How to fertilize a vegetable garden
There's a thin line between fertilizing and overfertilizing your food crops. Too few nutrients and your tomatoes and corn never really take off. Too many and your artichoke and broccoli can suffer. Since I tend to favor deep-rooted perennials, which are low-maintenance and more resistant to weather extremes, my particular challenge is making sure I don't overdo it. So I use fertilizer strategically, boosting plant growth largely by feeding my soil. I test my soil every other year so I know the nutrients my soil needs. And I rely on timing, knowing the right mix of fertilizer, and the best sources.
Time Your fertilizer use
The spring: Perennials benefit from a side dressing of nutrients in the spring just before new spring growth begins to push through the soil. In the early spring I like to add a nitrogen heavy fertilizer to really help my plants take off. You don’t need much or you’ll get leggy plants that flop over.
Mid-season: Sometimes plants need a little extra help when they start producing flowers and fruits. So much energy goes into ripening food that plants get stressed and weak plants emit distress signals that attract pests. Give your flowering perennials a boost in phosphorus to prevent stress and help fruits ripen faster.
The fall: Be careful not to add too many nutrients just before the winter rains set in or they’ll likely wash away. Excess nutrients drain into waterways, promoting harmful algal blooms that suck the oxygen out of water and kill aquatic life. If you do fertilize in the fall, ditch the nitrogen and phosphorus and focus on potassium based fertilizers.
know your numbers
If you walk down the aisle of a gardening center you'll likely be faced with a sea of numbers like 10-10-10 or 20-0-5. So what do they mean?
The three numbers on fertilizer bags represent the concentration of minerals in the mix, specifically nitrogen (N), phosphorus (P) and potassium (K). The higher the number, the higher the amount. A mixture of 20-0-5 has four times more nitrogen than potassium, for example, and no phosphorus to speak of. I keep the numbers straight by remembering the key phrase, “up, down, and all around.”
Up: The first number refers to the amount of available nitrogen, or the plant-boosting "up" factor. If you want to quickly add lush growth above ground, go for a nitrogen-heavy fertilizer. Be careful not to add too much or you could have vigorous growth at the expense of below-ground root development. One season I used too much alfalfa meal in my homemade fertilizer blend. My potatoes took off in a burst of green growth that had me dreaming of a bumper crop. But in late August, after I'd gleefully dug into my planting bed in search of tender fingerling potatoes to roast for dinner, I turned over the biggest plant and found only tiny little nubs. The problem? Too much nitrogen.
Down: The middle number signals the concentration of phosphorus. A fertilizer with more phosphorus will maximize fruit and flower production, giving you nicer rose blooms and tastier cucumbers. I switch from nitrogen-rich fertilizer to a high-phosphorus fertilizer like bone meal once the first flowers appear on my plants. This encourages my fruits and veggies to ripen faster, which is particularly important if you live where the growing season is short, the way it is here in Northern New England.
All around: The last number indicates the amount of potassium in the mix. Adding a potassium-rich fertilizer can help your plants fight disease, pests, and stresses from cold, heat and wind. I always use a potassium-rich fertilizer like greensand when I tuck my plants in for the winter to help them survive the cold.
Top Organic Fertilizers
For the gardener new to feeding soil, I've put together a list of my favorite organic fertilizers. To use them, simply mix your fertilizer into the first three inches of your soil before planting and once more midway through the growing season. Do the same for established plants, except add a little to your plant base in the spring.
Any of the following can be used as a stand-alone soil conditioner. You can use two or three different types of fertilizer that have particularly high concentrations in nitrogen, phosphorus, or potassium throughout the season to address different concerns. If you'd like to keep it simple and rely on one all-purpose blend, go with homemade compost or purchase an already made organic mix from your local garden center.
Good all-purpose fertilizers:
Homemade compost: Store-bought fertilizers are expensive but compost is always free. In fact, if you have to pay to get your trash hauled away, composting can save you money by reducing the amount of waste you throw out. See our composting guide for tips on how to create fertilizer from food, yard waste, and more.
