Soil pH Kits give one of the most misunderstood and misused measurements in the conventional and organic gardening worlds: pH.
It’s a fascinating topic and I’m excited to get into it here.
If you can get your head around the information in this article (it may be no problem for you, but it took me a while to get it), you will know more about pH than most gardeners, including most horticultural professionals, teachers and authors.
I must admit I now cringe every time I get to the pH test for soil discussion in most gardening books, because there is almost always bad advice. But I remind myself that I had the same beliefs about pH for most of my gardening life before I actually studied it in detail.
How Are Soil pH Kits Usually Used And What’s Wrong With It?
Here is how a pH test for soil normally goes. You determine the pH of your soil through soil pH kits or by sending a soil sample to a soil testing lab. If it’s “too low” (perhaps below 6 or so), you add lime, usually dolomite lime, to bring it up. If it’s “too high” (perhaps above 7), you add gypsum to bring it down. These are allowed in organic gardening, too.
But the soil pH kits don’t tell you how much calcium and magnesium (dolomite) or calcium and sulfur (gypsum) your soil has. You may very well have enough or too much of these things already and still have a pH that isn’t perfect.
Adding more just makes things worse.
Another problem with your average pH test for soil is that the results you get will change as much as 2 whole points over the course of 24 hours depending on rain and temperature and other variables. Taking them often and keeping track of the time of year and climate conditions when you take them can give hints as to the discrepancies you will find, but still, these discrepancies can be huge.
What Is pH?
Your soil is made up of sand, silt, and clay. There is also hopefully a small amount of organic matter such as leaves, microorganisms, insects and humus. Humus is organic matter that has been broken down almost as far as it can go, so it looks kind of like soil.
Of the sand, silt, clay and humus, only the clay and humus hold onto the elements such as hydrogen, calcium and magnesium. The clay and humus hold them on "exchange sites". If your soil was only sand and silt, those elements would leach out of your soil very quickly because there is nothing to hold onto them, and you might have perpetually infertile soil.
But back to the “exchange sites”. Soil pH kits measure what percentage of your exchange sites are occupied by hydrogen. The more hydrogen, the more “acidic” the soil becomes. Hydrogen and the other elements battle it out for exchange sites, so the more hydrogen there is, the less calcium, magnesium and other nutrients there are.
A pH of 1 is the most acidic and 14 is the most “alkaline”, but we don’t get to those extremes in the soil. In fact at a pH of approximately 4, all of the exchange sites are occupied by hydrogen anyway, and most soils don’t get much lower than that (if they do, there are other acids in the soil solution).
A pH of 7 is “neutral” and none of the exchange sites are occupied by hydrogen. Instead they are occupied by calcium, magnesium and other minerals. This doesn’t necessarily mean that your soil is totally awesome, because all of the sites could be occupied by just one mineral such as aluminum or calcium. Not very well-rounded nutrition and quite toxic to plants.
As an aside, the pH test for soil may go up to 8 or 9 when there are excess carbonates in the soil, such as in areas with low precipitation.
But soil pH kits don’t tell you what percentage of your soil is clay. You could have a pH of 7 and think that you must have lots of minerals in there, but if your soil is only 5% clay and organic matter (some of which is humus), both of which hold on to elements, and the rest is sand/silt, which doesn’t hold onto elements, you really don’t have many elements in your soil.
Conversely, you could have a pH of 5.5 and think your soil is low in fertility, but if you have 60% clay, you might be doing just fine.
Another way of looking at this is we could have two organic gardens in different areas both at a pH of 6, yet these soils may have very different mineral contents because the first soil could be low clay and the second soil could be high clay.
So, soil pH kits tell what percentage of your exchange sites are occupied by hydrogen, but they don’t tell you what other elements are there, and they don’t tell you how much clay and humus you have.
On Wednesday, I will discuss whether a pH test for soil is ever useful.
Please feel free to ask me any questions below. This is a bit of a tricky topic and definitely different than what you will read in most gardening books.