How and When to Fertilize Your Vegetable Garden

Have you ever walked by a neighbor’s vegetable garden and thought how remarkably lush and healthy all of their veggies looked? Do rows of juicy red tomatoes and bushy sweet peas vines make you feel green with envy and wonder, what is their secret? Well, we know the secret. It’s the fertilizer they use, of course!

Most of the popular vegetable plants that we grow in home gardens are big nutrient consumers. They need a lot of nitrogen and other nutrients to produce those large, healthy edibles that we all look forward to from the time we germinate those tiny seeds.

These nutrients are typically present in the soil but not always in the amounts needed to grow these common vegetables.

There are different ways of increasing your garden’s yield, such as crop rotation and companion planting, but fertilizing is a surefire method of getting your plants all of the necessary nutrients to perform at their peak. Let’s talk about how fertilizer works and what works best when fertilizing your vegetable garden.

About Fertilizers

Fertilizer is a material that contains the elements that give plants the energy to grow. In other words, it is food for plants. Just like people and animals, plants require a specific combination of nutrients to perform and produce the greatest amount and most nutrient-rich fruits and vegetables.

If you are just getting started planning your vegetable garden, a soil test is a valuable tool to determine when nutrients are present and in what amounts. This way, you have a jumping-off point. Once you understand the nutrient makeup, you can determine what needs to be added.

Fertilizer comes in many forms and formulas. The three primary nutrients in most commercial fertilizers are nitrogen, phosphorus, and potassium. In addition to these nutrients, some plants also need a combination of other nutrients, such as magnesium, calcium, sulfur, boron, manganese, zinc, iron, chlorine, nickel, iodine, boron, and copper. 

If you’ve ever been down the fertilizer aisle at your local nursery, you know that it can be overwhelming how many different types of fertilizers are available.

One brand will often have more than a dozen different formulations, and many of them do not indicate what plants they are best used on. If you haven’t been down this aisle or aren’t sure whether you’ve had this experience, you would know it because the smell is unmistakable.

Fertilizer Types

Most fertilizers will display a series of three numbers on the front of their packaging. This will be in the format “10-10-10”. A formula that has three of the same number is a balanced formula with the same amount of nitrogen, phosphorus, and potassium, in that order. It is best to head to the store armed with an idea of what type of fertilizer your plants need.

Fertilizers can be natural, organic, or synthetic. Seabird Guano tea sounds like something out of a horror movie, but I guarantee you that if you mist your garden with this solution, it will perk your plants up visibly, and pouring this treasure into the soil will improve the strength of your plant’s roots leading to stronger, faster-growing plants.

There are also other products, some are common household items, that work well as supplements to fertilizer once they decompose into the soil. Coffee grounds are full of nitrogen. Mixing in your used grounds will give plants a mild boost as the coffee grounds break down. Composting your coffee grounds is even better, as that will infuse your future compost with nitrogen that’s ready for plants to absorb immediately.

Bonemeal is a tried-and-true favorite among farmers. Commercial farmers commonly treat their crops with this phosphorus booster, which strengthens root systems as it decomposes. Healthy roots equal healthy plants.

The long and short of it is that fertilizers bridge the gaps in soil nutrient content. If you are a long-time gardener, it’s also not a bad idea to have a soil test done. Vegetable gardening can deplete the soil of nutrients over time, and your soil may need to be amended before you sow any new seeds.

A note to the reader: It is important to note that the overuse of fertilizers is as detrimental as not using them at all. Too much fertilizer can burn out a plant quickly. This is why it is a great idea to have a soil test conducted. Once finished, you will know which nutrients your soil needs and which it has plenty of.

Why Fertilize?

To put it simply, fertilizing is important because most vegetable plants require more nutrients than are available in most soil. Popular vegetables like broccoli, tomatoes, peppers, and kale are all heavy feeders that can deplete the soil of nutrients. If you are planting for the first time, fertilizing will give your plants a healthy start.