Mycorrhizal Inoculant (0-0-0): Mycorrhiza (My-cor-rye-zay) is a group of fungi that forms valuable relationships with plants roots. A network of mycelium or long white strings akin to fungal roots attaches to plant roots and enormously increases plants ability to absorb nutrients and water. This isn’t a true fertilizer because it has no nutritional content but it helps plants to get more out of the soil. I mix it into my compost and mulch piles so that when I spread them in the spring, I inoculate my whole garden.
Fish Emulsion (5-2-2): Fish emulsion is a liquid fertilizer made from byproducts of the fishing industry. I use this well balanced and fast acting fertilizer all season long on my heavy feeders. It does have a very fishy smell but a few drops of lavender oil can help to mask the odor.
Particularly good for jumpstarting spring growth:
Manure compost: Manure from herbivores and poultry is a great source of nitrogen and organic matter and generally packs more punch than homemade compost. I get mine from a nearby farm but you can also just buy ready made composted manure from your local garden store. Manure should be composted for at least three months to kill weed seeds and diseases before adding it to a garden. If that's not possible, you can minimize the risk of weeds by using manure from chickens instead of from horses or cows. If you have your own chickens, simply compost the manure with bed material like wood shavings or sawdust. The combination of green manure with brown bed material produces the perfect ratio of nitrogen to carbon for composting.
Blood Meal (13-0-0): Blood meal, a dry powder made from cows' blood, is a fast-acting source of nitrogen. I use it in soils with a serious lack of nitrogen to jumpstart spring growth. You can add too much, making your soil acidic, so always test your soil. Testing your soil is easy, look up your local university or extension center and send a request for a soil sample box. Once it arrives follow the instructions on the label and send it away. Your results should be ready in about 10 business days. If I am looking to boost nitrogen mid-season, I use plant-based alternatives like alfalfa meal (3-1-2) because it is gentle and supplies other beneficial nutrients that help feed soil microbes.
Particularly good for maximizing production:
Bone Meal (3-15-0): Bone meal is made from ground up animal bones and is widely used to replenish phosphorus and calcium. I like to mix bone meal with composted manure for a potent all around fertilizer for spring. In the fall when I plant flower bulbs and garlic, I add bone meal to the bottom of the holes to help promote fall root growth before the winter freeze.
Greensand (0-0-3): Greensand is a very popular fertilizer collected from the ocean floor or ancient seabeds. This dry powder is a great source of iron, potassium, magnesium and dozens or other trace minerals. Greensand is great for breaking up clay soils and adding water retention to sandy soils. It’s very gentle so you can’t add too much. You can even use it around seedlings and sensitive plants.
Guano (12-12-2.5): Guano is made from the droppings of seabirds and bats. It is usually harvested from coastal cliffs and dry caves where the droppings can sit and decompose. Not only does guano add nutrients but it is full of microbes that help to deter parasitic soil creatures like nematodes. This organic fertilizer has been collected for hundreds of years and you can buy it as an odorless powder.
Particularly good for fighting pests and climate stress:
Kelp meal (1-0-2): Kelp meal is made from dried ocean seaweed and is full of nutrients, especially potassium. You can by a bag of it or get permission to collect it from your local beach. Kelp is an exceptionally renewable source of potassium, growing up to three feet per day in ideal climates. I use kelp meal to treat tired soils that have been intensely cultivated and I spray it my plants to help them deal with heat stress in the summer and fight against pests.
Mulch: Organic mulches like leaves, wood chips, sawdust and even paper add nutrients to your soil while stamping out weeds and protecting your soil from sun, wind and erosion. Mulch materials are generally carbon rich so I always sprinkle some alfalfa or blood meal onto my soil before I layer on my mulch.
Acadia Tucker is a New Hampshire-based farmer and gardener, and author of Growing Perennial Foods: A guide to raising resilient herbs, fruits, and vegetables (Stone Pier Press, Spring 2019) and Growing Good Food: A citizen's guide to backyard carbon farming (Stone Pier Press, Fall 2019).
This is part of our series on radical gardening, or how to grow food to help the planet. To find out more about growing your own perennials. Check out our Perennial Profiles.