(Thank you to Laura Forlin, co-chair of the Merritt College Horticulture Dept, for teaching me all this stuff.)
We completed our Foliar Feeding Experiment yesterday in lab. The results were interesting, and I want to share them with you. But before I do that, we need to review a couple of things.
What do plants require to grow, thrive, and set seed?
An element is essential if: 1) A plant cannot complete it's life cycle without it; 2) no other element can perform the same function; 3) it is directly involved in the nutrition of the plant; and 4) missing or insufficient supplies adversely affect plant growth.
There are three macronutrients that everyone forgets, they are non-mineral, and they are required in larger quantities than any of the others. Can you guess what they are?
They are obtained from the atmosphere and water.
Yes! Oxygen, carbon, and hydrogen.
The next six elements are macronutrients and are the most important after the big three: Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and Sulfur (S). These are required in large quantities. N, P, and K more than the others.
The next eight are micronutrients - still essential, but required in much smaller amounts: Chloride (Cl), Iron (Fe), Boron (B), Manganese (Mn), Zinc (Zn), Copper (Cu), Nickel (Ni), and Molybdenum (Mo).
Many of these are present in the soil already and you may not need to supply them. Knowing your levels of nutrients can be important, and that's why folks do soil tests, to discover what is lacking and add it in appropriate amounts. Now this is important: TOO MUCH FERTILIZER CAN BE WORSE THAN TOO LITTLE. I cannot stress this enough. If you just add chemicals willy-nilly, they will either be leached out quickly (entering the water supply, affecting organisms downstream) or they will damage your soil and plants.
MOST SYNTHETIC FERTILIZERS HAVE HUGE AMOUNTS OF THE NUTRIENTS. You buy something like a 16-16-16, that's super hot. Putting that on your plants can be very damaging. Most synthetic fertilizers are completely water-soluble, meaning they enter soil solution quickly, are taken up quickly, and are leached quickly. It's wasteful, it's expensive, it can damage your plants, it can kill your soil microorganisms, and it damages the environment. If you've decided that you need to fertilize your plants, ORGANIC FERTILIZERS ARE THE WAY TO GO. They have a much smaller percentage of nutrients (like 3-3-3), most are only partly water soluble (which means some will be available right away, and some needs to be mineralized by the soil life first, giving you longer lasting nutrients; it also means less will be leached quickly from the soil and cause environmental problems), and they are made from products which are found in nature, and are often made out of waste-stream materials like feather meal, blood meal, bone meal. CHECK THE LABELS ON THE PACKAGES to know what you are getting and how high the percentages are.
Container plants will need regular feeding because they are disconnected from the soil life. BUT - if you are adding lots of organic matter to your in-ground beds, mulching, using cover crops, crop rotation, etc - you probably won't need much in the way of fertilizers at all. The only way to know for sure is to test your soil.
You can buy a simple home-testing kit for about $10, which is really all most of us need - they aren't perfectly accurate but will give you enough of a result to figure out your imbalances. However, if you're feeling adventurous and you want some serious results, you can send your soil away to be tested at a lab. For a more conventional nutrient test, you can send a sample to U Mass. It costs very little, and you'll get a very interesting report. They will also give you recommendations based on your nutrient levels and soil texture. This is a fine way to go, but it's maybe not the most important test you can do.
You see, the microorganisms present in your soil determine your soil health. They are the ones who process the minerals and make them available to your plants. If you really want to know how alive your soil is (and you want it to be very, very alive), you could send a sample to Earthfort. They will test to determine the amounts of bacteria, fungi, nematodes (beneficial and detrimental) and protozoa in your soil. They can also tell you the percentage of organic matter. This costs a bit more, but it's probably worth it. I haven't done this yet, but I very much want to.
So that's an overview of nutrition. Now I want to tell you the results of our foliar feeding experiment.
The reason we even did this experiment is because of all the hype surrounding foliar feeding. Search for it on the internet, and you'll get some fabulous claims. Let me be clear: the science is definitely not there to support those claims. There just isn't enough peer-reviewed data to say definitively that foliar feeding works. Plus, plant biology doesn't really support anything being taken up by the leaves. Each leaf has stomata, little openings on the undersides, which allow for gas exchange. But they don't take anything else up in there. So how does a foliar substance get in the leaf in the first place? It hasn't been discovered, if it exists. Also, there is a translocation of nutrients within the plant, but the nutrients themselves come from the roots. Photosynthate moves down, nutrients move up. Mostly. Not always. Things can be reallocated around the plant if need be. But there is no pathway from the stomata to the xylem or phloem from the stomata.
There IS evidence that spraying microbiology on the leaves, like with compost tea, provides a coating on the leaf that is helpful in many ways. The leaves need good biota just like the roots do. It can protect them from predators, keep them from being sunburned, allow good stuff to live on the surface. It's just that it isn't clear that anything actually ever gets in the leaves except gases.
Our teacher, Laura, wanted us to try an experiment to see if we could get any definitive results. Once the plants (chard for my team, again) germinated and had a couple of leaves, we began to cover the soil and spray only the leaves with macronutrients. We sprayed some with organic all-purpose fertilizer, and some with synthetic all-purpose fertilizer. We also had several controls that got soil drenches of these same fertilizers. We drenched the soil of each plant with micronutrients each week so that wouldn't be a limiting factor. The soil surface of each plant was covered with fresh plastic wrap each week, so no accidental dripping from the leaves would occur. The controls were covered as well so we could make a comparison.
Spoiler alert: They all did horribly. Even the controls weren't so hot, we think because the soil was covered each week and there was a lack of oxygen.
Here's our best-looking control:
Really floppy. Not good strength. Nice color, but not a lot of growth.
Here is an example of one of our foliar-sprayed plants. Remember, this plant got zero macronutrients other than on the leaves.
Just terrible. It's very chlorotic, and it didn't grow at all from the time we started spraying until the end of the experiment. What a total waste of a plant.
So there you have it. Foliar sprays are really a waste of time. Unless you feel that you need some good biota on the leaves, you can skip it. Add your inputs to the soil itself. Making compost tea is fine, but just drench the soil with it, where it can do some real good. Don't bother with the spraying.