Blueberry fertilizer: How and when to feed blueberries

Blueberries are among the easiest fruits for home gardeners to grow. They have few pests, don’t take up much room, and produce berries for many years. Blueberry plants are extremely cold hardy, and their care routine is not complicated. In a previous article, I shared information on proper blueberry pruning and received many follow-up comments and questions about how and when to apply blueberry fertilizer. This article offers insight on the best products to fertilize blueberries, the proper time to apply blueberry fertilizer, and how much of it to use.

Ideal conditions for growing blueberries

Before we talk specifics about blueberry fertilization, I’d like to touch on the importance of locating your blueberry bushes in ideal conditions for optimum growth and production. If your plants are sited properly and they receive enough sunlight and water, the need for fertilization won’t be as critical. And perhaps most importantly, if you don’t provide them with the right conditions, even if you properly fertilize, the plants won’t perform their best.

Here’s a quick rundown of the best conditions for growing blueberries:

• Site blueberry plants in full sun (a minimum of 8 hours per day).
• Grow at least two different varieties for maximum fruit set. Most blueberry varieties are not self-fertile, meaning their flowers cannot pollinate themselves. They need the pollen from a different variety to make quality fruits. If you only have one blueberry plant, you likely won’t get many fruits.
• Blueberries evolved in regions with nutrient-poor, acidic soils. Because of this, having the ideal soil pH level is key to plant health and productivity. Without the right soil pH, your blueberries won’t be able to access certain nutrients from the soil, regardless of you ensuring the plants are properly fertilized. The target soil acidity level for blueberries is between 4.5 and 5.1. Here’s an article on how to test and adjust your soil pH.
• The roots of blueberry plants do not like poorly-drained soils. Don’t plant them in low-lying areas that are boggy.
• Soils high in organic matter are best. Add composted leaves, compost, well-rotted manure or other organic matters to the soil prior to planting blueberries if possible.

When to use blueberry fertilizer

Blueberries do not have high nutrient needs. In fact, they are very sensitive to over-fertilization and excessive nutrient levels in the soil. Don’t overdo it. Most nutrition-related problems are easy to prevent by making sure your soil pH is correct. As mentioned above, blueberries require acidic soils. It is critical to their success. Before doing any blueberry fertilization, start by testing the soil pH. It’s an inexpensive, easy, and very important step. Here’s a simple home test you can use, or you may be able to purchase a soil test kit from your state’s cooperative extension service or your local governmental agricultural agency.

Be prepared to re-test the pH of the soil beneath your blueberries every 4 to 5 years. The results will not only tell you your existing soil pH level, but they will also provide suggested fertilizer types and amounts for the following growing season. The advice in this article will help you determine blueberry fertilizer needs for the years in between soil tests.

As mentioned, the most critical aspect of blueberry fertilization is making sure your soil stays in the ideal pH range. Soil pH is so important because it determines the availability of almost all of the nutrients contained within the soil. At certain pH levels different nutrients are either tied up and unavailable, or they are readily available for plant use.

Signs to watch for

The biggest sign that the soil beneath your blueberries does not have the ideal target pH is nutrient deficiency symptoms. The primary one for blueberries is chlorotic leaves. Chlorotic leaves exhibit what is called “interveinal chlorosis”. This means that the area in between the leaf veins is yellow or pale green while the leaf veins themselves are bright green (see photo below). Poor growth and poor yields can be another symptom of improper soil pH and related nutrient deficiencies. New growth should be lush and green, not tinged with red or yellow.

The best fertilizers for blueberries

There are three types of blueberry fertilizer commonly used. One works to solely to lower the soil’s pH into the target range; the second’s focus is exclusively on adding nutrients to the soil without altering the pH; and the third blueberry fertilizer is a combination product that both acidifies the soil pH and adds nutrients to the soil. Let’s talk about each of them in turn.

1. Blueberry fertilizer to acidify the soil.

If your soil pH is too alkaline and the soil needs to be acidified but your nutrient content is fine, these are the products to use.

