants can also absorb these nutrients through the leaves. This has led to numerous scientific studies, much of it agricultural based. After all, there is money to be made by increasing farm yields.
Most of the interest is focused on plant nutrients, but in recent years there has been some work on biostimulants. These are chemicals, other than nutrients, that play a role in plant growth, including things like, humic acid, proteins, beneficial microbes and plant hormones. This post will focus on nutrient sprays, but much of the content also applies to biostimulants, except that even less is known about their benefit to plants.
The process is fairly simple. Take a product, dilute it with water, and spray your plants. The plants will hopefully absorb the material into their leaves, where it can be of immediate benefit.
How Do Leaves Absorb Chemicals?
Leaves look fairly solid, so how would they absorb chemicals?
Plant leaves do have openings called stomata which are used to expel excess water and oxygen and absorb CO2. Compared to the size of most chemicals, these openings are quite large and for many years it was believed that foliar spray entered leaves through these openings. We now know that stomata are very waxy and repel water, and that foliar sprays are not absorbed through them. This is evidenced by the fact that foliar feeding works best at night when stomata are closed.
More recent research has reveled the existence of micro-pores on the surface of leaves. These openings are less than a nanometer in diameter with a density of 10 billion per cubic cm, and they are the most likely way that nutrients enter leaves.
These micro-pores are lined with negative charges which attract positively charged cations such as calcium (Ca+2), magnesium (Mg+2), potassium (K+) and ammonium (NH+).
Some nutrients are negatively charged, including phosphate (PO4-3), sulfate (SO4-2) and nitrate (NO3–). These anions are repelled by the walls of the pores but they can get in if the concentration gradient is high enough.
Organic molecules do not have a charge and can also enter the leaf provided that they are not to big. You can’t assume that all of these will be absorbed by the leaf.
Is Foliar Feeding More Efficient?
It has been claimed that foliar feeding is 10 times as efficient as fertilizing the ground. The accuracy of this statement depends on how you want to define things.
It is true that adding nutrients to soil encounters significant losses due to leaching, chemical reactions and use of them by microbes. However, the leaf can only absorb small amounts of nutrients. “Soybean will remove 3 to 4 lbs of nitrogen, 0.8 lbs of phosphorus, and 1.4 lbs of potassium per bushel of seed produced. Foliage fertilization could never provide these level of nutrients.” Roots are much better at absorbing large quantities of nutrients.
Nutrients can only flow through micro-pores, also called transcuticular pores, as ions in water. Sprayed leaves dry quickly and limit the time in which feeding takes place. This is why it is common in agriculture to keep wetting plants or to spray at night when dew sits on leaves for an extended period of time. It is also common to repeat applications using diluted solutions.
Only about 15 to 20 percent of the nutrients applied to leaves are absorbed.
Roots grow and function best at cooler temperatures and are less efficient in the middle of summer, whereas foliar absorption is most effective in summer when the plant has a full set of leaves. It is obvious, but worth stating; foliar feeding does not work in the absence of leaves.
When roots absorb nutrients they are able to send those to all part of the plant. Foliar feeding results in some nutrients being stuck in the leaves.
Given these facts, foliar feeding is not more efficient than fertilizing the soil.
Movement of Nutrients Once in the Plant
What happens when leaves absorb nutrients?
Small molecules or ones with a smaller positive charge move around a leaf and other parts of the plant more easily. These include things like ammonium (NH+), potassium (K+) and urea (NH2CONH2).
Larger molecules and those with a stronger positive charge tend to stay close to the point of entry. These immobile nutrients include calcium (Ca+2), iron (Fe+2), manganese (Mn+2), zinc (Zn+2), and copper (Cu+2). Leaves may be able to absorb these nutrients more efficiently than roots, but they are not as useful to the plant because they don’t move around as well.
A popular suggestion is to spray tomato plants with calcium nitrate to prevent blossom end rot. The problem with this suggestion is that the calcium will not move from the leaves to the fruit. It can only move from roots to the fruit. The second problem is that partially developed tomato fruit does not absorb calcium through the skin. Foliar feeding of calcium nitrate will not prevent BER unless it is sprayed on the fruit soon after fruit set and even then it has limited effect.
Nutrient Absorption Rate
How quickly do nutrients get into leaves after being sprayed. Gordon Johnson, Vegetable & Fruit Specialist, University of Delaware had this to say, “Urea, ammonium, potassium, and magnesium are normally absorbed within 12 hours. All other nutrients may take several days of wetting and rewetting to be absorbed. Therefore, it is recommended that foliar fertilizers be applied at dusk or early evening when dew is on the leaves, in high volume water, and using smaller droplets to cover more of the leaf. Applications should also be made when temperatures are moderate and wind is low.
Does Foliar Feeding Work?
Plant leaves do absorb nutrients that are sprayed on them but that does not really answer since we have not defined the word “works.”
In agriculture, “works” is defined as either increasing yield or increasing quality.
