The technical definition of pH is the negative logarithm of the hydrogen ion concentration. In English, soil pH is a measure of how acid or alkaline the soil is. The pH scale goes from 0 to 14. Zero is the most acid, with the fewest hydrogen ions. Fourteen is the most alkaline, with the most hydrogen ions. Seven is the neutral pH. Each number on the scale represents ten times as much acid or alkaline as the number below or above it. For example, soil that is a four is ten times as acidic as soil that is a five. Most plants grow best at 6.0-6.5.
Finding Your Soil pH
There are three ways to find your soil pH. The best way is to send a soil sample from the area you are going to plant to the soil lab for your state. Soil test sample bags, instructions, and the sheet to send in with your soil sample are available from your county Extension agent. In Texas, the soil lab is at Texas A&M University. The testing costs around $12 currently and you get your results in a week or two.
Soil pH meters are not as accurate as soil tests, but are usually accurate enough. The meter has a long probe you insert in the soil. The least accurate way of testing pH is with home test kits that use dyes and reagents.
Why pH Matters
The pH of the soil affects what nutrients are bioavailable. For example, acidic soil makes it hard for plants to use phosphorus, magnesium, and calcium. It may make too much aluminum and manganese available, which can poison the plant. If the soil pH is above 6.5, phosphorus and many micronutrients become unavailable.
Changing the pH
It is possible to gradually change the pH of the soil. Agricultural lime can be added to make soil more alkaline. Sulfur can be added to make the soil more acidic. However, there is a practical limit on how much the soil pH can be changed because of the amount of lime or sulfur it takes and the length of time involved.
Grow Plants Suited to Your pH
Those arbor day groups that promise to send you ten saplings suited to your USDA hardiness zone do just that. However, the saplings have different pH requirements, so they won’t all grow in any one place. For example, dogwood needs aa soil pH of 6.0-7.0, while redbuds grow best when the soil pH is 6.6-7.8.
Raised Beds Help
While raised beds do not help much with trees and shrubs, they can help quite a bit for perrenials and annuals. These include most ornamentals, vegetables, herbs, and ground covers. Raised beds are commonly twelve inches tall and are filled with a mixture of sandy loam soil and compost. They can be built higher so that it is easier to reach in the bed from a chair. Raised beds also improve drainage. It is best to build a raised bed no wider than four feet so you can reach the middle without entering the bed. Standing on the soil compacts it and makes it harder for roots to grow.
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