We use fertilizer to make our plants grow better, but when is the best time to apply fertilizer? And how much of what kind of fertilizer do we apply? In one page, we’ll cover the basics of using fertilizers in your garden.
What Is Fertilizer?
As well as having soil that’s rich in organic matter (compost!), plants often need an application of fertilizer to get the nutrients that need. Think of fertilizers as nutritional supplements. For example, tomatoes need to grow in soil that has plenty of calcium.
If you’ve grown and harvested plants in your garden in the past, these crops took up 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 replace lost 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 gardener takes the time to do a soil test, but we recommend it; you may even find that the fertility level of your garden is already adequate.
Apply fertilizer with caution, though: The only thing worse than starving a plant of nutrients is to accidentally overfertilize it. Plants use only the nutrients that they need. Absorbing more than necessary can result in abnormal growth or adverse effects.
When to Fertilize Your Garden
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 the fertilizer.
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 by frost.
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 will need extra fertilizer mid-season as the plants take up and use existing nutrients. When tomatoes start producing flowers, switch to a low-nitrogen fertilizer in order to encourage more flowers and fruit rather than foliage.
- 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 the harvest.
- Ornamental trees, shrubs, and perennials are often fertilized at the beginning of their growing season, as dormancy breaks.
Always Take a Soil Test!
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 ever year. The goal is to understand your soil, build it up, and then simply apply fertilizer to the soil ever year to maintain the basic fertility level.
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 Much of What Kind of Fertilizer Should I Use?
A fertilizer bag will be labeled with a combination of numbers such as 3-4-4 or 8-24-8, or +61404532026. These three numbers refer to the three most important nutrients plants need: Nitrogen (N), Phosphorus (P), and Potassium (K). The numbers refer to the percentage of 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, and manganese.
To start your garden, use a general vegetable fertilizer. For vegetables, we use an herb and vegetable plant food with a 3-4-4 number. For tomatoes, we use a separate fertlize with a 3-4-6 ration which also contains calcium to help prevent blossom-end rot.
Phosporous is important because this is what’s needed for root development and growth. Potassium strengthen 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 leafy growth.
- Vegetable crops require most of their nitrogen after they have made considerable growth or have already begun to fruit. To have too much nitrogen before this time delays maturity and reduces flowering and yields. Your plants will also get nitrogen from the breakdown of organic matter in your soil.
Later in the season, some plants benefit from a nitrogen side dressings (sprinkled in 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 the vegetable.
- Cabbage, cauliflower, broccoli can benefit from more fertlizer three weeks after transplanting.
- Peas, beans, cucumbers, and muskmelons can benefit after blossoming begins.
- Peppers, eggplants, and tomatoes benefit after first fruit sets and tomatoes could use more about two weeks after picking your first tomato, and then again a month later.
- Sweet corn can benefit when plants are 8 to 10 inches tall and then one week after tassels appear.
- Spinach, kale, mustard, and turnip greens can benefit when plants are about one-third grown.
- These vegetables should NOT have added nitrogen: sweet potatoes, watermelons, carrots, beets, turnips, parsnips, lettuce.
The fertilizer bag should tell you the amount to use per 1,000 square feet of garden area. You can always ask the nursery staff for help translating to your garden space.
Read more in our article on fertilizer basics and the NPK ratio.
Processed vs. Organic Fertilizers
The fertilizer guidelines apply to both processed or 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.
How to Apply Granular Fertilizers
For that first “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 3 to 5 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, potentially “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!
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