When you’re new to growing vegetables and herbs in pots, figuring out the how, when, and what of fertilizing container gardens can feel overwhelming. This article is the third in a three-part series that offers a beginner-friendly guide to feeding your potted edibles.
In part 1 of the series, I talk about fertilizing basics, including the special challenges plants face when growing in containers and why feeding them is essential to their health.
In part 2, I give my four-part recommendation for fertilizing and offer three sample schedules you can follow depending on the plants you’re growing and your gardening goals.
This article, part 3 of the series, is designed to give you in-depth information about the four types of fertilizers and supplements: liquid fertilizers, liquid supplements, granular fertilizers, and granular supplements.
When you’re looking for a liquid fertilizer, I think that you will get the best and safest results by selecting a simple, well-balanced, organic product.
Fish emulsion is the best option I have found, no question.
It comes labeled with many different names like “Liquid Fish,” “Organic Fish Concentrate,” and just plain old “Fish.” As I understand it, fish emulsion is basically just a by-product of commercial fish processing—it’s the rest of the fish, liquefied in a blender.
Fish emulsion typically analyses at 5-1-1, N-P-K. So, it’s great for growing your plants large, and great for leafy green growth (lots of nitrogen). But, you’ll want to add a phosphorous source for vigorous root growth, budding, and fruiting. Or a potassium source for plants with general health and growth issues.
There are many blends available on the market that use fish emulsion as a base and incorporate other ingredients for a more well-balanced effect.
A good way to choose is to make a trip to a local organic garden center and ask them what they have available. Specify that you want single digit N-P-K concentrations, and specify organic. They should be able to help you make a good choice.
If you don’t have a good organic garden center nearby, Amazon may be your best bet. A simple search of “organic liquid fertilizer” yields many good results.
My favorite solution for a liquid organic fertilizer is a locally made blend that includes fish emulsion, liquid seaweed, humic acid, molasses, fermentation extracts, magnesium chloride, iron sulfate, zinc chloride, and water. This product is labeled as 3 – 1.5 – 2, N-P-K. I have used this solution on a huge variety of plants, with good success and no burning,ever.
Any product that you buy should have instructions for diluting. Follow the manufacturer’s instructions to dilute the concentrated fertilizer—never apply directly without diluting.
A common dilution rate is 1 ounce (2 Tablespoons) per gallon. If my label has worn off, or someone gave me a sample with no instructions, that is the dilution rate I normally use.
I tended bar when I was in college, so sometimes when I’m in a rush I trust my steady hand to pour an even 1 ounce without measuring. It’s better to keep some measuring spoons and a conversion chart on hand in the area where you will be mixing fertilizers. You can get cheap plastic measuring spoons from the dollar store. (When you smell the fish fertilizer, you’ll understand why you probably don’t want to use your regular kitchen measures for this task!)
Shake the liquid concentrate well before measuring out the desired amount—ingredients like molasses have a strong tendency to separate and settle on the bottom of the container. You want to make sure you remix the concentration each time you use it.
I add a little water to a watering can first, pour the measured concentrate into the watering can second, and then fill the watering can third.
You should always stir the dilution well, so that each plant gets an equal dose of the fertilizer. I use a bamboo stick to reach into the watering can, past the handle. I stir counterclockwise before reversing the direction and stirring again, this time clockwise, until a vortex occurs (a little water whirlwind).
The correct amount to apply per plant should be given by the manufacturer, either on the product’s label or on the manufacturer’s website.
If they do not supply this information, a rule of thumb I use is to apply one ounce of diluted fertilizer solution per gallon of soil in the container.
So, a 1 gallon pot gets 1 ounce from the watering can. A 5 gallon pot gets 5 ounces, and so on.
For very small containers, I just give a quick splash. If I get liquid fertilizer on the plant’s leaves, I shake the leaf or rinse it off to get rid of standing drops of fertilizer.
You can also apply liquid fertilizers as a foliar spray, and your plants will probably love you for it. It does well when applied as a mist, but can hurt if it’s collected in puddles on the leaves. Apply it lightly, with a mister/sprayer, when the wind is low.
