Foliar Feeding - Does it Work?

In the past couple of weeks, I have toured several different farms and gardens where the caretaker sung the praises of foliar feeding. “The plants absorb the nutrients so much faster and growth just explodes!” said one farmer enthusiastically. Every time I hear a comment or recommendation regarding foliar feeding, my inner bullshit alarm goes off. I just don’t believe it actually works. But is my hunch true, or am I just stuck in old thinking?

Years ago, I was enamored with the idea of making compost tea and then spraying it as a foliar feed on my veg and fruits. I was sure it would improve the health of my plants, deter pests, and increase yields. Making compost tea is a lot of work; it requires a bubbler in order to oxygenate the mixture as it’s steeping, keeping the organic matter in a mesh bag so it doesn’t clog the bubbler, then decanting the mixture into a sprayer, etc etc etc. And then the spraying itself, messy and time-consuming. All of this has associated costs, too - you need to buy a big sprayer, a bubbler and tubing, several containers to hold everything, and mesh bags or cheesecloth. This is all after the regular work of making compost, which is noble and important but not exactly a hands-off project. After all this fuss, I really didn’t notice much of a difference in my plants when I performed foliar feeding.

Not long after, I took a Plant Nutrition class as part of my ‘nursery management’ coursework at Merritt. Several students were convinced that foliar feeding worked better for plant nutrition than any other method. My instructor was dubious. She explained that leaves are not designed to take in nutrients. There are nearly microscopic holes on the bottoms of leaves, called ‘stomata,’ which evolved to provide gas exchange - these holes allow for oxygen and carbon dioxide to move between the plant and air as part of the process of photosynthesis. They can expel water, but actually repel water that’s trying to come in, using a waxy coating for that purpose.

More recent research has revealed the existence of micro-pores on the surface of leaves, and they are lined with negative charges which attract positively charged cations (things like calcium, magnesium, and potassium). This research has shown that nutrients can enter these pores as ions in water, but sprayed leaves dry very quickly and limit much absorption, and only about 15% of the nutrients applied this way are absorbed.

And as my professor then told us, even if leaves do take in nutrients in this way, the plant is really not designed to move nutrients other than sugars from the leaves to the roots, or to any other part of the plant. The whole system is designed to draw nutrients from the soil solution (a pool of available water at the rhizosphere that contains nutrients) up through the roots and into the xylem, which is a kind of fluid river that moves water-soluble nutrients from the roots to the rest of the plant. There’s a downward river, too - the phloem - but it’s carrying sugars made in the photosynthetic process down from the leaves to the root zone. That’s what it’s designed to do, not carry nutrients from the stomata down to the roots. So even if nutrients are entering the leaves, they are stuck in the leaves. They are immobile due to that stronger positive charge.

Let’s take calcium for instance. A gardener recently told my class that she sprayed her tomatoes using a foliar feed of liquid calcium to prevent blossom end rot. But calcium will not move from the leaves to the fruit. It can only move from the roots to the fruit. Tomatoes also do not absorb calcium through the skin. So this is arguably not helping the plant avoid blossom end rot. (And if you’re a long-time reader of this blog, you already know that blossom end rot is not really a calcium issue.)

Remember my plant nutrition class? My professor posited that what was really happening in foliar feeding was that the nutrients were dripping off the plant and down into the soil, thereby entering the soil solution. But she was not married to that theory and was game to experiment. So, in lab, we planted a bunch of leafy vegetables in 4” pots. Then we covered the whole surface of the soil with plastic wrap. Then we turned the plant on its side and sprayed the leaves with a foliar feed (I can’t remember what the actual fertilizer was, unfortunately). This way the feed dripped off the leaves onto the ground, instead of down into the pot. And the plastic wrap was there as insurance to make sure that nothing entered the soil. We let the plants drip dry before righting them, removing the wrap, and watering them with plain water before replacing the wrap. We cared for these plants over the course of the whole semester.

Care to guess the result? The plants in 4” pots with only foliar feed did very poorly indeed. They were yellowed, diseased, and wilted. We concluded that foliar feeding didn’t work, or at least it didn’t work well enough to supply enough nutrients to the plant.

