Why I don't worry about soil pH

Few gardening topics are as widely discussed, and as fundamentally misunderstood, as soil pH testing. Well, do we really need more confusion and uncertainty in the world? Certainly not! So here's a little treat for you: A free excerpt from Pantry founder Phil's book, "Building Soils Naturally", where he deals with the subject in a beautifully succinct, non-drama way (thank you, Phil).

Hope you'll enjoy!

"Soil pH is talked about a lot in the gardening world, but most people don't understand it and the number garnered from a pH test is generally misused. After this short section you will know how to use pH data better than most garden experts. Now, pH can get very technical, but I'm going to stick to what is relevant to us in the garden, yet try not to oversimplify too much for the scientists out there.

For our purposes, pH is basically a measurement comparing how many positively charged hydrogen ions we have in our soil versus the other cations - calcium, magnesium, potassium, sodium and aluminum. The more hydrogen ions we have, the lower our pH is and the more acidic it is. The more of the other cations we have, the higher our pH is and the more alkaline it is. The scale goes from 0 to 14, with 7 being neutral, but most soils are between 4 and 9. At 4, all cation exchange sites are occupied by hydrogen, so there's not much in the way of nutrition for plants.

At just under pH 7, all cation exchange sites have cations other than hydrogen, which doesn't necessarily mean the soil is highly fertile because we still don't know which cations are there. In theory, it could be all magnesium, which would not be a very balanced soil. Also, what if you're gardening on a soil with 98% sand and silt, both of which have no CEC [cation exchange capacity]? In this example, even if all of your clay and organic matter exchange sites are occupied by cations, your soil is probably very low in cations because you don't have many exchange sites. A pH below 4 involves additional organic acids and above 7 or 8 involves additional carbonates. Usually the pH is somewhere in between, which means we have some hydrogen and some of the other cations.

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. Some micronutrients become more available outside this range, especially in low pH soil, potentially to toxic quantities. So it's not that the acidity of a 4.5 pH soil is harmful to the soil, it's that most nutrients aren't as available to plants, and a few may be too available. Further, many microbes can't live at an extreme pH, so the soil food web will be lacking.

So we can see it's best to have a pH somewhere in the middle. Actually, between 6 and 7 is generally considered ideal, which may be true, but this is where a mistake is often made. Let's say we take a soil sample and determine the pH is 5.5. We will be told to add lime to raise the pH of our 5.5 soil, usually this is dolomite lime.

The reason we are told to do this is because, as discussed earlier, cations can knock each other off exchange sites. All things being equal, the hydrogen ion is the most attracted to a cation exchange site. If we look at the major cations in decreasing order of their affinity for cation exchange sites - that is, how strongly attracted they are to the cation exchange sites - it goes hydrogen (H+), calcium (Ca2+), magnesium (Mg2+), potassium (K+) and sodium (Na+). There are many other micronutrients in the full list, but these are the major ones.

Hydrogen has the greatest affinity for a cation exchange site, but there's one other factor that influences which cations will bind to exchange sites. If there's an abundance of some other cation, such as calcium (Ca2+), some of it can knock off some of the hydrogen. This is called the “mass effect”.

Back to our example of adding dolomite lime to raise pH. Dolomite is calcium carbonate and magnesium carbonate. The calcium and magnesium in the lime will knock some of the hydrogen off the cation exchange sites. Some of that hydrogen will combine with the carbonate, and some of it will go elsewhere. That will give us less hydrogen and more cations, therefore raising the pH. This may happen in the short term, and even in the long term if done annually for a number of years, although the soil will tend to move back towards its starting pH. Still, it works for now.

So the problem is not that dolomite lime won't raise the pH, but that our pH test did not tell us if we actually needed calcium and magnesium. Perhaps we already have too much magnesium, or too much calcium. It's almost certain that we don't need both in the ratio that dolomite gives us. On the other hand, some high pH soils are due mostly to sodium and potassium, and they actually still need calcium and perhaps magnesium. We wouldn't know that if we just used the pH number as our basis for liming. Adding more of the wrong nutrient is just going to make things worse, as we’ll see later.

It is the pH that gives us a clue that we may have a nutritional and microbial imbalance in the soil, but this gives us no information as to why that may be so. As such, it's of very little use to us. If we do regular pH testing over a number of years, making sure we always take our soil sample from the same place, at the same time of day and year, in the same conditions, the only thing it can help tell us is if our soil management practices are working, since pH will move towards neutral when we balance the nutrients in our soil and increase the organic matter content. Besides, plants seem to grow well in soils of various pH levels given plenty of organic matter.

It's not that pH isn't important to plants and microbes. For the most part, we’re happy to have it be between 6 and 7 to have the healthiest plants. Remember, hydrogen is used in cation exchange and for mining certain minerals from the soil, so a slightly acidic pH is ideal. It can be difficult for plants to get phosphorus out of an alkaline soil. Knowing the pH value, however, doesn't help us 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. From Charles Walters’ Eco Farm: An Acres U.S.A. Primer, “excess acidity is nothing more than the reciprocal of fertility depletion.”

Plants that are considered “acid-loving” may just need certain trace minerals in abundance, and those trace minerals are more available in acidic soil. 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. Other acid-loving plants 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.

Trying to make your soil acidic by applying peat moss or chemicals doesn’t give the plants the nutrients they need or the biology they need. What we need to do is focus on all of the soil management practices we will be looking at, 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. When all of these factors are brought in line, the pH will follow.

Even if we have a perfect pH, we don't actually know which nutrients we have and how available they are to plants. This is where soil testing comes in, which is coming up later in this book."

(Excerpt from “Building Soils Naturally” by Phil Nauta, Acres U.S.A., Austin, Texas, 2012. Used with permission.)