Ammonium sulfate

Ammonium sulfate is the most commonly recommended blueberry fertilizer for ensuring the pH of the soil remains acidic. How much to initially apply depends, of course, on how acidic your soil is to begin with. Typically, 2 to 4 ounces per bush per year is adequate to maintain an established pH between 4.5 and 5.1. That being said, I’m not a fan of ammonium sulfate. It’s all-to-easy to overdo it, and because it is an artificial commercial fertilizer, it is prohibited for use in organic agriculture.

Elemental sulfur

My favorite blueberry fertilizer to acidify the soil is elemental sulfur. It’s a product that is fairly slow-acting, meaning it takes time for the soil microbes to process it and lead to an effective pH change. However, the risk of root burning or over-fertilization is much smaller than it is when using ammonium sulfate. It is approved for use in certified organic farming under the USDA’s National Organic Standards Program.

My soil here in Pennsylvania is naturally acidic (5.8 to 5.2). I add about 1 cup of elemental sulfur to each of my blueberry shrubs every year or two to keep the pH within the target range. You may need to add more or less sulfur than I do based on your starting pH. Again, your soil test results will tell you how much is best. Re-test your soil every 4-5 years for an accurate assessment. Two of my favorite brands of elemental sulfur include Jobe’s Organics Soil Acidifier and Espoma Organic Soil Acidifier. Both have recommended rates listed on their labels.

Eventually soils that have been acidified revert back to their original pH, regardless of which product you use. Because of this, you’ll need to add sulfur or another soil acidifying fertilizer every year or two.

2. Blueberry fertilizer to add nutrients

If your soil pH is within the target range but your soil test shows nutrient deficiencies, then you’ll only need to provide nutrients, not alter the pH. Young blueberries are damaged by fertilizers containing a high amount of nitrogen (they are sensitive to the salts in synthetic fertilizers). Avoid fertilizers that contain nitrates or chlorides. Blueberries prefer their nitrogen be provided in a slow-release form. Organic granular fertilizers that contain nitrogen (N), phosphorous (P), potassium (K), and micronutrients are particularly good choices. Urea is often recommended as a source of nitrogen, but it is prohibited for use on organic farms since it is a synthetic product.

Interestingly, phosphorous is less available to plants in soils with an acidic pH, yet blueberries require a lot of phosphorous for good fruit set. And to make matters worse, the root systems of blueberries are very shallow and very fibrous, which prevents them from acquiring all the nutrients they need. To overcome this, they have formed a mutually beneficial relationship with a particular mycorrhizal fungus in the soil that helps supply nutrients to the plants. Synthetic chemical fertilizers can harm these beneficial fungi, which is another reason to avoid them. Blueberry bushes that are deficient in phosphorous, have leaves that are a darker, almost purplish, green.

To supply adequate nitrogen, phosphorus, potassium, and micronutrients (such as zinc (Zn), calcium (Ca), manganese (Mn), magnesium (Mg), copper (Cu) and others) to your blueberries, use a balanced organic granular fertilizer made from ingredients like blood meal, seaweed, bone meal, potash, and the like. For 2- and 3-year-old blueberry plants, apply ¼ – ½ cup per plant per year. For mature, full-size blueberry bushes, apply 2 to 3 cups per bush per year to supply adequate nutrients.

3. Blueberry fertilizer that acidifies the soil AND adds nutrients

In my experience, combination fertilizers that both acidify the soil and add nutrients are the best blueberry fertilizer. Fertilizers intended for acid-loving plants like rhododendrons, azaleas, hollies, and other evergreens are my top choice. Holly-tone is a favorite brand of mine because the nutrients are released slowly over time, it’s safe for beneficial soil microbes, and it contains nitrogen, potassium, phosphorous, calcium, magnesium, and other elements in addition to containing elemental sulfur that acidifies the soil. It’s made from natural ingredients like feather meal, poultry manure, alfalfa meal, and of course, the elemental sulfur needed to keep the soil pH acidic. If you use it, you do not need to add a separate sulfur fertilizer to further acidify the soil unless your pH is very alkaline.

