Let's Grow! How and When to Fertilize Your Vegetable Garden (2024)

We use fertilizer to make our plants grow better, but when is the best time toapply fertilizer? And how much of what kind of fertilizer do we apply?In one page, we’ll cover everything you need to know about using fertilizers in yourgarden.

What IsFertilizer?

As well as having soil that’s rich in organic matter (compost!), plants often need anapplication of fertilizer to get the nutrients that they need.Think of fertilizersas nutritional supplements. For example, tomatoes need to grow in soil with plenty of calcium.

If you’ve grown and harvested plants in your garden in the past, these cropstakeup the nutrients from the soil, and those nutrients should be replaced in the spring before more plants are grown there. This is where fertilizer (organic or processed) plays a role. Fertilizers replacelost nutrients, which ensures that soil nutrient levels are at an acceptable level for healthy growth.

If you are a brand-new gardener, the ideal first step is to get a basic soil test in the autumn to see what kind and amount of fertilizer to apply to get to a “basic fertility” level. (See more about testing below.) We recognize that not every gardenertakes the time to do a soil test, but we recommend it; you may even find that thefertility level of your garden is alreadyadequate.

Apply fertilizer with caution, though: The only thing worse thanstarving a plant of nutrients is to overfertilize it accidentally.Plants use only the nutrients that they need. Absorbing more than necessary can result in abnormal growth or adverseeffects.

When to Fertilize YourGarden

For edible crops, fertilizer is usually applied in the spring and mixed into the garden soil before planting.If you’ve already sown your seeds or planted seedlings, you can still gently work in granular fertilizer (not liquid fertilizer, which can burn young roots) around the plants. It need not be worked deeply into the soil—the upper 3 to 5 inches will do. Then, water in thefertilizer.

For perennial flowering plants, fertilize before growth begins in the spring. Wait until the ground is no longer frozen and the date of your last frost is only a week or so away. This ensures that there is less of a chance of the tender new growth brought about by the fertilizer getting immediately killed byfrost.

While a spring application is a good general rule, understand that what plants really need is help when they are growing themost.

• This occurs earlier for spring plantings of lettuce,arugula, kale, and other leafy greens.
• Rapid growth occurs in midsummer for corn and squash. So, for a long-season crop such as corn, many gardeners apply a small amount of fertilizer as a starter at the time of seeding and then also add a larger amount in early summer, just before the period of rapid foliargrowth.
• Tomatoes and potatoeswill need extra fertilizermid-season as the plants takeup and use existing nutrients. When tomatoes start producing flowers, switch to a low-nitrogen fertilizer in order to encourage more flowers and fruit rather thanfoliage.
• For perennial plants, the timing depends on the plant’s growth cycle. Blueberries, for example, benefit when fertilizer is applied early in the season at bud break while June-bearing strawberries benefit most when fertilized after theharvest.
• Ornamental trees, shrubs, and perennials are often fertilized at the beginning of their growing season as dormancybreaks.

Always Take a SoilTest!

The only way to truly determine the level of nutrients in your soil is a “soil test.”Testing in autumn will give you plenty of time to receive the results. Soil tests are usually available for free or low cost from your local cooperative extension.You will not need to do this every year. The goal is to understand your soil, build it up, and then simply apply fertilizer to the soil every year to maintain the basic fertilitylevel.

You may even find thatif your garden has been fertilized for years, you have high levels of nutrients. You do not want to add nutrients to your soil if they’re already available in high amounts; this may actually inhibit your plants’ growth. Read more about how to take a soil test.

How Much of What Kind of Fertilizer Should IUse?

A fertilizer bag will be labeled with a combination of numbers such as 3-4-4 or 8-24-8, or 12-12-12.These three numbers refer to the three most importantnutrients plants need:Nitrogen (N), Phosphorus (P),and Potassium (K). The numbers referto the percentage of the weight of each nutrient in the bag.If you add up the numbers, they are the percentage of the bag’s total weight (the rest is simply filler to make it easy to apply). There may also be other nutrients, including calcium, magnesium, iron, andmanganese.

To start your garden, use a general vegetable fertilizer. For vegetables, we use herb and vegetable plant food with a3-4-4 number. For tomatoes, we use a separate fertilizer with a 3-4-6 ration, which also contains calcium to help prevent blossom-endrot.

Phosphorus isimportant because this is what’s needed for root development and growth.Potassium strengthens plants’ abilities to resist disease. Note the nitrogen (first number) is lower. Ever seen tomato plants that have lush leaves but no flowers or fruit? That’s due to too much nitrogen, which encourages leafygrowth.

• Vegetable crops require most of their nitrogen after they have made considerable growth or have already begun to fruit. Having too much nitrogen beforethis time delays maturity and reduces flowering and yields.Your plants will also get nitrogen from the breakdown of organic matter in yoursoil.

Later in the season, some plants benefit from nitrogen side dressings (sprinkled in the middle of rows). The demand of the plant for nitrogen often exceeds that supplied by the first two, and a nitrogen side-dressing is needed. But it depends on thevegetable.

• Cabbage, cauliflower, and broccoli can benefit from more fertilizer threeweeksaftertransplanting.
• Peas, beans, cucumbers, and muskmelonscan benefit after blossomingbegins.
• Peppers, eggplants, and tomatoes benefit after first fruit sets, and tomatoes could be used more about two weeks after picking your first tomato, and then again a monthlater.
• Sweet corn can benefit whenplants are 8 to 10 inches tall and then oneweek after tasselsappear.
• Spinach, kale, mustard, and turnip greens can benefit when plants are about one-thirdgrown.
• These vegetables should NOT have added nitrogen: sweet potatoes, watermelons, carrots, beets, turnips, parsnips, andlettuce.

The fertilizer bag should tell youthe amount to use per 1,000 square feet of garden area. You can always ask the nursery staff for help translating to your gardenspace.

Read more in our article on fertilizer basics and theNPK ratio.

Processed vs. OrganicFertilizers

The fertilizer guidelines apply to both processed or organicfertilizers.

• Processed fertilizers(also called “synthetic”or “chemical” fertilizers) are manufactured from natural ingredients such as phosphate rock (P), sodium chloride (NaCl), and potassium chloride (KCl) salts, but these are refined to be made more concentrated. Most (but not all) processed fertilizers are quick-release in a water-soluble form to deliver nutrients quickly to the plant, which can be useful in some situations. (Some processed fertilizers are coated to slow down the release.)
  
• Organic fertilizers are materials derived from plants that slowly release nutrients as the micro-organisms in the soil break down. Often applied in granular form (spread over the soil), most organic nutrients are slow-release, adding organic material to the soil so that you don’t need to apply it nearly as often. (Plus, they don’t leach into and pollute waterways, as do many of the synthetic, water-soluble fertilizers, which plants can’t fully absorb.)While most organic fertilizers are slow-release products, some release a portion of their nutrients quickly (examples are animal manure, biosolids, and fishemulsion).

Chemically, the nutrients for processed and organic fertilizers are the same. Ideally, slow is the way to go. Slow-release granular fertilizers measure nutrients in a controlled, “digestible,” and safe manner, as opposed to fast-acting, synthetic, water-soluble fertilizers, which are, in essence, anoverdose.

In terms of cost: While organic fertilizers can be more expensive upfront than processed fertilizers, they are often still economical for small gardens. Plus, you don’t need to apply as often. When you add the long-term benefits to your soil, organic outweighsprocessed.

How to Apply GranularFertilizers

For thatfirst “starter” fertilizer application of the season, apply granular fertilizers by broadcasting them either by hand or with a spreader over a large area. Or, if you’ve already planted, side-dress the fertilizer alongside your rows. All dry fertilizers should be worked or watered into the top 3to 5inches of soil with hoe or spade work after being applied to help the fertilizer leach down toward the plants’ root zones. If your plants are already growing, cultivate them gently so that you do not damage anyroots.

During the growing season, lighter supplemental applications can be made to the top inch of soil in crop rows and perennial beds and around the drip lines of trees or shrubs. (Read the label to determine how often applications should bemade.)

In general, applying granular fertilizers just before a good rain can be beneficial, as it aids in working the fertilizer down into the soil where roots can accessit.

How to Apply LiquidFertilizers

All water-soluble fertilizers are applied by dissolving the product in irrigation water and then applying it to the leaves of the plant and the soil around theplant.

Warning: Do not apply liquid fertilizer at the same time that you plant!No matter how carefully you remove plants from their containers and place them in the ground, some root hairs will break. The fertilizer will reach the roots immediately and enter them at the broken points, potentially “burning” them and causing furtherdie-back.

Many gardeners wait 2 to 3 weeks after planting before fertilizing with liquid solutions; by then, the newly set-out plants should have recovered from any rootdamage.

It is important to water plants thoroughly with plain water before applying the liquid fertilizer to avoid burning the roots if the soil is dry. Also, take care that the fertilizer is indeed diluted based on instructions, or you could burn the leaves. If you have a watering system, you can use an injector device to run the fertilizer through thesystem.

In the case of liquid sprays, it is best to apply them on dry days in either the early morning or the early evening, when the leaves will have time to absorb the material. Avoid extremely hot days when foliage is subject toburning.

Learn More About Fertilizing YourGarden

If you have more questions about fertilizers, please ask below, or we encourage gardeners to call their country’s free cooperative extension office for localadvice.

We hope you’ve learned a lot about fertilizers!When is your soil ready to plant in the spring? See our minimum temperature for seeds to germinate.

Learn how to make your own organic fertilizer at home—fromweeds!

Free Online GardeningGuides

We’ve gathered all of our best beginner gardening guides into a step-by-step series designed tohelp you learn how to garden! Visit our complete Gardening for Everyonehub,where you’ll find a series of guides—all free! From selecting the right gardening spot to choosing the bestvegetables to grow, our Almanac gardening experts are excited to teach gardening to everyone—whether it’s your 1st or 40thgarden.

Do you have questions about how to properly use fertilizer in your garden? What fertilizer do youuse?