Everything you need to know about fertilizing your potted vegetables
Story and Photography by Kathryn Fontenot
Vegetable gardens come in all sizes and shapes. One of the lowest maintenance vegetable gardens is one growing in a container. In an age where working people are constantly “at work” thanks to email and text messages and parents and children are overloaded with extracurricular activities, growing in containers may be easier and less daunting compared to large in-ground gardens. This article will help you better understand fertilizer and as a result, increase yields of your potted vegetables.
Before going any further, remember a few golden rules for planting veggies in containers:
1. Make sure the container is at least 5 gallons (especially important for larger vegetable plants such as asparagus and eggplant).
2. Containers can be any material: plastic, cloth, clay, tin, etc., as long as there are holes for drainage.
3. Leave room for mulch. Weeds blow into every garden – large ones and tiny ones.
Once you’ve selected your container, it’s time to add soil. Soils are vastly different depending on where you reside in Louisiana. Generally speaking, most gardeners do not use native soils for container gardens; they purchase growing medium – either in bags or loose, sold in bulk. If you only need to fill one or two pots, purchase a bagged medium. If you are filling many pots, a loose medium is the most economical choice. Only you’ll need a pickup truck, as it’s usually sold by the yard. (Note: a standard 6-foot truck bed can hold about 1½ yards of media.) So why am I stating this? Everyone knows to add soil before placing a plant in a pot. Soil feeds our plants right? Well in some cases, but most soils need additional nutrients to keep plants robust. Remember plants obtain their food from the sun through the process of photosynthesis. Fertilizer is like a multivitamin, making the plant better able to grow and better able to conduct photosynthesis.
The reason I begin by asking, “What soil are you using?” is because some bagged media already has fertilizer in it. If so, you do not need to add any fertilizer until the plant begins to grow. How do you determine if the bagged medium includes fertilizer? Often the bag will read “pre-charged,” meaning includes fertilizer. Or you will see the three numbers on the bag indicating the percentage of nitrogen, phosphorous, and potassium (N-P-K) incorporated into the medium. Or, the bag will have a statement along the lines of “feeds 5-6 months.” If that is the case, simply fill your container, plant, and water.
If the bagged medium or loose soil you purchased does not include fertilizer, it is best to incorporate some, water the container, plant, and then water again.
All vegetables can be grown successfully in containers. Some like more fertilizer than others.
OK, that sounds great, but what kind of fertilizer should you use? There are many, many types of fertilizers on the market: synthetic, organic, slow release, fast release, from animal origin, with micronutrients, without micronutrients, etc. It can be overwhelming. But don’t fret. First ask yourself, “Am I trying to grow this vegetable organically?”
If the answer is yes, stick with naturally mined or derived fertilizers, such as aged manures, compost (some is organic some is not be careful), bone meal, cottonseed meal, fish emulsion, compost teas, worm castings, kelps, nitrate of soda, and more.
If the answer is yes, then the sky is the limit: use any of the above or fertilizers such as Osmocote (many formulations), Miracle-Gro (many formulations), 13-13-13, 8-8-8, 8-24-24, Epsom salts, ammonium sulfate, calcium nitrate, potassium nitrate and the list goes on and on.
I recommend slow-release fertilizers when growing vegetables in containers. Slow-release fertilizer will stay in the container a bit longer than quick release, which is extremely important if you find yourself watering the containers daily and leaching many nutrients with the water. NOTE: Do not assume the “Feeds for 3-4 months” statement on the fertilizer or bagged medium will really feed for the listed number of months. Fertilizer age or release is based on laboratory experiments where temperatures are an average of 70 F with lower humidity. This is just not the case here in Louisiana.
I also prefer using fertilizers that include micronutrients. Soilless soils are often very deficient in micronutrients such as boron (B), sulfur (S), zinc (Zn), magnesium (Mg), manganese (Mn), copper (Cu), iron (Fe), calcium (Ca), and molybdenum (Mo).
So now that you have some choices for pre-plant fertilizing your container, how much do you apply? It all depends on which type of fertilizer you choose. This is where you’ll need to read the label. Almost all fertilizers have rates listed. You’ll see water-soluble fertilizers giving rates such as “1 tablespoon or 1 teaspoon per gallon of water will cover 100 square feet.” Whereas granular fertilizers or plastic-coated slow-release fertilizers provide the purchaser with a spoon to properly measure an amount determined to adequately cover a certain square footage of garden space. Many will now even list the number of “spoonfuls” per 5 gallon, 7 gallon, and larger containers.
