What’s Soil pH and Why Do I Care?

Soil pH is a measure of acidity. It is important because it affects the availability of nutrients for your plants. Soil pH is supposed to be in a certain range, like a speed limit on a road. Go too far over or under, and the plants will let you know. The cops will give you a ticket, and the plants will give you a warning by growing poorly. Proper soil pH is important to your livestock too. If your soil is too acidic, trace minerals become less available for plant intake, so your animals don’t get all the benefits from grazing them that they might have.

What are we measuring when we look at pH?

What we’re looking at is a scale of the concentration of hydrogen ions present. That’s the H. The p of pH lets us know the scale is logarithmic, or that instead of counting 1, 2, 3, it would be like counting 1, 10, 100. The center of the scale, 7,  is neutral and is based on the number of ions present in pure water.  Each step to the left of the 7 shows an increase of 10 times in the number of hydrogen ions present.  (“But wait!” you say. “Why does the number get smaller if the number of hydrogen ions is increasing?”  We could tell you, but then you’d want to kill us. So let’s just go with it.)

Some folks describe soil pH by taste. Acidic soil is sour or bitter, and soil becomes “sweeter” as you move closer to neutral. But this isn’t a very precise, or particularly palatable way to test for soil pH. Usually we send it to a soil test lab instead of eating it, and they let us know our pH. Based on the results the lab can tell what we should add to our soil get it to the goal pH, usually 6-6.5 or 7.

Why are we looking for a pH score between 6 and 7?

That’s the soil sweet spot. It’s where most nutrients are available, and where plants and soil organisms do best. You can see it best by taking a look at the chart on the right. Notice how nitrogen begins to drop off when pH hits 5.5, and the impacts on phosphorus when pH is 6.

Here’s how it happens. Imagine a particle of soil as a kitchen table with only a certain number of chairs around it. Each chair is a seat for an element of your soil. If the chairs are all taken up by acidic hydrogen ions, there’s no room for anyone else to sit down. If you want to kick some of those acidic fellows out of their chairs, you’ll add lime to your pastures.

If you have sandy soil, your soil particle kitchen table has fewer chairs, so it takes less lime to improve the pH. If you have clay-based soils, your soil particle kitchen table has LOTS of chairs, so the soil testing lab will tell you to add quite a bit more lime.

Where does soil acidity come from, anyway?

Soil acidity comes from numerous sources, and some are simply unavoidable.

1) Hydrogen ions can be a result of the breakdown of carbon dioxide gas in soil water. Roots and soil organisms release carbon dioxide. The carbon dioxide and soil water react, releasing H⁺ ions, and lowering the pH.

2) Another situation is a result of our good friend organic matter. You still want to invite organic matter to the party, but you should know that sometimes organic matter will pick up some of those nonacidic cations and swoop them off through the soil profile. Oops. Also, soil organic matter tends to have lots of bits and pieces, and those bits and pieces break off easily, releasing hydrogen ions.

3) A third way that soil gets more acidic is when things oxidize. When ammonium (NH₄⁺) gets turned to nitrate NO₃^–, all those H⁺s go into the soil. Half end up as water and half as ions- which adds 2 more hydrogen ions to the concentration for every ammonium ion that gets oxidized.

4) When plants take up ions through their roots, and we want them to do that so they grow, they release a cation or take up an additional negatively charged ion as well to maintain the same charge (ratio of pluses to minuses). Lots of times, they release hydrogen ions, lowering the pH.

5) Finally, there’s acid rain. Not just rain, but dust, fog, snow, all have acids in them that add hydrogen ions to the soil. Rain falling through the sky may start out pure, and just the simple fact that there is carbon dioxide, not pollution mind you, can drop the pH from 7 to 5.6. This is just like what happens in the first source for acidity, above.

All these ways to lower soil pH, and soil acidity is one of the biggest obstacles to production. What to do about pH depends on the soil you have. The more you know, the more you understand your soil and soil test results, the better decisions you can make. The better your grass will grow.