Blueberry Soil Chemistry
Oh, no, chemistry! Fear not, this explanation is in simple terms to help you understand the environment around the root system of a blueberry plant. In the introduction you saw that blueberry plant health is all about the root zone. If the plant can not take up what it needs, it will not thrive. Most failed blueberry plantings can be traced back to improper soil conditions, and the most important one to get right is the pH of the soil.
There is also a little information here about root mycorrhizae, these are the little white fungi you find when you dig up a plant's roots. They're the good guys, and your blueberry benefits from them.
Explaining Root Zone pH
Orange juice stings when you get it up your nose. It does that because it is acidic. Vinegar is acidic. So is soda pop and battery acid. The acidity of the plant's root zone is measured with a pH test kit available at the garden center. After mixing a liquid with your sample you will see where it falls on the pH scale. It is as easy as reading a thermometer. A reading below 7 is acidic, a reading above 7 is not, it's alkaline.
This means that making a soil more acid LOWERS the pH reading. OK, now I'm really confused, we lower the pH by increasing the acidity? That's correct, remember that 7 is neutral and the acidity increases as that number drops. Pure water has a pH of 7.0, tea is 5.5, beer is 4.5, orange juice is 3.5, a cola drink is 2.5, and battery acid is 1.0 on a logarithmic scale. Blueberries have their comfort zone between the beer and the tea.
Only the blueberry root zone needs these acid conditions, it doesn't matter what the rest of the garden is like. Soil pH controls the chemical reactions and processes going on in the root zone. It strongly influences the availability of plant nutrients and can have a drastic effect on fruit production. You can make the zone more acidic by adding certain organic materials or by adding certain chemicals. Some fertilizers, both organic and non-organic, will change the soil pH. But the soil's pH changes slowly, and you may not see results for some time. Natural forces also change the pH over time, back to where it was originally. If you are planting a brand new bed it is wise to do an annual pH test for at least the first few years.
Organic matter (good stuff) in the root zone breaks down, forming a weak carbonic acid which helps keep the zone at a stable acidic level. Rubble in the soil, especially from new construction, is bad stuff and may leach out components that will make the soil less acidic. This leaching may go on for years (see buffering below), so building rubble should be removed from the root zone. For the same reason, try to place the plants at least six feet from new sidewalks and fresh concrete foundations.
Some components in your soil will cause pH "buffering". This is the reservoir of materials in the soil which makes it resistant to change. A pH test will not show you the amount of buffer in the soil, and for this reason it is important in the beginning to do an annual soil test. Clay has high buffering capacity and is highly resistant to pH change.
The rainfall here on the North Coast of California already makes the soil slightly acidic, so you might not need to change it much. But the only way to tell is to do a soil pH test. Blueberry roots do best in an organic material with a pH of 4.5 to 5.5. If the pH of the root zone is much above or below this range, certain nutrients will not be available. For instance, iron and zinc are important micronutrients for a blueberry, but they will become deficient if the soil is too acidic. Some nutrients may even become toxic to the plant.
You may have heard you can determine your soil pH by looking for certain weeds or by the color of a hydrangea flower. This is true, but only for certain hydrangeas, and only directly under that plant, and you still will not know where the soil falls on the pH scale. Old-time farmers might have pulled down their pantaloons and sat on the soil to determine proper soil temperature for seeding, but you can't measure soil pH that way or any other physical way. Do the litmus test, you're just wasting time and money by guessing. The soil pH depends entirely on how its previous owners treated that patch of soil.
But what to do if you don't want to deal with soil tests and all that? Follow these general directions, or buy a couple of bags of acid planting mix and a half-barrel at the garden center. That might work for you.
And since we're talking about roots...
This section is about beneficial ericoid ectomycorrhizal fungal symbiosis, and I had to slowly read that out to get the pronunciation right. But you can ignore the big words and read on.
Real simply, almost every plant has mycorrhiza. It is a tiny fungus that grows on and sometimes into a plant's roots. It also extends itself into the immediate soil layer and takes up foods from it. Those nutrients are then passed to the plant roots in exchange for sugars and/or starches that the fungus uses to further its own growth. This symbiotic win/win relationship benefits both organisms and helps the blueberry plant immensely since its roots have extremely tiny or non-existent root hairs. Agronomists are discovering that plant root zones contain a huge and diversified universe of life, and are striving to understand how these tiny forms of life influence the growth and health of their hosts.
When we planted our apple trees we acquired some soil from around the tree roots of a friend's old apple orchard. This soil was rich in the type of mycorrhiza that colonizes apple roots, and we mixed it with (inoculated) the soil immediately around our new apple tree roots. But each type of plant has its own type of mycorrhiza fungus, and there hasn't been all that much research yet about blueberry root mycorrhizhae. But they do exist, predominantly as ascomycetes, and they, as well as the beneficial bacteria, play a large part in assisting the plant in absorbing nutrients in soils.
Does the plant need it and is this critical to add during planting? No, these microbes will show up on their own as long as you go easy on the fertilizers and chemicals. This is also why some growers prefer to prepare the planting area a year or more ahead of time. But where could one get this fungus and good bacteria to add to a new sterile garden mix? The same way we got our apple mycorrhiza, but now from some soil of a nearby blueberry or huckleberry patch, or maybe from around existing heather or rhododendron plant roots. Stay local, ask first, and don't let it dry out. Here's a very scientific USDA PDF on blueberry mycorrhizae, skip to the very last part titled "Discussion". The first paragraph pretty much sums up what it's all about.