But that’s not a rigorous study, so I consulted some other recent research. According to a 2016 study by the University of Wisconsin-Madison’s Agriculture Department, “for many nutrients, there is little or no published information showing a valid relationship between crop yield increase and tissue nutrient concentration that provides good reason for making widespread recommendations to apply a foliar- or soil-applied fertilizer that includes that nutrient,” and, “for many of these essential nutrients, we lack proven research that defines the exact minimum nutrient concentration below which yield is harmed and verifies that a beneficial yield response to foliar feeding occurs.” And according to a 2020 article from Ohio State University’s College of Food, Agriculture, and Environmental Services program, “results (in foliar feeding) were inconsistent and didn’t reveal a cohesive pattern of increased yield or… plant health for the sites in this study.”

I suppose it’s possible that spraying the leaves with a nice compost tea might help protect the leaves from some fungal infections, or pest predation. But as a method of actually feeding the plant, I think it’s an inferior choice. The best thing to do for any plant is to provide it with plenty of organic matter, which allows the soil biology to process all the nutrients in the organic matter and make them available in soil solution for the plant roots.

Reference: “Why I Typically Do Not Recommend Foliar Fertilization,” by Caleb P. Goossen, Ph.D, MOFGA Crop Specialist, June 2023.

Don't be Afraid to Edit

Pictured above is a flower I’ve been excited to see this year - a Sherbet-toned Chantilly Mix snapdragon, from Floret Farms. They’ve just begun to bloom, and I anticipate a fabulous display. I am growing them in a container this year, and if they do well, I’ll collect the seed and scatter it elsewhere next year. I’ve never been particularly keen on snapdragons, but I think this mix is lovely and delicate, and adds to the cottage-y feel of the garden.

I’ve had several friends (the desire to make a beautiful and productive garden, during this time of shelter in place, is definitely widespread, and I’ve had more interest in my garden and my services then I’ve ever had before), express a recurrent theme that I want to discuss: Don’t be Afraid to Edit.

You may have an established garden that you planted yourself, or you may have a garden you’ve inherited; you may be new to gardening, or an experienced gardener; the advice is the same regardless. If something in your garden isn’t pleasing you, and you’ve had it there for a long time, and you’ve tried many times to make it look good by pruning or feeding, and it still bothers you - TAKE IT OUT and start again. You do not have any obligation to change your feelings about this particular plant. You may have loved it once, you may have always hated it. No one is going to come into your garden and say “Oh my God what did you DO?” or if someone does, just tell them you decided to make a change. It’s YOUR garden, after all.

Sometimes we’ve looked at the same things over and over again for years and we can’t imagine the space looking different. Maybe our eyes have always drifted past that place and settled on a more pleasing grouping. I would advise that you allow yourself to really look at that space. Ask yourself a few questions: What is it that isn’t working? Why does it bother you? or alternatively, why are you ignoring it? If you could dream up anything, what would it be? Is there something you’ve always wanted to add to your garden, but haven’t had room? Don’t edit your thoughts, just let yourself dream. You may just come up with an exciting new plan.

See, the thing is, right now (and conceivably in the future as well), if we’re fortunate enough to have any kind of outdoor space, be it a back patio, a front porch, a balcony, or a garden - it’s all we have. Along with our homes or apartments, this is our safe place. You may be, like me, taking a fresh look at the way the interior of the house works and looks, and thinking of things to make it more comfortable and efficient. Why would your garden be any different? In many seasons, your garden can be an extension of your living space, another room that is underutilized at the moment. And I’m not talking about enormous expensive changes, though if you have the money and desire, more power to you (although I’m a big fan of doing it yourself). I’m talking about making these spaces pleasant for those who want to dwell in them.

And if something bothers you, why live with it? If you have an old, woody lavender that barely blooms anymore, and you feel like you should leave it because the blooms are good for pollinators, and you like the smell, why not chop it back and see if it improves? Or maybe, take it out, increase the drainage in the soil by mixing in compost and grit, and plant three different kinds of lavenders in the same spot? Or maybe that’s the ideal place for several Mediterranean herbs like lavender, rosemary, and thyme, which can be beautiful AND useful, both for you and the pollinators?

