Chemistry for Preppers: Soil pH

Spring is finally here (at least according to the calendar), the time of year when some of us start to pull out the plans we made over the winter and start working on our gardens. Seeds should have been ordered months ago -- although there are still plenty of high-priced packets in the stores -- so it's now time to start playing in the dirt.

Since I don't live where you do, I can't tell you what or how you should grow in your garden, but there are some basics that apply. Soil pH is one of those basics and is chemistry-related, so I'm going to blend a gardening/farming post into a chemistry post. I'll even try to keep the math to a minimum.

What Is pH?

It is a number that represents whether something is acidic, neutral, or basic. Technically, that number is the negative logarithm (base 10) of the concentration of H+ ions present in a solution measured in moles per liter.

To break that down, if you measured how many moles (explained a few weeks ago) of H+ ions were present in a liter of a solution, you'd get a number. That number is commonly expressed in “scientific form” as a small number times ten raised to some power, like 1.8 x 10^6 which is a short way to write 1,800,000. Logarithms were used before the invention of calculators and computers to make working with large numbers easier and quicker. Multiplying two numbers is the same as adding their logarithms; dividing two numbers is the same as subtracting their logarithms.

You can find log tables in old math books, but the concept is similar to the “scientific form” of expression: every number can be expressed as a power of ten (1 = 10^0, 10 = 10^1, 100 = 10^2, 1000 = 10^3 and so on). For numbers between 1 and 10, the exponent (the number that ten is being raised to) is going to be a fraction of one. Lets go back to that example of 1,800,000 and find the logarithm for it.

1,800,000 = 1.8 x 10^6

Since there is a multiplication sign in there, we can find the logarithms for the two parts and add them together. The log of 10^6 is simply 6, and by looking on a table I see that the log of 1.8 is 0.255. Add the two together and we get 6.255 as the logarithm.

When dealing with pH, the number of moles of H+ ions per liter of solution is going to be a lot smaller -- generally between 1 and 10 mol/l. The pH scale is the inverse log of that concentration, so we need to flip the fraction upside down. Say we have pure water, which will have some free H+ and OH- ions floating around in it. They balance each other out at about 1.38mol/l H+. The log of 1.38 = 0.14, which is about 1/7, so the inverse log is 7/1, which is how we get the “neutral” pH of 7.0 for pure water.

Acids will have a lot more H+ ions present, so the inverse log will be smaller; alkalies will have fewer H+ ions and have a larger inverse log.

How pH Affects Plants

As plants grow, they take nutrients out of the soil. This can change the pH of the soil enough to hinder the growth of future plants. Fertilizer, especially manures that are too “green”*, can swing the soil pH enough to slow or stop plant growth. The dreaded “acid rain” that we were all warned about in the 70s and 80s can affect topsoil over time, as can ashes from forest fires carried on the wind. Trying to plant a garden in a new spot, or on soil that has been moved, can be a challenge if you don't make sure the soil is ready for planting. All of these, and more, are reasons to check the pH of your soil.

Plants generally prefer a slightly acidic soil. According to the Old Farmer's Almanac and various other sources, a pH of 6.0 to 7.0 works best for most crops. If you need to fine-tune soil for a specific crop, the optimum pH range for several common plants are found on this site, but I'll list a few of the common ones.

• Corn: pH  6.0-7.0
• Beans: pH   6.0-7.0
• Potatoes: pH   5.8-6.5
• Tomatoes: pH   5.5-7.0
• Onions: pH   6.2-6.8
• Garlic: pH   5.0-6.0
• Carrots: pH   5.0-6.0
• Cucumbers: pH   5.0-6.0
• Cabbage: pH   5.6-6.6

How Do I Check Soil pH?

There are several types of electronic soil testers on the market, and they all work about the same. If you have access to one, they make testing a lot quicker. If you don't have one, then we need to find another way to check pH.

• Test strips of dyed paper have an indefinite shelf-life and are cheap.
• If you have red cabbage on hand, you can use the juice to make a versatile pH indicator due to the presence of natural dyes in the leaves.
• Red radish skins and elderberries also make natural pH indicators, but without the range of red cabbage.

How Do I Adjust Soil pH?

Common lime (crushed gravel or limestone) is used to raise the pH of soil.

Powdered sulfur was once the preferred method of lowering the pH of soil, but with various government entities trying to keep all of the dangerous things away from us, sulfur is getting harder to find. (I work with agricultural chemicals, and our insurance companies are really getting anxious for us to stop selling it, due to the fire hazard and liability if someone uses it to make a bomb). Aluminum sulfate seems to be the most common, current choice for lowering the pH; organic matter will break down and cause a lowering of pH as the acid-forming bacteria digest it, but it takes a long time.

Nothing is going to change the pH of your soil overnight. Adding lime or sulfur should be done at least two or three weeks before planting, since it is going to take some time for the microbes in the soil and the chemical reactions within the components of the soil to make use of it, so till it in and wait. Aluminum sulfate does work quicker, taking days instead of weeks, but the effects may not last (the pH may trend upwards) as the growing season progresses.


I hope I didn't make too many eyes glaze over with the explanation of pH and logarithms. Logarithms and how to use them may come in handy if you find your self in a situation where you have reference books, but the batteries on your phone/calculator/computer have died. We sent men to the moon using slide-rules, which are physical logarithm tables for doing multiplication, division, and some geometry functions. Old tech is still good tech to know.

* Manure has to be aged for at least a year to break down properly and provide nutrients. More on that in a separate post.