Can Human Food Also Be Plant Food?

Interview with Robert Pavlis of GardenMyths.com:

Can Human Food also be Plant Food? (It depends how you apply it.)

(originally aired in Episode 126 of the Garden Basics Podcast)

Farmer Fred: What do you give plants to drink besides water and plant food? This question actually has been asked on many social media outlets, and in many gardening groups. And you might be surprised at the number of household kitchen products that are given to plants. Are they worth it? There is one gentleman who actually tackles a lot of garden myths. He's even written a book on the subject. It's Robert Pavlis. He's a well known speaker-educator, with over 40 years of gardening experience. He's the author of several books including "Building Natural Ponds", "Soil Science for Gardeners", and "Garden Myths". And he publishes the popular gardening blog, gardenmyths.com as well as garden fundamentals.com . He gardens in Ontario, Canada, and Robert Pavlis, it's a pleasure to have you on the program. Tell us, for those of us who aren't familiar with gardening conditions in Ontario, Canada, exactly what your seasons are.

Robert Pavlis: Well, we have a USDA Zone five garden, so we have a fairly long winter. Around here we don't get great snow cover but we do have snow and then we have hail and sleet and everything in between. And then it gets warm and then it gets cold. So even though our winters aren't that cold, the problem here is that we have this freeze-thaw problem. Then spring comes, the summers are quite humid, quite hot. So today is 29 degrees centigrade (84 F). And fall comes pretty quick. And you know, by middle September, while things are finished in the garden by October, we could have frost.

Farmer Fred: So if you're going to grow a summer vegetable garden there, you start around Mother's Day and wrap it up by, I guess, the end of September.

Robert Pavlis: Yeah, well, our traditional last frost date is May 24. That's moved back now to around May 10. So a lot of gardeners up here will try to extend that season. So we use things like rowcovers, or we gamble, what I call gambling in the garden. And I plant things two or three weeks early and hope it doesn't freeze. But that doesn't always work. And we try various other ways to keep things a little warmer. But yeah, early May. Tomatoes and warm season crops, they probably won't go out until late, middle-late May, depending on the year.

Farmer Fred: All right. So a bit shorter than here in California, to say the least.

Robert Pavlis: Just a little bit. Right.

Farmer Fred: All right. Well, I love your GardenMyths.com column that you post, and people can subscribe to it. It's free, and you get email notifications when they are released. And awhile back, you had one on feeding plants from the kitchen, discussing which products actually work. And for those of you who are short of time, the quick answer is, "not many". But I am amazed, though, at what food scraps people are feeding to plants that they think will work. I guess we should start off with discussing exactly how plants absorb nutrients.

Robert Pavlis: Plant roots only absorb certain things. And they have to be what I call small molecules. So nitrogen, potassium, phosphate, these are all small molecules. But what you have in most plants is large molecules, proteins, carbohydrates, and so on. And you can put all that you want on a plant, it can't use it until it decomposes. That's why that composting process is so important. We're taking large molecules, converting them to these small little nutrients that plants can use. And until that happens, plants get no benefit. And they need a certain amount of those, they need a fair amount of nitrogen. They use a fair amount of potassium and phosphate, and then they use about a half a dozen other minor nutrients like calcium, magnesium, manganese, and sulfur. And they need all of those. So one of the things I look at when we're talking about the stuff we're putting on our plants, the first question you really need to ask is, what nutrients are in there? And by the way, the one nutrient that most soil is missing is nitrogen. And I won't get into why,  just accept the fact that most of the other nutrients are probably there in the soil. But nitrogen can go away very quickly. It doesn't stick to soil,  it runs away. So in most cases, growth is controlled by the amount of nitrogen you give your plants. Now, if you are deficient in one of those other things, you also need to add that one, but nitrogen is a key one. So the key question is how much nitrogen is in what I called food scraps, adding to my plants, and if it's very minor, then it's really not going to do much for your plants.

Farmer Fred: And besides, it has to be converted by the soil biology into a form that the plant can use.

Robert Pavlis: That's right, and nitrogen is used mostly as nitrates and some as urea. But plants pretty much use nitrates. So if I take a big protein molecule, which has lots of nitrogen in it, and I give it to my plant, it's absolutely useless, the plant can't use it. Now, the bacteria in soil may come along and degrade that protein and eventually release the nitrogen. But until the nitrogen is converted into nitrate, plants can't use it.

Farmer Fred: Well, speaking of high protein foods that people like to put on plants for one reason or another, is milk. And this is something that people have been trying for years. Does milk do any good on a plant?

Robert Pavlis: Well, I guess it depends on how you ask the question, right? So milk is mostly water, the second biggest ingredient in clean whole milk is fats. So now there's a tiny, tiny little bit of other stuff there. And some of that is protein, and protein does contain nitrogen. So milk does have a very low level of nitrogen in it. It also has things like calcium and magnesium and some of the other nutrients. So yes, adding milk to plants will provide nutrients to that plant. The question you really need to ask: is that enough nitrogen to actually make my plant grow? Well, and quite honestly, if you have spent milk, you know, you can't drink it. Okay, put it in your garden, put it on some plants, and it will do some good. But milk is an extremely expensive way to get nitrogen. So if you're using good milk that you could drink, I would never put that on the soil. That's just a waste of money.

Farmer Fred: Exactly. It's a  very expensive source of nutrients, if you dare. All right. Now, people also want to think that they can get their plants to grow faster if they give them some coffee. And I don't think plants absorb caffeine.

Robert Pavlis: Yeah, this is something that people like to do. So they've finished drinking their coffee, there's a bit left on, it's got to be good for them, let's put it in the plants. So what I did in that blog post is I took a slightly different approach, because so many people are against chemicals, and pro organic. And this concept never makes any sense to me. I'm a chemist, so I understand what's going on here. And coffee is full of all kinds of carcinogens, and I listed them in the blog. And, I said, would you give that to your plants? I mean, would you even drink it yourself? All right. Well, that's the key here. And the thing that people just don't understand is that there's lots of chemicals out there that are toxic, and most of those, in fact, come from plants. Remember, coffee is a plant extract, it is coming from plants. Every plant we eat is full of toxins. The key question is how much of that toxin is in the plant? And what does it do to our bodies, and in most cases, these levels are very low. You don't have to be concerned about them. The chemical names just sound really nasty. They are carcinogens without question, but their levels are low. And the other thing is our bodies have been doing this for millions of years and we're very adapted to eating these toxins as long as they stay in low levels. So coffee doesn't hurt us, and coffee will probably not hurt most pl…

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