If you’re a gardener you have probably ran into the term soil pH. It may be used in reference books or on plant tags. But just what is soil pH and how important is it to your Michigan garden?
The term pH stands for the potential or percentage of Hydrogen ions in a solution. (The correct way to write this term is lower case p, upper case H.) In soil, the hydrogen ions are in the water that fills the pores between soil particles. The more Hydrogen ions in the soil the more alkaline it is.
Soil is composed of minerals from dissolved and crushed rocks, and of organic matter, air and water. The types of rocks that formed the soil in an area added different minerals to the soil. Hydrogen gets into soil from the rocks, from the breakdown of organic matter and from chemical reactions in plant root systems.
A pH scale is a way to rate how acidic or alkaline soil is. It ranges from 0-14. Seven is considered neutral. Above 7 is alkaline and below 7 is acidic. The pH increases or decreases by ten times for each point on the scale. A pH of 6 is ten times more acidic than 7 and a pH of 5 is ten times that or one hundred times more acidic than a pH of seven.
What pH means to gardeners
How acidic or alkaline the water in soil is determines what kinds of mineral elements get dissolved and become available to plants or get bound up in complex reactions and become unavailable. Most plants grow best at pH levels of 6.5 to 7.5. That is where most beneficial minerals become available to them. A few plants have adapted to survive in pH levels slightly higher or lower than that.
In addition the pH level in soil also affects the microbes and micro-organisms that break down organic matter that adds nutrients to soil. They like to grow at about the same pH level that plants do.
The soil your plants are in can have all the essential elements the plants need but if they are unavailable because of too high or too low pH the plants can’t use them and will suffer. Certain plants are more sensitive to the loss of certain minerals such as iron. When the pH of soil goes above 6.5, iron becomes largely unavailable to plants. Some plants like Pin oaks will quickly decline. In other cases the pH level may cause toxic elements, such as aluminum, to become available to plants in amounts that can harm them.
Your soil pH may determine whether or not some species of plants will survive in your garden. But just because plants survive doesn’t always mean all is well. Marginal deficiencies caused by pH problems may cause your plants to become more susceptible to disease and insect attack because their immune systems aren’t as effective.
Pesticides may not work effectively if your soil pH is too high or low either. They are developed for use in the pH range that most plants prefer. The chemicals in them may react with elements available in too high or low pH conditions.
Finding your soil pH
You can get your soil tested at almost any County Extension office in the United States. Many garden and farm service stores also offer the service. They will tell you how they want you to collect and submit the specimen. These places will generally give you recommendations with the results for fertilizing or changing the soil pH.
There are small kits that have you mix water and soil and test the pH but these are not very reliable. If you are a person who likes to do it yourself you can purchase a small meter that has probes that go into moist soil and reads the pH. The more expensive ones used by professional growers are pretty accurate.
Changing soil pH
Soil pH can be raised or made more alkaline much more easily than lowered or made acidic. If your results are only slightly out of optimum range and your plants seem to be growing well don’t worry too much. Adding lots of organic matter and working it into the soil will help balance soil pH.
Lime can be added to soil that is too acidic to raise the pH. This works fairly quickly. Don’t add lime until you know your soil needs it and then follow the recommendations for your situation. Soil may be acidic if it is too wet from poor drainage or lacking air from soil compaction. Changing these conditions may help.
Lowering soil pH is harder and takes longer than raising it. Too much lime or too many wood ashes can raise soil pH and make soil too alkaline. Soil can be naturally alkaline from the type of rocks which helped form the soil, such as limestone. Adding organic matter helps. Fertilizers such as ammonium nitrate or ammonium sulfate will tend to lower pH. These are often marketed as fertilizers for acid loving plants.
If the drainage is good lots of rainwater, which is generally a neutral pH, may leach out some of the alkaline elements over time if they were the result of human error, such as adding agricultural lime to soil that didn’t need it.
For small areas aluminum sulfate or sulfur can be tried as a means to lower pH and make soil more acidic. You will need to consult with your county Extension office or other garden expert to find out how much to use on your soil. This method will be quite expensive.
Choosing plants that like the soil pH you have in your yard is another good way to minimize plant stress, providing the soil pH is not too high or low. There are plants that like either more acidic or more alkaline conditions.
Knowing what soil pH is and how it affects your garden plants makes you a better gardener. Knowing your soils pH starts with a soil test.
For more garden articles by Kimberley Willis click on her name at the top of the page.