What is soil pH?

Basically, pH is a measure of the concentration of hydrogen atoms in a solution; in this case, the soil.  The pH scale ranges from 0 to 14, with 0 being the most acidic and 14 being the most basic, or alkaline; a pH of 7 is neutral.  Here in Utah, and in most of the Intermountain West, pH is slightly alkaline.  Areas that get significant rainfall (think the East Coast and the Pacific Northwest) tend to have more acidic soils.

Why should I care?

stawflower, nutrient deficiency

This strawflower is showing symptoms typical of an iron deficiency.

If you’re not too interested in chemistry (you should be! it’s great!), you might be wondering why pH matters to you.  Just like Hardiness Zones can help determine best plant selection for your area, pH is also a good indicator of what may thrive and what will not.  Hydrangeas are a neat example because in alkaline soils they tend to be pink, while in acidic soils the color changes to blue.  On the other hand, plants like rhododendrons and blueberries, which grow really well in New England, don’t grow as well here in Utah.

A lot of that has to do with how pH affects nutrient availability to the plant.  At certain pH levels, some nutrients are much more tightly bound to the soil particles and so aren’t able to be taken up by the plant, so even though there may be a good amount of a certain nutrient in the soil, deficiencies still show up.  A good example of that here in Utah is iron.  Though adequate iron is often present, the higher pH makes it so the plant can’t take it up.  Simply adding more iron would be futile; a chelate, or a binding agent, is needed.

soil pH chart

This chart shows the relationship of soil pH with the nutrient availability. The fatter the line, the more available it is to the plant.

Can I change the pH of my soil?

If you live in an area where you have acidic soils, you can adjust the pH of your soil to be slightly more alkaline by adding lime.  No, not that little green citrus fruit.  Rather, lime as in pulverized limestone.  On the other hand, if you have an alkaline soil, you can add elemental sulfur, but this is a slow process and really only affects the immediate root zone.  Adding more organic matter into your soil can help lower the pH and create a healthier soil over all.