Purdue Extension Educators frequently encourage homeowners to get soil tests when they want to grow new plants in an area, or try to diagnose what’s wrong with plants currently growing. One key component of a laboratory soil test is soil pH. Do you know what soil pH means and why it is important?
Purdue experts authored an Extension publication entitled “Soil pH,” from which I’ll be quoting occasionally.
“Soil pH is a measure of soil acidity or alkalinity,” they said. That’s important because plants have their own preferred pH at which they thrive. If the soil is too acidic or alkaline from the preferred pH, plants will begin to suffer, and they may also exhibit nutrient deficiencies or toxicities.
To better understand pH, we need to understand the pH scale on which it is based. “The pH scale extends from 0 (a very strong acid) to 14 (a very strong alkaline or base), but most soils will have a pH range of 4 to 8.5,” they said. “Pure water is neither an acid nor a base and is considered neutral, right in the middle of the scale at 7.”
A little more difficult to understand is that the pH scale is logarithmic, meaning that a pH of 6 is 10 times more acidic than pH 7, and 100 times more acidic than pH 8.
“Soil pH directly affects nutrient availability and can influence plant growth,” they said. For most landscape and garden plants, a slightly acidic soil is preferred, from about 6.0 to 7.0. However, some plants prefer more acidic soil, such as potatoes, blueberries, and azaleas.
To measure pH, you can use a pH meter or submit a soil sample to a laboratory for testing. pH meters range from cheaper models to very expensive, and you generally get what you pay for. Follow the manufacturer’s instructions on proper use of the meter. Meters may alert you to a potential issue, prompting further justification for laboratory tests. For the accurate information they provide, lab tests are relatively inexpensive.
Another thing that soil tests provide is to give us more of a prescription for nutrients we may need to supply via fertilizers. Some home gardeners have fallen into the trap of throwing another sprinkling of “triple 12” on the garden without regard to what nutrients may already be in the soil in sufficient or high quantities.
Plants require 17 essential nutrients to grow. “Soil pH affects whether a given nutrient is more or less available to the plant,” they said. “Both too little and too much of a nutrient can cause problems.” Plants absorb most nutrients from the soil through their roots. Soil pH affects the availability of nutrients in the soil. If the soil is too acidic or alkaline, certain nutrients are held tightly to clay and organic matter, and they aren’t available to plants for nutrient uptake. For example, alkaline soils hold more tightly to essential nutrients like iron and manganese, and certain plant leaves may begin to turn yellow, indicating a nutrient deficiency. In higher pH soils, pin oak and river birch trees are notorious for exhibiting iron deficiency symptoms, and red maples can show manganese deficiency symptoms. There may not be a true deficiency of these nutrients in the soil, but a high pH makes these nutrients less available to the plants.
If you see that your soil is too acidic or alkaline, you may need to modify the soil pH to grow the plants you wish to grow. Adding lime, an alkaline material, will raise soil pH. Adding a product containing sulfur will lower soil pH. However, do not expect an immediate change. Altering pH is a slow process in soils that may take many months. One mistake some homeowners make is to add lime when the soil pH is already high (approaching 8 or more). This can hinder you more than help you.
For more information, access “Soil pH,” “Soil Acidity and Liming of Indiana Soils,” and “Lowering Soil pH for Horticultural Crops.” The free publications are available at https://edustore.purdue.edu/.