When to apply blueberry fertilizer

Regardless of whether you are fertilizing to adjust the soil pH, add nutrients, or do both, blueberry bush fertilization can take place in the spring or the fall. You can also split the application in half, applying one half in the spring and the other half in the autumn.

If you’re fertilizing blueberries in the spring, do it any time before the buds begin to swell (but if you forget and run a little late, it’s really not that big of a deal). Remember, organic fertilizers and elemental sulfur take time to be processed by soil microbes and go to work. You’re in it for the long-fix, not for the quick-fix. If you choose to apply blueberry fertilizer in the fall, apply it about 4 weeks before your first expected frost. Truthfully, when using the right fertilizer that won’t burn tender roots (meaning a slow-release, long-acting organic fertilizer), the timing isn’t all that critical. The most important thing is to be consistent and fertilize every year or two, and re-check your pH every 4 to 5 years.

How to apply blueberry fertilizer

Regardless of which of the above products you’re adding to your berry bushes, sprinkle it around the base of the plants, extending it out to the outermost edge of the bush’s leaf canopy. Remember, blueberry roots are very shallow and fibrous, not deep. Don’t dump the fertilizer in one lump at the base of the plant. Distribute it evenly using your gloved hand or a container. The granules don’t have to be uniformly spaced but do try to spread them as evenly as you can.

When I apply my blueberry fertilizer, I either distribute it directly from the measuring cup or I hold a handful of it in my partially closed hand and shake my hand from side to side, letting the granules slide out between my fingers and fall to the soil below. It should only take a few seconds to apply blueberry fertilizer. It’s not a big or time-consuming chore.

Do you need to scratch or turn blueberry fertilizer into the soil?

Because of those shallow roots, don’t turn the soil over, till it, or deeply cultivate after applying blueberry fertilizer. The roots are easily damaged, and the mycorrhizal fungal network associated with them is very fragile. If you must, you can use a pronged cultivator or your fingers to gently scratch the fertilizer into the top inch of the soil, but it really isn’t necessary. With time, microbes, and water, the fertilizers will easily make their way down through the soil profile and to the root zone just fine on their own. There’s no need to help them along; you’ll be doing more harm than good.

Should you mulch blueberries after fertilizing?

Mulching is a useful way to insulate blueberry plant roots, stabilize soil temperatures around them, and reduce weeds. But because of their shallow root systems, don’t go crazy with the mulch or you’ll smother them. One to two inches of pine straw, wood chips, or shredded leaves is plenty. Add it on top of the fertilizer if you’d like. Or put the fertilizer right on top of the mulch. Either choice is a good one.

Extra blueberry growing tips

• Do not fertilize the first year after planting blueberries. Their roots are not yet established and are susceptible to root burn. Water soluble fertilizers are a safer option during this first year, but they are not necessary.
• Do not use peat moss/sphagnum peat to fertilize or mulch blueberries. Yes, peat moss is acidic, which they do like. But it is devoid of nutrients (making it a lousy fertilizer), and peat moss repels water when dry (making it a lousy mulch). If you insist on using peat moss as part of your blueberry growing journey, use it only as a soil amendment by mixing it into the soil before planting your bushes. But contrary to popular opinion, you do not need peat moss to grow blueberries.
• As important as learning how to fertilize blueberries is, learning how to prune blueberry bushes is equally as valuable. Thankfully, this comprehensive article sharing step-by-step instructions on how to properly prune blueberry bushes is here to help.
• Choose the right blueberry varieties for your climate. Not all types grow well in all climates. You’ll find a list of the best blueberry types for each growing zone in this article.

Fertilizing blueberries properly is a significant step toward growing healthy, productive plants. I hope this advice enables you to have the blueberry harvest of your dreams.

For more on growing backyard fruits, please visit the following articles:

• Growing kiwi fruit
• Transplanting raspberries
• Choosing the right fruit trees for your climate
• Growing watermelons in pots
• Alpine strawberries
• Growing berries in containers

Read more: