Everyday gardener // Fertilizer 101

Raise your hand if you don’t feed your plants like you know you probably should?

Yeah, my hand is up, too. There’s no question that plants need nutrients to thrive but, somehow, fertilizing is always low on my list of gardening tasks.

Part of the reason for this is that the garden really doesn’t look like it’s struggling, so I put off feeding for another day. But left on their own year after year, plants will deplete the soil of primary macronutrients nitrogen (N), phosphorous (P) and potassium (K), which is why you find these most often in packaged fertilizers you buy off the shelf.

To a lesser degree, and at a much slower rate, secondary and micronutrients like sulfur, calcium, magnesium, iron, copper and manganese may also be depleted over time.

Plants need these nutrients in order to do things like grow strong, produce fruit and stave off pests and diseases. So it isn’t enough to say (as I do), “Ah, well, my plants look good enough.” If you don’t feed them anything, they’re probably in need of a good meal. Some signs of hungry plants are: off-color leaves, yellowing leaf veins and browning of leaf margins (which may mean potassium deficiency).

It is nitrogen that helps promote growth of leaves and stems. Phosphorous is responsible for root growth and helps plants set their buds and blooms. Potassium helps promote health and disease resistance in a variety of ways. When you see the NPK ratio listed on fertilizer labels in numbers, such as 10-10-10, those numbers are telling you the percentage of each nutrient in that particular product. In the case of 10-10-10 you’ve got what is called a “balanced” fertilizer, which is perfect for routine feeding of many plants but in some cases you may want something specifically tailored for, say, tomatoes or certain shrubs of flowers.

Though I’ve never gotten a soil test myself (I’m more of a try-it-and-see-how-things-go gardener), these tests are a good way to find out quickly and inexpensively exactly what nutrients your soil needs. The University of Minnesota has a great soil testing laboratory and you can find out more about how the process of submitting samples works here: hsoiltest.cfans.umn.edu/submitti.htm.

Whether you do a test and know precisely what to add to your soil, or you’re just looking to provide nutrients to plants on a regular basis to keep them healthy, you’ll need to decide what type of fertilizer to use.

Organic vs. synthetic fertilizers

This is where things get confusing. I’ve only recently taken time to read up on fertilizers in the hopes of better understanding what they are, what they do and how they do it. Without getting too boring and wonky (I hope), I’d like to share some of what I’ve learned with you.

In a broad sense, fertilizers can be separated into two categories: organic and synthetic (chemical).

Organic fertilizers are essentially organic matter derived from plant and animal sources. Compost is probably the most common organic soil amendment used to feed plants. About a half an inch of compost spread out over existing gardens or turned into the soil each year will break down slowly, providing much-needed nutrients to plants and helping to improve soil structure so it can better retain water and nutrients and get good air circulation.

Be aware, though, that whether you make your own compost or buy it in bags or bulk, compost provides different nutrients depending on its composition so you will likely need to provide other organic food sources, too. Some of those might include fish emulsion, mulched leaves, manure (composted, not fresh), worm castings (worm poo), dried blood (rich in nitrogen), bonemeal (rich in phosphorous) and fish meal (which offers nitrogen and phosphorous). Add these according to package directions with the exception of leaves, which can be spread out over garden beds and allowed to decompose if they’re mulched up small enough.

Synthetic fertilizers are a different matter entirely. Though they come from natural sources so you would think they would be organic, they are chemically processed in ways that are far from eco-friendly. Phosphorous and potassium are mined, so they are ultimately destructive to the earth and turning nitrogen into useable fertilizer takes a great deal of fossil fuel, which is unsustainable.

Why do we use these then? Well, on the plus side the nitrogen, phosphorous and potassium in synthetic fertilizers are almost immediately soluble (unless you buy a slow-release type) in water meaning plants can take them up as soon as they are applied. Organic fertilizers, on the other hand, are usually insoluble. They work by stimulating beneficial soil microorganisms that convert organic fertilizers into soluble nutrients that be absorbed by plants over several months or even years.

Synthetic fertilizers like Miracle Grow and Osmocote (just to name a couple of popular brands) are popular because they’re easy to use and produce results quickly. Liquid synthetic fertilizers offer a quick boost of nutrients while granulated products (known as slow-release) provide plants with nutrients over a longer period of time because the coating keeps the fertilizer from washing away too quickly.

For this reason, slow-release synthetic fertilizers are usually the better choice (if you’re going the synthetic route), particularly those that are grown in containers because containers need to be watered so much during hot summer months and all that watering depletes nutrients fast.

At this point I know it sounds like I’m demonizing synthetic fertilizers but honestly, that is not my intent. They do have their place.

For example, in early spring when microbes in the soil are not yet active because the soil is still cold, synthetic fertilizers are a very sensible choice for plants that need early feeding like lettuce. They also make good sense for container plants, as I said above. Though you can also use organic options, such as fish emulsion for containers.

Aside from their sustainability issues, the biggest drawback to synthetic fertilizers is really that they don’t improve soil structure over time the way organic fertilizers do. In fact, long-term use of synthetic or “inorganic” fertilizers has been shown to upset the natural ecosystems in the soil changing the balance of the organisms that live there. Some studies suggest they are also harmful to earthworms, which as I noted in my last column, are hugely beneficial to have in the garden because their castings are loaded with nutrients and they help improve aeration by creating burrows that allow more air and water to penetrate the soil.  

The upshot? Let’s face it. Synthetic fertilizers are much more convenient to use than organic fertilizers, so it’s likely that gardeners (including me) are going to go on using them in one way or another.

When we do use them in the garden, though, it seems wise to use them judiciously. And it would be a good idea to also add compost and other organic matter as often as possible to ensure the health of the soil and all of the organisms and plants that dwell there.

Meleah Maynard is a Master Gardener and freelance writer. If you’ve got a gardening question you’d like her to address in her column, you can email it to [email protected]