After meeting with some clients who are new to a property and have no attachment to anything already there, I am feeling emboldened and ready to make a few changes of my own. I also recently read ‘Planting: A New Perspective’ by Piet Oudolf and Noel Kingsbury, and it inspired me to try this sort of ‘modern meadow’ somewhere in my own garden. I am thinking of replacing the largest pollinator garden with a planting of this nature. Something like this:

image credit: detroit.curbed.com

or this:

image credit: Town and Country Magazine

It won’t be able to look exactly like this in my California garden, but I can come close; I can mimic the ‘dune’ look with these mounded plantings and grasses and perennials, all of them adapted to my area. This will take less water overall than the planting scheme I have now, and decidedly less maintenance. It will require that I move some plants that I really like, and remove some others. It will require that I think highly about structure and form. It will require that I add a lot of amendments to improve the soil texture, as this kind of planting requires excellent drainage. In other words, it will require me to look at this space with fresh eyes and do the work required to make a change. Any of us can do this. Many plants can be started from seed, which keeps the cost down. The thing it will require more than anything else is a willingness to edit and change.

So I would encourage you to do the same. Don’t feel you have to live with something that isn’t working for you. It can absolutely stymie the creative flow of the gardener.

In other news, I tried making my own low-nutrient fertilizer from comfrey leaves. This is something that is recommended by one of my idols, Monty Don, and he makes and uses it every year in his own extensive and beautiful garden. I was and still am skeptical of this fertilizer because I don’t understand how it works and I would like a scientific study that tells me that information, but I have searched and searched and cannot find anything, so I decided I would just try it. It requires lots of comfrey leaves, which luckily I have in abundance, which you chop and cover with water, and then simply let sit and fester for three weeks. Strain, dilute, and apply.

This is a picture I took at the beginning of the process. I could not bear to take a picture at the end. The mass was slimy and brown, and the resulting ‘water’ was fetid, and the smell, oh my Lord the smell, like a manure lagoon at a concentrated animal feeding operation. Just simply terrible. Of course that dissipated as soon as I diluted it and added it to the soil around my tomatoes and peppers, thank goodness. It smells because it’s an anaerobic concoction, and it’s this that makes me skeptical: Compost works because it is aerobic. Oxygen is neccessary for the good microbes to live. This was the exact opposite and I’ve heard only bad things grow in anaerobic conditions. And yet Monty (and so many British gardeners) swear by it! So I am just befuddled. Of course I have no way of knowing if it is improving anything, and it’s all circumstantial anyway, as I haven’t done any kind of scientific trial.

Oh yeah, did I mention the smell? My family always complains about the stench of my usual fish and seaweed fertilizer, but that smell is positively pleasant compared to the comfrey feed, and methinks they will not complain in future.

One other item today, and that is those plastic clips that I got for the tomato vines.

As you know, I’m not a fan of plastic, but I needed these specially designed clips (which are normally used in greenhouse growing) to help my tomatoes grow ‘up’ their strings. And they work great for that, are easy to use, and can be reused next season. What I’m discovering is that these clips are quite handy for other purposes, like clipping cucumber vines to the trellis, and clipping raspberry vines to their stakes, and clipping squash vines to the strings on the teepees. I guess, basically, they are good for securing various vines to their supports.

I have found in the past that using string or twine can sometimes hurt the stems of plants - it can cut into them and damage them. These clips allow for smooth movement between the vegetation and the support. They are cheap, and reusable. They will just take a little more work at the end of the season, because I usually just cut down the vines with their strings and throw the whole lot into the compost.

Now that we are ‘at the doorstep of summer’ (as my pal Monty likes to say), how is your garden looking? Are there places that you’d like to make a change? Are their places that are really succeeding and you want to share your methods? Please feel free to add your comments! I love to know how your gardens are growing.

