Soil pH is talked about a lot in the gardening world, but most people don’t understand it, so it’s generally misused. To simplify what pH is, it is basically a measurement comparing how much hydrogen you have in our soil versus a handful of other nutrients — mainly calcium, magnesium, potassium, sodium and aluminum. The more hydrogen you have, the lower the pH is – the more “acidic” it is. The more of the other nutrients there are, the higher the pH is – the more “alkaline” it is.
The scale in measuring pH goes from 0 to 14, with 7 being neutral. Most soils are between 4 and 9. It’s usually best to have a pH somewhere in the middle. Actually, between 6 and 7 is generally considered ideal, which is often true, but this is also where a mistake is often made. If your soil for example is pH 5.5, the common advice would be to add lime to raise the pH of our soil. Most often this would be dolomite lime.
Dolomite is calcium carbonate and magnesium carbonate. The calcium and magnesium in the lime will probably knock some of the hydrogen out of the way. This will give you less hydrogen and more other minerals, therefore raising the pH, at least in the short term. So the problem is not that dolomite lime won’t raise the pH, but that your pH test did not tell you if you actually needed calcium and magnesium. Perhaps you already have too much magnesium, or too much calcium. It’s almost certain that you don’t need both in the ratio that dolomite lime gives you. Adding more of the wrong nutrient is just going to make things worse. For example, too much magnesium causes some major compaction in your crops, among other things.
In this manner you are effectively slowly destroying the soil, with annual applications of lime, as recommended in in so many of the gardening books.
Looking at the other end of the scale, some high pH soils are due mostly to sodium and potassium, they actually still need calcium and perhaps magnesium. You wouldn’t know that if you just used the pH number as a basis for liming.
The pH does give you a clue that you may have a nutritional and microbial imbalance in your soil, but this gives you no information as to why that may be the case. As such, it’s of very little use. Not that pH isn’t important to plants and microbes. For the most part, you are happy to have it be between 6 and 7 to have the healthiest plants. However, knowing the pH value doesn’t help you much with soil management decisions, and it certainly shouldn’t be used to determine how much lime to add to the soil. pH is the result of the elements in our soil, not the cause.
Now, the reason we are happy with a relatively neutral pH is that most nutrients, particularly the most essential nutrients, are most readily available to plants somewhere in the 6-7 pH range, gradually decreasing as the pH gets further up or down the scale. Another potential problem is that some micro-nutrients become more available outside this range, especially in low pH soil, sometimes to toxic quantities.
So it’s not that the acidity of a 4.5 pH soil is harmful in itself; it’s that most nutrients aren’t as available to plants, while a few may be too available. Further, many microbes can’t live at an extreme pH, so the soil food web will be lacking. But plants that are considered “acid-loving” don’t actually love hydrogen. Instead, there are various benefits they may get out of a lower pH soil. They may just need certain trace minerals in abundance, and those trace minerals are more available in acidic soil. Or they may just need a fungal-dominated soil – fungi decrease soil pH, so it may be that these plants don’t care at all about the pH, and they just want their fungi.
Rhododendrons, for example, are often thought of as acid-loving. In reality, they love magnesium, which is sometimes more available at a low pH, and they aren’t particularly fond of calcium. They’ll grow just fine in a high pH soil if they have sufficient magnesium and lots of organic matter.
Trying to make your soil acidic by applying peat moss or perhaps chemicals doesn’t give the plants the nutrients they need or the biology they need. Trying to make it more alkaline by applying lime will often give the wrong nutrients, causing serious problems.
The best way is to focus on a more holistic approach to soil management, such as creating high quality compost and using things like rock dust and seaweed in order to give the plants the chelated minerals they need. The other important step is soil testing. A soil test will not only tell you your pH, but also which minerals need to be added back. It will rarely be lime. When all of these factors are brought in line, the pH will follow.