Keys to Establishing a Successful Blueberry Planting
The first part of this article should help with understanding the blueberry plant and its needs, and the second part describes the most important steps for establishing a planting successfully.
Understanding the Blueberry Plant
Blueberries fall into an entirely different plant family from most other food crops. This plant family, the Ericaceae, includes azaleas, rhododendrons, mountain laurel, and teaberries (wintergreen). These species are often found growing together in nature, as their needs are similar. The native soils in which they thrive are typically low in pH (i.e., acidic), high in organic matter, low in phosphorus and calcium, and porous. Forest understories, bog areas, and pine barrens are all common locations where wild blueberries grow, though the species vary depending on location and conditions.
Most of the roots on a blueberry plant are extremely fine (only about the width of a human hair), so they are fragile, and are usually shallow. These roots can easily find their way through porous and high organic matter soils such as the ones in their native ecosystems, but they don't make their way into heavy soils that are low in organic matter. They also form associations with specific types of mycorrhizal fungi that act as an extension of their root systems, and that help them take up nitrogen from organic sources like decomposed organic matter. These species of fungi are not the same types that are frequently commercially sold.
Blueberry plants are native to North America and are ancient plants, but they have been commercially cultivated for only a little over 100 years. That is only a moment in agricultural history compared to most other crop plants. Thus, they still have essentially the same needs as wild plants. However, observing where they grow in nature can result in misunderstandings of what they need. The fact that blueberries are found in boggy areas makes it seem like they should grow well in wet spots in the field, but that's not the case. When blueberries are growing in bogs, they are often on small hills made up mainly of sphagnum moss or other organic matter. There is an entire ecosystem of aquatic plants and wetland creatures, and they are still getting plenty of oxygen to their roots. I equate these natural systems with being nature's hydroponics setup. This is unlike a poorly aerated, waterlogged part of the field where water molds like Pythium and Phytophthora grow best.
Blueberry Nutrition Basics
Earlier we mentioned that native blueberries are found in soils that are low in certain nutrients. However, the most common problems we see with blueberries are nutrient deficiencies. If they grow in low-nutrient soils, why would this be? The answer lies in the conditions we try to grow them in, and in differences in nutrient availability and uptake.
The optimum pH for blueberry growth is 4.5 to 5.0, or up to 5.2 if in a clay soil. Often, we try to grow blueberries in soils with a pH level that is much too high for them and are puzzled when the plants fail—after all, other plants grew there perfectly fine. However, consider that we wouldn't expect our vegetable crops to grow well in soils with a pH of 4.5 to 5.0. Neither will the blueberry plants thrive in a high pH soil that is similarly outside of their range. The nutrients readily available in these different pH ranges are much different due to soil chemistry, and blueberries need the complement of nutrients that are more available from low pH soils.
One nutrient deficiency we commonly see in the plants is low nitrogen. A big difference between blueberry plants and other crops is that blueberries use the ammonium form of nitrogen, while other crop plants use the nitrate form. Under low pH conditions, more of the nitrogen is in the ammonium form, but when the soil pH is higher, more of the nitrogen is in the nitrate form. In studies in Florida and Oregon, when nitrogen was supplied in the ammonium form, blueberries absorbed twice as much nitrogen as when it was in the nitrate form and moved it into their shoots four times faster. This is one reason why it is important to keep the soil pH low, and is also is why we recommend only nitrogen fertilizers for blueberries that supply nitrogen in the ammonium form. These would be ammonium sulfate (21-0-0) or urea (46-0-0), or other fertilizers with the word "ammonium" in their name - when other nutrients are also needed. Nitrogen tie-up when too much undecomposed organic matter is applied to the soil is another reason why we sometimes see nitrogen deficiencies.
