Brent Crain is a Consumer Horticulture Educator with Michigan State University Extension.
JW: Brent, let's talk about plant food-- fertilizer? Do all plants need additional fertilizer?
BC: Oh—a trick question right off the bat! Depends on where you got them planted, but absolutely not. If you have something like orchids or some other kind of plant that doesn't grow in the soil, it’s different than if you have something that's planted in regular soil. And if you've got it in a soilless, potting mix, like what you bought from the store versus in the ground, it's going to be different as well. But no, all plants don't need more supplemental fertilizer.
JW: How do we know when they do?
That's a great question, there's a few ways you can find out. What I usually say is if your plant is growing well, and it's producing the way you want it to whether that's just vegetative growth, or whether it's flowers or fruit, then I wouldn't worry about it. But if you start noticing that the plant looks stunted, or something's off, or it's not just doing what it used to do, then the best thing to do is to get a soil test. And that's where you send in a soil test sample, and you find out what's going on in the nutrients in your soil. And that's the best way to guide when to add fertilizer.
So you have to think about what is your outcome, and what are you looking for. And that really makes a difference. But, if you have something growing in a container, it's going to use up the nutrients in that container mix more rapidly than something growing in the ground where there's plenty of nutrients in the ground for it to select from.
JW: When we do want to feed our plants-- organic versus non- organic fertilizer?
BC: There are pros and cons. And it kind of depends on what you're what you're interested in doing. So one of the benefits of organic is it's natural, right, so we think about how great that is. We're not using the petroleum inputs and all these sorts of things to produce the organic materials. But they do, tend to have a very low nutrient content, which means that you'll need to use quite a bit of it, and it releases very slowly. So if you're growing something like tomatoes, or cucumbers, and you're trying to grow them with just adding organic nitrogen, for example, you may have a very difficult time getting the amount of nitrogen that that plant needs. And by the time you get enough nitrogen, you're probably going to be too high on phosphorus. And that causes all kinds of problems because it gets into our surface waters, causing algae blooms and things like that. On the inorganic side, you know exactly what you're getting. So-- there'll be a bag and it will say something like 12-12-12. And what that means is that the three main nutrients, there's nitrogen, there's phosphorus, there's potassium, those are the three numbers, it's the percent of that in that bag. So that means if it's if it's a 12 at the beginning, that means that it's 12% nitrogen. So if you have a 100 pound bag, you would have 12 pounds of nitrogen. When you get the organic sources say you get a composted manure, you have no way of knowing what the nutrient contents are in that unless you do a compost analysis. Now if you're buying compost from a landscaper, you can usually ask if they have a nutrient analysis that's been done. And they will typically share that with you. And then you know exactly what's in that. A lot of people love to use bonemeal, and bloodmeal, because they think it’s natural and great for the environment. But you might want to think about what’s in that. It is usually leftover material from slaughterhouses. So they're actually taking waste byproducts-- animal blood and animal parts and drying and grinding these down. So if you didn't know that, and that's important to you, then you might want to think about that. So there's definitely pros and cons to both. The plants don't care about the source of the nutrient. But the main thing is you get slow release with these organic ones. So you know they might release for weeks and weeks to over a year. Typically, you get rapid release unless you're using a slow-release formula for inorganic ones.
There is a lot to consider and look online in the MSU resources. You'll find that we have a lot of information, so there's plenty of resources available.
Additional Resources: https://homesoiltest.msu.edu/tools/fertilizer-calculator
http://msue.anr.msu.edu/uploads/files/Fertilizing_newly_planted_perennials_Finneran.pdf
http://msue.anr.msu.edu/uploads/files/Nutrition_and_fertilization_BertCreggfinal.pdf