If you have previously grown vegetables in the same place, they have probably depleted your soil of these necessary nutrients. Fertilizing adds nutrients back into the soil for the continued growth of your vegetable garden. Failing to fertilize your vegetable garden will ultimately result in slower growth and less yield.

When to Fertilize

Typically, the best time to fertilize vegetables is in the spring, before you plant them! That’s right! Fertilizer should be mixed into your garden beds before you put your plants in the ground. This makes sure that your baby plants get the strong start that they need to produce lots of big, healthy, flavorful vegetables.

If you’ve already planted, that’s ok, too. Just work some fertilizer in lightly around your plants.

Granular organic fertilizer is best for this first round of fertilizing, as liquid fertilizers can be too harsh for young plants and can burn out their roots. Spread your fertilizer around your garden bed and work it into the soil about 4”-5” deep. You’ll have to do this carefully if you’ve already planted. Try not to jostle your seedlings around much.

Certain types of vegetables are heavy feeders and will need repeated applications of fertilizer throughout their growing season. Vegetables in the Brassica and Nightshade families are all heavy feeders.

These include (Brassicas) broccoli, cauliflower, kale, brussels sprouts, and cabbage, as well as (Nightshade) tomatoes, peppers, eggplants, and potatoes. These vegetables are full of nutrients and are considered some of the healthiest foods you can consume.

Below, we have compiled a chart of some of the most popular vegetables planted in home gardens, along with their specific needs and the type of fertilizer that works best for each.

You can bookmark this chart to refer back to as the season progresses, a reminder of which vegetables need extra fertilizer and when.

Tomatoes

Scientific Name: Solanum lycopersicum

• Family: Solanaceae
• Planting Season: March/April
• Harvest: May-November (depending on growing region and ending at first frost)
• Growth Rate: Fast
• Light Requirements: Full Sun
• Water Requirements: Moderate to high
• Fertilizer Requirements: 3-4-6 provides a good balance for tomatoes. Provide a small amount of fertilizer in the planting hole at the time of transplant. Fertilize lightly every 2-3 weeks after plants set fruit. For these, a granular slow-release organic fertilizer is ideal; a light fertilizer application every few weeks will ensure continual nutrient coverage.

Peppers

Scientific Name: Capsicum annum

• Family: Capsicum
• Planting Season: 2 weeks after the threat of frost
• Harvest: May-November
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Moderate
• Fertilizer Requirements: 3-5-5 if using a granular organic fertilizer. Provide a small amount of fertilizer in the planting hole at the time of transplant. Light application every 2-3 weeks throughout the growing season ensures continual plant nutrition.

Lettuce

Scientific Name: Lactuca sativa

• Family: Asteraceae
• Planting Season: March to August (in cooler regions), October to April (in warmer ones)
• Harvest: 30-70 days for full heads or continuously for individual leaves
• Growth Rate: Fast
• Light Requirements: Full Sun
• Water Requirements: High
• Fertilizer Requirements: Lettuce typically requires very few nutrients in its early stages of life (seed to newly-germinated seed). Once plants reach the rosette stage (a few young leaves in a cluster), they need increased nitrogen. Applying a granular organic nitrogen source such as alfalfa meal or feather meal at planting time ensures it has time to decompose as the seedling germinates and reaches the rosette stage. Applying additional nitrogen every 2 weeks from planting will ensure a continual supply for the heads from the rosette stage through maturity. Phosphorus is only necessary if a soil test shows low phosphorus.

Green Beans

Scientific Name: Phaseolus vulgaris

• Family: Legume
• Planting Season: Spring, after the threat of frost
• Days to Harvest: 45-60
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Moderate
• Fertilizer Requirements: Some legumes absorb nitrogen from the air and fix it in their root system for later use. Due to their heavy production of long pods, green beans usually like a tiny bit of extra nitrogen but don’t need much; their primary need is for phosphorus or potassium if those are deficient in the soil. A 3-5-5 granular organic fertilizer applied at planting time and monthly thereafter is ample.