If your growing media does include fertilizer, there is no need to add any – the first season only. Assume the second time you plant your container – say you are planting a tomato plant this June into fresh medium that includes fertilizer – you might want to replace it with a broccoli plant in October – you will need to add pre-plant fertilizer, assuming the container has lost its charge.
After using the container for several years, you may consider completely renovating the pot with new soil, or adding organic matter, such as compost, to build the soil back up.
Pre-plant fertilizer alone is not enough to obtain the best yields ever. As plants grow, they begin to need nitrogen. Nitrogen is important for plant growth because it is a major component of chlorophyll (used in photosynthesis) and nitrogen is a component of amino acids, which build plant proteins, which aid in growth and development of cell membranes in plant tissue. Deficiencies in nitrogen show up in the lower foliage first. Nitrogen is a mobile nutrient, meaning it will physically move toward new growth at the top of the plant. Therefore, we side-dress fertilizer vegetable crops with fertilizers high in nitrogen (N) and lower in phosphorus (P), and potassium (K).
The term “side-dressing” when applied to vegetables means the fertilizer is placed to the side of the row, 6-8 inches from the trunk of the plant so not to burn it. With in-ground vegetable gardens, producers use ammonium nitrate, calcium nitrate, potassium nitrate, or other forms of fertilizer that are high in nitrogen but low in P and K. But in containers, it is really hard to place a granular fertilizer 6-8 inches from the base of the plant, so we typically recommend using water-soluble fertilizers and applying them evenly around the entire container. Again read the label to determine the rate per pot.
In the case of vegetables that bloom and then produce a fruit (tomato, eggplant, peppers, etc.) side dressing occurs as soon as the plant begins to bloom using water-soluble fertilizers and continues until harvest. Many gardeners fertilize in this fashion weekly. In the case of leafy vegetables (broccoli, lettuce, kale, spinach, cauliflower, etc.) side-dress applications begin two to three weeks after transplanting and again two to three weeks after the first side-dress application. If the leafy greens were direct seeded, wait until they are at least 2-3 inches tall before side dressing. When direct seeding any crop (say you are planting beans, cucurbits, or even the new sweet corn hybrids specifically bred for containers), never apply a side-dress fertilizer before the first true leaf emerges. Hint: Cotyledons – the two opposite initial leaves that unfold as the seedling emerges from the soil – are not true leaves. Wait until the third leaf is fully unfolded before applying additional fertilizers.
Side-dress tips for vegetables in containers:
• Don’t use fresh manures. These have the potential to carry human pathogens, such as E. coli, listeria, and salmonella. If you do not carefully or properly wash your produce, you may get sick from fresh manure coming in contact with your uncooked produce.
• In containers, use water-soluble fertilizer as a side-dress application to avoid accidentally burning the plants.
• Some vegetables do poorly when overfertilized. Do not side dress okra, beans, or peas unless they are displaying deficiencies.
All vegetables can be grown successfully in containers. Some like more fertilizer than others. The next list will help you determine if your container vegetable plant needs more or less fertilizer.
Heavy feeders:
Artichoke, asparagus, Brussels sprout, sweet corn, tomatoes, strawberries, bell peppers, hot peppers, eggplant, Irish potatoes, onions, garlic
Medium feeders:
Beets, carrots, radish, cabbage, broccoli, cauliflower, squash, cantaloupes, watermelon, cucumbers, greens, parsley, pumpkins, lettuce, spinach
Light feeders:
Peas, beans, okra
LISTEN TO YOUR PLANTS
Pre-plant and side-dress fertilizer applications will keep your vegetable plants healthy. However, sometimes deficiencies occur. Here is a brief list of plant symptoms and which nutrient is most likely deficient.
Nitrogen – Older and lower leaves are yellow. Plant size is stunted.
Phosphorus – Purple to dark green foliage, especially on older lower leaves. Fruiting is slow to occur.
Potassium – Older leaves look burned along the margins. Noticeable uneven ripening of fruit.
Iron – New foliage is white or pale yellow with green veins.
Sulfur – New foliage begins to turn pale, followed by the entire plant.
Calcium – New foliage is distorted and smaller than normal. Fruit is rotting at the blossom end.
Boron – New foliage is distorted, sometimes the plant will have a purplish cast, broccoli will have hollow stems and turnip roots are hollow.
Magnesium – Lower and middle foliage is yellow with dark green veins.
Molybdenum – Older foliage yellows from the margins moving inward.
For more information on fertilizing your container grown vegetables, read The Louisiana Urban Gardener: A Beginner’s Guide to Growing Vegetables and Herbs published by LSU Press and written by Kathryn Fontenot. (Pick up a copy here.)