You should always apply liquid fertilizers to soil that has been moistened—but not saturated—with water, so that the fertilizer solution will distribute itself throughout the soil. If you apply liquid fertilizer to dry soil, it can absorb into one small area. If you apply liquid fertilizer to the saturated soil, it can drain out through the bottom of the container. When I remember to, I water containers late in the day, on the day before they will get fertilized.
I’ve been told that the best time of day to apply fertilizer is very early in the morning when a plant is beginning its rhythmic growth cycle for the day. However, if you’re a late riser, any time is better than never.
What defines a liquid fertilizer is that it has a significant concentration of three key macro nutrients required for plant growth and fertility—nitrogen (N), phosphorus (P), and potassium (K). These three macro nutrients are key to growing healthy plants, but there are several other things you can do to help your plants grow big and strong.
These supplemental products include other macronutrients, lesser-known micronutrients, beneficial trace minerals, beneficial fungi, natural enzymes, living microbes, and a whole slew of other goodies.
Seaweed is like magic for some plants.
It doesn’t have high concentrations of the three key macronutrients—Maxicrop’s popular seaweed product analyzes at 0-0-1. But seaweed is great for heat and drought resistance, and it helps with many common problems.
If your plants are suffering from chlorosis, with yellow leaves and poor growth, seaweed can get them back to green and growing quickly (although you may be overwatering, so be sure to address the root problem).
Strawberries and spinach love seaweed.
Some plants require a more acidic soil in order to thrive and fruit. You can use liquid supplements to manage the pH level of soil in a container. Seaweed with added iron is a great supplement for acid-loving plants. You can also acidify your regular water by adding a little vinegar to the watering can—you can safely use up to 1 cup of vinegar per gallon of water.
Aerobic Compost Tea
Using aerobic compost tea is a little bit like cheating. You take water that contains thriving colonies of microscopic life, and pour it onto your garden soil.
This is an especially effective tactic for containers, where the normal soil biology likely doesn’t exist yet.
You can inoculate the soil with many desirable microorganisms by applying aerobic compost tea. You can brew this miracle tonic yourself at home, or you might find an organic garden center nearby that offers it for sale by the gallon. If there is an active garden club in your area, ask around and find out if anyone else is sharing their brew or can show you how to make it yourself.
If you’re confused about what exactly compost tea is, read this: “Leachate, Worm Tea, and Aerobic Compost Tea–A Clarification.”
Any time you’re using a supplement or additive that has a living component, it’s important to pay attention to the water you’re using to dilute the additive.
Tap water has chemicals in it that are specifically chosen to kill microbial life, so it’s easy to torpedo your efforts by diluting aerobic compost tea with tap water. You will still likely get trace concentrations of seaweed, molasses, humate, and the other good ingredients that went into the tea while it was brewing, but you won’t get the beneficial microorganisms, and those are what you’re really looking for with this particular supplement.
For more information on this, see Leslie Parson’s article, “A Guide for Using Tap Water in Your Garden.”
Enzyme and Vitamin Solutions
There are some good products on the market that contain natural enzymes and vitamins that are organically manufactured by biological processes. I think this basically means that the products are bottled microbe poo.
I have used a few of these, and I feel like they do improve the size and vigor of treated plants. I would consider using these on stressed plants, or on plants that need to have an especially beautiful appearance, like flowers or edibles in a visible place.
Some examples are Vitazyme, AgriGro, and SUPERthrive.
Granular fertilizers are generally stronger and longer lasting than liquid fertilizers.
As always, a good rule of thumb is to stick with products that have single-digit concentrations of the three key macronutrients—nitrogen, phosphorous, and potassium. You’re still looking for something like 4-4-4, not 20-20-20 (N-P-K).
You will find granular fertilizers made from an assortment of different ingredients—mostly animal poop.
I have used packaged fertilizer products that were made from chicken poop, cow poop, turkey poop, rabbit poop . . . the list goes on. As with liquid fertilizers, a good course of action for a first-timer is to walk into a brick-and-mortar garden center and talk with the staff there about what is available and what meets your needs.