Peas

Scientific Name: Pisum sativum

• Family: legumes
• Planting Season: February/March for spring, October in warm areas for fall crop
• Days to Harvest: 80-100
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Moderate
• Fertilizer Requirements: Similar to beans, peas may fix some supply of nitrogen in their roots for later use. 3-5-5 granular organic fertilizer applied at planting time and monthly thereafter should be ample.

Carrots

Scientific Name: Daucus carota

• Family: Apiaceae
• Planting Season: Spring and Fall
• Days to Harvest: 60-80
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Moderate
• Fertilizer Requirements: A soil test is best before planting carrots to know what’s already in your soil. Excess nitrogen can cause carrots to fork. If NPK levels in your soil are good already, you should not need to add additional fertilizer. If your P or K is low, apply these once before planting. Most carrots use N only when they’re bulking the root size and putting up tall leafy greens. It’s okay to do a light side-dressing at that time, but they don’t strictly need it.

Kale

Scientific Name: Brassica oleracea var. sabellica

• Family: Brassicaceae
• Planting Season: Spring and Fall
• Days to Harvest: 95
• Growth Rate: Slow to Moderate
• Light Requirements: Full to Part Sun
• Water Requirements: Moderate to High
• Fertilizer Requirements: Kale, like lettuce, prefers a high level of nitrogen from its rosette stage through maturity. Apply a granular organic nitrogen source at the time of thinning direct-sown plants, and then every 2 weeks thereafter through the end of the season. If transplanting, add nitrogen to the beds approximately 3 weeks prior to transplant to prepare the soil for the young plants, and use a diluted liquid fish fertilizer on your young plants once prior to transplant.

Cucumbers

Scientific Name: Cucumis sativus

• Family: Cucurbitaceae
• Planting Season: April-June
• Days to Harvest: 50-70
• Growth Rate: Fast
• Light Requirements: Full Sun
• Water Requirements: Moderate
• Fertilizer Requirements: If a soil test shows that P & K are both fine, apply a N-based fertilizer (such as alfalfa meal or feather meal) mixed into the surface soil around the plant every 2 weeks from planting through the end of the season. A general-purpose 3-3-2 liquid fertilizer can also be used, diluted with water per the manufacturer’s recommendations

Radishes

Scientific Name: Raphanus sativus

• Family: Brassicaceae
• Planting Season: March-June and again from September through November
• Days to Harvest: 3-5 weeks
• Growth Rate: Fast
• Light Requirements: Full to Part Sun
• Water Requirements: Moderate
• Fertilizer Requirements: Most potassium and phosphorus fertilizers should be applied before planting time. A balanced fertilizer once prior to planting should be sufficient for their needs (and possibly for the needs of a second crop of radishes too).

Zucchini

Scientific Name: Cucurbita pepo

• Family: Cucurbitaceae
• Planting Season: April-July
• Days to Harvest: 45-55
• Growth Rate: Moderate
• Light Requirements: Full sun
• Water Requirements: Moderate to High
• Fertilizer Requirements: 10-10-10 worked through the soil before planting, then once again as flowers start to set as a side-dressing scratched into the soil’s surface. Squash tend to be heavy feeders, so if you have a long season, apply a side-dressing monthly through the growing season.

Onions

Scientific Name: Allium cepa

• Family: Allium
• Planting Season: Spring or Fall
• Days to Harvest: 100-125
• Growth Rate: Slow
• Light Requirements: Full Sun
• Water Requirements: High
• Fertilizer Requirements: Soil for onions should have phosphorus applied in advance of planting, as this is necessary for the young plant’s development. In addition, adding nitrogen an inch or two below where the seed is planted will encourage the onion to develop a few deep taproots to reach it, resulting in hardier onions. Most of what they need will be nitrogen from the start of bulb formation through harvest, and this can be a slow-release organic form or a liquid form as a side-dressing.