We also see low magnesium and occasionally low potassium. Soil magnesium, potassium, and calcium compete with each other for exchange sites on the soil and the roots, so too much of one negatively affects availability of the others. Thus, when our soils are high in calcium, as many of our best agricultural soils are, we tend to see deficiencies of mainly magnesium. Foliar sprays of magnesium, usually in the form of Epsom salts (magnesium sulfate), are helpful, and soil applications of Epsom salts are as well, but much more needs to be applied. Eventually, if the soil pH is acidic, some of the calcium will be 'bumped off' of the soil exchange sites by hydrogen, but this will take several years. If potassium is low, potassium sulfate or sul-po-mag (if magnesium is also low) can be used.
We see low iron and zinc mainly when the soil pH is too high (i.e., close to neutral or basic), as these micronutrients are less available from the soil at higher pH levels. However, we also see these deficiencies when the soil pH is in the correct range, but soil phosphorus levels are excessive. At low soil pH levels, phosphorus can form complexes with iron and zinc, so if an excessive amount of phosphorus is available and the soil pH is low (as we want it to be with blueberries), iron and zinc can be tied up. To correct these issues, it is first most important to get the soil pH into the correct range before the plants are planted. If soil phosphorus levels are high, foliar sprays or regular soil applications of micronutrients may be needed.
Keys to Successful Establishment
First, for all of the reasons outlined above, it is important to get the soil pH into the correct range. A soil test is the best first step you can take, preferably at least a year before planting. The results will provide a recommendation for adding sulfur to lower the soil pH. Elemental sulfur, which is a yellow powder, is the least expensive form. Soil microbe activity is needed to start the process, so you won't see much of a pH change unless the soils are warm enough for the microbes to be active. Pelleted sulfur is easy to work with, but we've found that the pellets sometimes don't degrade, so the entire process is slowed down. You may want to check the soil pH again before planting to see if additional sulfur is needed.
Second, incorporate plenty of peat moss when you fill in the planting hole – enough to replace about half the volume of the soil. A cubic-foot-sized planting hole works well, though some growers get very large bales and work the peat into the entire row length. That is even better. The pH of peat moss ranges from 4.3 to 4.8, and by mixing the peat in, you are immediately making an environment in which the soil pH is close to the correct range, and the soil in the shallow root zone is more like a native "blueberry soil." Further, the peat makes it easier for the blueberry roots to grow in the surrounding environment, and it helps hold water and nutrients that the plants can more easily access. Decomposed softwood sawdust (preferably) and decomposed hardwood sawdust are much better than no organic matter, and they help with soil structure, and water and nutrient-holding capacity, but both have a higher pH.
Third, mulch with organic matter about four inches deep, and keep the depth maintained over time. For this purpose, peat won't work very well, so you will need to use another material. Pine straw is great, but availability is an issue. Aged sawdust, bark mulch, a mixture of the two, or chipped materials from line-clearing work are much better than nothing, but we are finding that the pH of all of these materials can be very high. The pH of the soil creeps back up over time anyway, so you will need to keep track of the pH in your planting, and you probably will need to make additional sulfur applications in the future.
Fourth, make sure you have trickle irrigation available for dry spells. Those fine blueberry roots mentioned in the third paragraph can dry up and die quickly.
Fifth, assuming you've incorporated fertilizer as recommended by the soil test results before planting, only apply light doses of nitrogen in the year of planting – about 10 pounds per acre of actual nitrogen when the plants put on their second flush of growth, which should happen about two weeks after they first leaf out, and another 5 to 10 pounds per acre about 6 weeks later. This is only about a half-tablespoon of ammonium sulfate per plant at a time, applied in a circle about 6 inches from the stem. After all of this work, you certainly don't want to burn your roots, which is surprisingly easy to do with rates any higher than this. You will make similar applications in year two – 10 pounds of N at bud break and then 6 weeks later. Then gradually increase the nitrogen rate by about 10 pounds of actual nitrogen per acre per year after this until you reach a maximum of 60 to 65 pounds per acre, split into two applications – the first at budbreak and the second six weeks later.
And one final point—be sure to remove flower buds in at least the first two years, and possibly some of them in the third year. Based on our experiences, you only delay reaching full harvests if you don't. Leaving them on is tempting, but as in so many aspects of life, patience will pay off down the road.
