How and When to Fertilize Your Vegetable Garden

Learn All About Using Fertilizer in the Garden

February 25, 2021
Fertilizer for Tomato Plants

We use fertilizer to make our plants grow better, but when is the best time to apply fertilizer? And how do we apply fertilizer to the garden correctly? In one page, we’ll cover the basics of using fertilizer in your garden.

What Is Fertilizer?

Even garden soil that is rich in organic matter and nutrients can benefit from applications of certain types of fertilizer.

Think of fertilizers as nutritional supplements that improve a plant’s health, growth, and productivity. You can often tell which nutrients your soil is lacking by the deficiency symptoms plants display, which can range from yellow leaves (lack of nitrogen) to reduced flowering (lack of phosphorus) to weak stems (lack of potassium) to blossom-end rot (lack of calcium). 

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

Too Much of a Good Thing

More fertilizer is not always better! Plants use only the nutrients that they need. Absorbing more than necessary can result in abnormal growth or adverse effects. In addition, it’s important to add any fertilizer at the right time when the plant needs it. For example, you don’t want to spur new foliage growth right before a cold winter, when the plant should be conserving its energy!

When to Fertilize Your Garden

The general rule is to fertilize in the spring before planting most annual flowers and vegetables. For perennial 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 getting immediately killed by frost.

Many gardeners use a general-purpose fertilizer in the spring (either an evenly-balanced fertilizer or one that’s slightly higher in nitrogen). Incorporate fertilizer into the soil several inches deep for annuals and vegetables. For perennials, work fertilizer lightly into the soil around the plants. 

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

  • 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 foliar growth. 
  • Tomatoes and potatoes also will need extra fertilizer (N) mid-season as the plants take up nutrients. When tomatoes start producing flowers, switch to a low-nitrogen fertilizer in order to encourage more flowers and fruit.
  • 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 harvest. 
  • Ornamental trees, shrubs, and perennials are often fertilized at the beginning of their growing season, as dormancy breaks. 

Soil shovel

Always Take a Soil Test!

The only way to truly determine the level of nutrients in your soil is to test it. Soil tests are usually available for free or low-cost from your local cooperative extension. A soil test is easy to do and the results guide your fertilizer applications. You may even find that if your garden has been fertilized for years, you have high levels of nutrients. You do not want to add nutrients to your soil if it’s already available in high amounts; this may actually inhibit your plants’ growth. Read more about how to take a soil test.

How to Read Fertilizer Labels

Ever seen those confusing labels on fertilizer bags? The numbers can seem daunting at first, but once you know what they mean, they tell you exactly what you need to know about a fertilizer.

A fertilizer label on a package will have three numbers, such as 5-10-10. These numbers refer to the percentage of Nitrogen (N), Phosphorus (P), and Potassium (K), the three nutrients that plants need the most. 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 handle). There may also be other nutrients, including calcium, magnesium, iron, and manganese. 

You can find these nutrients in many strengths; they can be processed or organic, and may come in liquid or granular formulations.

“Complete” fertilizers contain all three nutrients (such as 5-10-10) and “balanced” fertilizers have equal levels of each nutrient (10-10-10). Sometimes, the nutrient ratios are important. For example, if you’ve ever experienced lush green growth without flower blooms, you may have too much nitrogen. You might choose a fertilizer label with 3-20-20 (low in nitrogen). Alternatively, vegetables planted in cold soil may need extra phosphorus for root growth; you might choose a fertilizer labeled 10-50-10.

Read more in our article on fertilizer basics and the NPK ratio.

Processed vs. Organic Fertilizers

  • Processed fertilizers (also called “synthetic” or “chemical” fertilizers) are manufactured from natural ingredients such as phosphate rock (P) and 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. (There are some processed fertilizers that 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 fish emulsion).

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

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 outweighs processed.

fresh vegetables

How to Apply Granular Fertilizers

For the first fertilizer application of the season, apply granular fertilizers by broadcasting them either by hand or with a spreader over a large area. Or, side-dress the fertilizer alongside your rows or plants or seeds. All dry fertilizers should be worked or watered into the top 4 to 6 inches 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 gently so that you do not damage any roots.

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 find out how often applications should be made.)

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 access it.

How to Apply Liquid Fertilizers

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 the plant.

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, “burning” them and causing further die-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 root damage.

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 the system.

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 to burning.

Learn More About Fertilizing Your Garden

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

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—from weeds!

Free Online Gardening Guides

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

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Reader Comments

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Use of fertilizer on potatoes

How do you apply fertilizer on potatoes from planting to harvest?

Fertilizer for potatoes

The Editors's picture

Potatoes require more fertilizer than most garden vegetables. For the home garden (not commercial growing):

  1. Add lots of aged manure/compost to the soil (50/50%). 
  2. Side-dress along the row about four weeks after planting. As you hill up soil around the plants, incorporate 0.15 pounds actual nitrogen per 50 feet of row.
  3. Repeat the hilling and fertilization two weeks later.
  4. Do not use any fertilizer containing a weed killer (“Weed and Feed”), as it may kill your vegetable plants.
     

Fertilizers

How shall I use 17-17-17 liquid fertilizers for plants ?? I have only this liquor fertilizer.

Fertilizers

Is it ok to apply organic granular fertilizer at the same time a soluble fertilizer is used?

Dolomite

I like how you mentioned that nitrogen and phosphorus are life-giving nutrients that plants need in order to be maintained and grown. My sister is thinking of looking for dolomite because she's considering planting tomatoes in her garden to increase the variety of homegrown food she has. I think it's a good idea for my sister to consider buying the necessary fertilizer she needs from a reputable supplier so that her crops can grow as best as possible.

editing

To absorb more than are unnecessary can result in abnormal growth.

Fertilizers

It is not just the timing that is important when applying a fertilzer, the weather should be considered as well just like what we do in my garden.