Broccoli

Scientific Name: Brassica oleracea

• Family: Brassicaceae
• Planting Season: Spring or Fall
• Days to Harvest: 50-60
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Moderate to High
• Fertilizer Requirements: Broccoli requires little nitrogen until it reaches the rosette stage (a clump of young true leaves), but suddenly becomes a heavy nitrogen feeder at that time. Apply an organic nitrogen source (such as feather meal or alfalfa meal) at the time of thinning seedlings and then every 2 weeks throughout the season for good growth. Phosphorus and potassium should only be applied if the soil is deficient.

Corn

Scientific Name: Zea mays

• Family: Poaceae
• Planting Season: Spring, 2-4 weeks after the last frost
• Days to Harvest: 75-85 days
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: High
• Fertilizer Requirements: An application of a 10-10-10 fertilizer before planting establishes most of the plant’s needed phosphorus and potassium for the early season. From the six-leaf stage through maturity, corn is almost solely a nitrogen feeder. Side-dressing the nitrogen with an organic option, like feather meal or alfalfa meal, every 2 weeks from the seedling stage onward should supply enough to meet its needs.

Cabbage

Scientific Name: Brassica oleracea var. capitata

• Family: Brassicaceae
• Planting Season: Fall, 6 weeks before the first frost
• Days to Harvest: 60-100
• Growth Rate: Moderate
• Light Requirements: Full Sun
• Water Requirements: Low
• Fertilizer Requirements: Cabbage requires little nitrogen until it reaches the rosette stage (a clump of young true leaves). Once it reaches that stage, it becomes a voracious nitrogen feeder. Apply an organic nitrogen source (such as feather meal or alfalfa meal) at the time of thinning seedlings and then every 2 weeks throughout the season for good growth. Phosphorus and potassium should only be applied if the soil is deficient.

Types of Fertilizers and Their Application

Organic – Organic fertilizers are made from organic materials like compost and manure, or from plant or animal sources such as feather meal or alfalfa meal. These are typically regarded as more environmentally friendly and can produce long-term positive effects on the soil.

The downside to these is that they are slower-acting, as they must decompose before their nutrients become plant-available. This means you need to apply the fertilizer you need 2-4 weeks before it’s necessary to ensure a continual supply. On the bright side, these organic materials are less likely to cause fertilizer to leach through the soil into groundwater supplies.

Organic fertilizers also contain organic matter that adds to the quality of the soil as it decomposes. This can provide beneficial results for your soil life; the microbiology in the soil feeds on some of the organic materials and can improve your soil over time.

Some common materials that are used to make organic fertilizers include bat guano, chicken litter compost, or various types of meals like cottonseed or kelp meal. Check the NPK ratios of your organic inputs if you’re going for single-nutrient fertilizers. The Espoma brand makes an excellent line of organic fertilizers if you want a simpler, general-use fertilizer that covers a broad range of vegetables.

Inorganic – Inorganic or chemical fertilizers are useful for quick, plant-available nutrients. As these do not need to break down before they’re plant-available, they are often widespread in use and easy to apply. Many are powders that can be added to water and sprayed onto the soil.

The downside to these fertilizers is that they are often quite strong – perhaps too strong. The sudden release of nutrients on tender plant roots or basal foliage can cause fertilizer burn damage to the plant.

In addition, many of these soluble formulas can be washed through your soil during regular irrigation and can collect in the local water table, causing pollution to groundwater. It is better with these types of fertilizer to do a light occasional application rather than a single heavy application.

Final Thoughts

Fertilizing is an important factor in vegetable gardening that promotes faster and more vigorous growth and more nutrient-dense, tastier vegetables. Vegetable gardens should begin with fertilization to give plants a strong start, and fertilizing at regular intervals will encourage plenty of healthy growth.