What is pH?
pH is a measurement that indicates the acidity or basicity of a solution. pH is expressed on a scale from 0 to 14, where lower numbers indicate an acidic substance and higher numbers indicate a basic substance. A pH of 3 is very acidic and a pH of 12 is very basic.
More scientifically, pH is a measure of the amount of free hydrogen and free hydroxyl ions in water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Distilled water, without hydrogen or hydroxyl ions has a neutral pH of 7.
The pH scale is logarithmic. This means that each number represents a 10-fold change in the acidity or basicity of the solution. For example, water with a pH of 5 is ten times more acidic than water with a pH of 6.
pH changes when minerals become dissolved in water. The pH of soil is largely dependent on the makeup of parent soil in your yard or the soil mix that you purchased for your beds. However, it is also largely impacted by the pH of rainwater. Rainwater is nearly always acidic, because it has naturally occurring minerals dissolved in it. A normal pH for rainfall is 5 or 6.
Why does soil pH matter?
The pH of your soil affects the availability of nutrients to your plants. Even if you add compost and fertilizer, if your pH is too high or too low, your plants may not be able to absorb and use the nutrients you provided.
Nutrients become more available or less available according to the soil’s pH level. The availability range of each nutrient is slightly different, but most essential plant nutrients are maximally available to vegetable crops at a slightly acidic pH, between 6.0 and 6.9.
As an example, an iron deficiency in a crop is indicated by yellowing between the veins of its leaves. However, this condition might not actually be a result of a lack of iron in the soil, but from soil that is too basic for the crop to effectively absorb the iron that is present.
Alternatively, elements in the soil can become toxically available in very high or very low pH ranges. For example, an extremely acidic pH can make manganese available at levels that are actually damaging to your crops; or even allow plants to absorb aluminum from the soil.
How do you test soil pH?
Fortunately, testing your pH is pretty simple. You have three basic options. You can use a pH testing kit that uses litmus paper; you can use an electronic pH tester; or you can send a soil sample into a lab.
For the average home gardener, I usually recommend using a litmus paper kit and sending in a soil sample to a lab. Electronic testers are also great, but in order to get one that is really accurate, you may have to spend quite a bit of money. A soil testing lab will provide a very accurate reading and a litmus paper kit can be used for readings between soil tests.
The cool thing about getting a soil test from a lab is that you also get a lot of other useful information. A good soil lab will not only tell you the pH, it will also give specific recommendations about what to add to your soil to adjust the pH into the proper range.
Once you get your results, you’ll add lime (to raise pH) or elemental sulfur (to lower pH). Keep in mind that these additions will not change your soil overnight. Typically 3-4 months are needed in order for the amendment to take effect. This is why the end of the season is a great time to adjust your pH. With a fall application, your soil should be at the right level by the beginning of next season.
Liming Your Soil
In most yards, soil pH is acidic and lime is the most common amendment to help make it more basic. Lime is “basically” what it sounds like, pulverized limestone rock, also known as calcium carbonate.
There are two main types of lime available: Calcitic and Dolomitic. Calcitic lime is straight calcium carbonate; Whereas Dolomitic lime also has magnesium carbonate mixed in. The advantage of Dolomitic lime is that it also provides magnesium and calcium to your soil, both essential plant nutrients. However, too much Dolomitic lime can throw your calcium-magnesium balance out of whack, which can lead to all sorts of plant health issues. A good soil test result will tell you which one to use, and when in doubt, choose Calcitic lime because there are fewer risks from over-application.
If your soil happens to be basic, you can add elemental sulfur, an organic amendment, to make the soil more acidic.
The Lazy Approach: * Please note that I don’t officially condone this approach, but have seen it performed with good results.
If you aren’t planning to take a soil test, it’s usually safe in our region to add Calcitic lime to your vegetable garden beds once per year. Just pick up a few bags of lime and spread it on top of the beds. Any product should have a recommended application rate listed in pounds or cups to apply per square foot. A typical application might be 1 pound per 25 square feet; one pound of lime is approximately 2 cups.
As a very rough benchmark, a standard lime application may be 3-4 cups of lime per 4’x8’ raised bed. For reference, a standard yogurt container is 4 cups, so applying a yogurt cup of lime to each of your raised beds would be a common yearly liming procedure.
Of course, the best practice is to check your soil pH at least once a year, and add amendments as needed. I think you’ll be amazed at what a little bit of lime can do.