Foliar feeding does not show an increase in yield for crops that are grown in suitable soil, that has been adequately fertilized. Testing at the University of Minnesota looked at 6 commercial foliar fertilizers at 46 sites in 16 states, and found no benefit for soybean production.
Foliar chelated iron was not effective on soybeans. Citrus leaves absorb zinc and iron much better as free ions, than ions chelated with EDTA. The EDTA chelating agent is negatively charged, and relatively large, two characteristics that slow down movement through leaf pores. This finding is the exact opposite of what chelated iron products claim.
There are many examples where foliar feeding does not work; here are a couple. Foliar feeding with boron on rice and wheat was not effective. Foliar feeding of cotton, grown on suitable soil, is not effective.
I found fewer studies that looked at quality but this one was interesting. Dairy farmers have been using foliar feeding on their feed crops for a number of years; based mostly on their anecdotal observations and product marketing. A study that took a more scientific approach and looked at 19 different farms found no benefit from foliar feeds. They looked at both yield and quality factors.
There are certainly cases where foliar feeding does enhance plant growth but most of these involve plants growing in less than ideal situations. For example foliar potassium produced a higher cotton yield compared to soil fertilizer, in plants showing a potassium deficiency. And foliar manganese can resolve a deficiency in soybean due to a high pH soil.
Iron and manganese are tied up in alkaline soils, preventing plants from absorbing enough through roots. Foliar sprays can alleviate such a deficiency. Zinc, magnesium and boron have been applied to foliage to solve fruit disorders. It is also commonly used for citrus fruits.
The Downside of Foliar Feeding
Spray too much of a nutrient and it will damage the foliage. Getting the balance right between too little and too much is not easy and it varies depending on plant species, the growing conditions, growth stage and even the time of day.
A Lot of Marketing Hype
When there is money to be made, there will be fake products to entice you. Many product claims are not based on research. They are just a collection of statements that try to convince you to buy.
How do you know a claim is valid? The company should provide a reference to a published research study. The claim should be specific to a plant type and clearly define the deficiency that will be corrected by the product. It should also define the soil and environmental conditions in which the product will produce results.
Statements like, this product will produce higher yields, bigger root systems and more flowers are useless. When plants are grown in good, suitable soil with adequate fertilization, foliar feeding offers none of these benefits. At best, products may meet these claims for some plants grown in less than ideal conditions.
Some specific agricultural products may provide this information, but good luck finding any product that will do this for gardens.
As you read this post you will realize that any legitimate foliar solution must provide the right nutrient(s) to solve a specific problem. These are not “general, do-everything tonics” as so many products claim. The reality is that most gardening products tell you nothing about their content. One product said:
“With this product, the company assembled our most advanced technologies for organic nutrients and micronized algae with other natural ingredients to formulate our proprietary, foliar organic nitrogen fertilizer, and beneficial microorganisms.”
That’s it. They provide no additional information on their website. You have no idea what you are buying, but it is claimed to solve a long list of common garden issues.
Stop buying products from companies that treat their customers like idiots!
Homeopathics for the Garden
Product manufacturers don’t want to recommend a dose that will harm plants so they tend to suggest low doses.
Recommendations for nitrogen foliar feeding products are around 1 quart per acre, which adds 0.2475 pounds of nitrogen per acre. Seedling plants may cover 1% of the area resulting in 0.003 pounds of nitrogen per acre, actually landing on plants, and remember that less than 20% of this will be absorbed. This reminds me of homeopathic medicine. Nothing in the bottle …. but it works magic.
Many home garden recommendations are so low that they’re a waste of time.
Is Foliar Feeding Better Than Fertilizing the Soil?
After 60 years of researching this topic, the scientific consensus on this answer, is clear, “feed the roots and not the foliage.” Foliar feeding is never an alternative to building good, healthy soil.
Foliar feeding can be effective in one of two cases.
- You have a specific situation where the research is clear and supports the use of foliar feeding. This will be situational-based, for a specific plant species and a specific set of soil and environmental conditions.
- Your plants have a known micro-nutrient deficiency that requires a quick fix.
In all other cases it is better to apply the right fertilizer directly to the soil.
Is Foliar Feeding Suitable for Gardeners?
Foliar feeding is used in agriculture, so why not use it in gardens?
To make foliar feeding work, farmers do extensive soil and plant tissue testing. It is crucial that they correctly identify the nutrient deficiency and apply the right kind of feed in the right amount. Gardeners try to identify nutrient deficiencies by looking at foliage, which simply does not work. Even worse, is advice from strangers on social media.
There is second part to this issue and that is knowing the right target nutrient level. What is the best nutrient level for a plant? Even in agriculture where they research specific crops, the answer to this question is not always clear. It is now known that much of the data we have is not correct. For the gardener who wants to solve a problem with their summer phlox, there is no data. Even if you know which nutrient is deficient, nobody can tell you how much to spray.
Using foliar spray on a landscape planting where you have many different kinds of plants makes no sense at all.
Sure, you can follow the directions on a bottle, or some advice on social media, or just guess, but it is unlikely that you will solve the problem. Home gardeners should stay away from foliar feeding.