If you don’t have a good organic garden center within driving distance, check on Amazon. Shipping costs could be a factor—the smallest bags I’ve seen available are in the 5-6 pound range.
If you are a vegan, your options will be a little more limited. There are some good fertilizers available made from cottonseed meal, alfalfa meal, and some others. Search online or consult with your local experts, and you should be able to find a solution that works for you.
My favorite granular fertilizer is a local product that analyzes at 8-2-4 N-P-K. It is made from feather meal, turkey compost, sulfate of potash, and molasses. Most plants use the basic macronutrients at the ratio of 4:1:2, so it’s handy to have a good all-purpose fertilizer like this that can be used on everything from the lawn to flowers to the vegetable garden.
Applying granular fertilizers is easy.
Because these fertilizers are stronger and last longer, you will probably only need to apply these once or twice over the course of a growing season.
For the first application, at planting, I mix the recommended amount into the potting soil that I will use to fill my container.
Follow the manufacturer’s recommendation for how much to use. If no recommendation is given on the product packaging, I use one large handful for a 5-gallon container, and adjust proportionately for smaller or larger containers. In the bottom of the planting hole, I sprinkle another teaspoon or so of fertilizer, for immediate availability to the plant’s roots.
Be sure to mix the fertilizer well into the potting soil, so that it is evenly distributed throughout. I usually get two buckets (or trugs) side by side, and pass the soil/fertilizer mix back and forth between the two buckets, completely dumping the contents each time. After dumping the mixture back into the empty bucket 10 or 12 times, I can visually see when the fertilizer has been thoroughly mixed.
Sometimes I will come back with a second helping of granular fertilizer, either at fruit set or when I notice that a plant’s growth has slowed down significantly.
For the second application, I don’t dig down into the soil—I don’t want to damage the existing root system. I evenly scatter the granular fertilizer over the top of the soil in the container. Then I “scratch” the fertilizer down into the top 1 or 2 inches of the soil. Sometimes I use a hand rake to scratch the fertilizer in, but I usually just use my fingertips.
After the fertilizer is scratched in, give the container a thorough watering to activate the fertilizer.
If the container is in an area where pests are a problem, cover the top of the soil with some sort of mulch after adding the second helping of granular fertilizer. Cats and dogs are very interested in organic fertilizers, and they will dig up your plant just as it is starting to fruit. If your pot is in a sitting area, flies and fruit flies are less likely to gather in the area if a thin layer of mulch separates the soil from the open air.
My favorite mulch to use is organic compost from the backyard pile. If I need the pot to look especially presentable, I will use a small, pea-sized stone instead. Expanded shale makes a great mulch and has a clean, neutral appearance.
In addition to the three key macronutrients, there are lots of beneficial soil amendments that you can mix in to your potting mix at planting time.
Mycorrhizae are fungi that have a symbiotic relationship with plant roots. The fungi form a web in the soil, tying in with the plant roots through a physical connection. As the fungus collects nutrients from the soil, it passes water and minerals up to the plant’s roots. In return, the plant passes sugars down to the fungi.
Mycorrhizae exist naturally in healthy soil in the ground everywhere, but not necessarily in potting soil. There are several products available from garden centers and online that allow you to inject these fungi into your potting soil. You can also get soluble mycorrhizae that can be watered into the soil.
While N, P, and K are the three big nutrients required for plant growth, there are many other nutrients that contribute to a plant’s overall health.
A good way of looking at this is to compare a plant’s diet to a human’s diet—if a person ate nothing but eggs, kale, and carrots every day, they would eventually develop a nutrient deficiency. Those three foods are healthy, but alone they do not constitute a well-balanced diet.
To round out your plants’ diets, you can add mineral sand to the potting mix at planting time. I use a mineral sand product that includes decomposed granite sand, lava sand, granite sand, basalt, soft rock phosphate with colloidal clay, humate, greensand, and montmorillonite.
I hope the information in this three-part series is more helpful than it is confusing. If you have specific questions that you want to ask, feel free to use the comments section below. There’s a whole community of people here who can help!
(This is an updated version of a post that was originally published in October 2015.)
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