Eggshells and Coffee Grounds

As you all know, I took a Plant Nutrition class last semester. It was really interesting and I learned a lot about the biology of how plants take up nutrients, and the chemistry of the soil solution. Each of us had to choose two forms of fertilizer to research and speak about to the rest of the class. It was a fascinating couple of weeks as folks talked about everything from worm castings to mined phosphorus from Florida.

I wanted to research two things I have in abundance (and I'm guessing you do, too): Coffee grounds and eggshells. There are a lot of myths surrounding both, and in the course of my research, I was able to determine the truth of these two products and how they work in the soil.

Let's start with coffee grounds. When you google "how to use coffee grounds in your garden," you will see fantastical claims: They improve disease resistance, repel ants and slugs, acidify your soil (lower the pH), add nutrients, earthworms love them, and they add tilth to your soil texture. 

Here's the truth of it: 

Coffee grounds DO have a nice nutrient profile, with an NPK of 2-.06-.6. That's a nice, low amount of nutrients, with a good hit of nitrogen, some of it soluble. So they are definitely good for your soil in this regard. It's still best to add them to your compost first, so that they can be broken down before adding to your soil. However, you can sprinkle them around your plants. Just don't add a thick layer, because that will form a mat that air and water cannot penetrate.

Coffee grounds are acidic, but they don't lower your pH. You would need an enormous volume of coffee grounds to change the pH of your soil. I remember adding them to my blueberries to acidify the soil; but that doesn't work.

It is also a myth that snails or ants will be repelled by grounds. There is some evidence that they help to repel fire ants, but not Argentine ants, which is the variety most of us have in our yards. Grounds do not cause any barrier to snails or slugs at all.

Worms will process coffee grounds just as they do any other food.

And finally, grounds do nothing for disease resistance.

So, to conclude: Definitely add your coffee grounds (and filters!) to your compost pile, where they will improve the nutrient content of your finished compost.

Eggshells are a bit more complicated, because they are less well studied. But I was able to find pretty decisive research papers to help us figure out how to use them.

One of the biggest claims is that slugs don't like to travel over the sharp edges, so the shells can protect your plants if sprinkled around them. This one has been debunked quite thoroughly. In fact I found a study where the conclusion was the slugs were actually attracted to the eggshells and decimated the plants even more readily. So don't use them for this purpose!

The other big claim is that eggshells add lots of calcium to the soil. I can't tell you how many posts I've sen about adding eggshells to prevent blossom end rot in tomatoes. I assumed that was true and have used them myself for this purpose.

Well, each eggshell does contain about 2000 mg of calcium, so if there was a way to put this back into the soil, that would be terrific. However, they don't readily break down. In fact, studies show that it takes many hundreds of years before they break down completely.

So then you might ask, do they break down in water? That's been studied, and it's been found that they do not. So don't bother boiling your eggshells first.

So then you might ask, do worms eat them? They do not; they don't have teeth and can't bite.

So then you might ask, what about other microbes? Bacteria? Fungi? Protozoa? Well, there is very little research on this, surprisingly, but I did find a study where differing amounts of eggshells were composted, and the calcium content of the finished compost was higher in the piles that had greater amounts of eggshells. So that makes me think that something breaks them down. Eventually. Here's the deal: You won't reap the benefits, but your descendants might. 

HOWEVER. There is one way to get calcium from eggshells, and that's to pulverize them. Once pulverized, the shells can break down into soil solution and become available to the plants. Plus, worms can eat them in this form, and add them to their gizzards (worms digest food like chickens do, in the gizzard). 

So here's what we need to do: Put your eggshells on a baking sheet to dry. (DO NOT RINSE THEM. Eggshells have two layers - one hard, one thin and soft. This inner layer is full of organic matter that you want in your soil. ) When dry, run them through your blender, or pound them with a mortar and pestle. Then sprinkle on your soil or add to your compost.

That's if you want an immediate calcium hit. If that's not a concern to you, you can just continue to add them to your compost in chunks. They do help to aggregate the soil, making nice channels for air and water, and eventually, they do become a part of the soil. It'll just take a long, long, long time.