Monitoring and adjusting soil pH is important for plant health. Many homeowners have stated that they understand the pH of the soil is important, but are not sure why or what to do about it. If you find yourself in that category, then this page was written for you. It contains basic pH soil science and materials used to adjust it.
Introduction to Soil pH
The entire world’s plants, including flowers, trees, and food crops, (yes, even turfgrass) have a preferred pH range they will grow best in. In the wetter states east of the Mississippi, the tendency will be toward the acidic side of the scale.
This is due in part to sulfur found in rain water. The more arid states west of the Mississippi are slightly alkaline. Exceptions in the West are the wetter, mountain areas, which often have a slightly lower pH.
Most plants will grow best in the mid-range of 6.0 – 7.0 on the pH scale, but there are many exceptions. When the soil pH is outside of the plant’s tolerance range, the plant’s health will decline and may even die in some situations.
The information below will help you understand the pH scale and the relationship it has with your plants.
The letters pH stand for "potential Hydrogen" and represents the measure of Hydrogen ions in the soil. The soil pH is measured on a scale of 0 to 14. A reading of 7.0 in the middle represents "neutral".
A reading of below 7 is considered to be acidic (sometimes called "sour" soil) while any measurement above 7 is basic or alkaline (sometimes called "sweet" soil). Most all plants will only grow within a range of a couple degrees either side of neutral.
Here’s why? Each whole number on the pH scale represents a factor of 10. That means that a soil pH of 6 is ten times more acidic than a soil pH of 7. A soil pH of 5 is 100 times more acidic than it would be at 7.
As will be explained later, the pH affects the soil’s chemistry. When the soil pH drops below 5.5, certain vital nutrients are blocked from being taken up into the roots. It is the same with high alkaline soils.
Active pH and Buffer pH
If you have submitted a soil sample through the county extension office or directly to a testing lab, you will receive a soil analysis report. The report will indicate the "active pH" as well as the "buffer pH".
Although I have found that some labs may not actually have the words "buffer pH" in the report, what the buffer pH reveals will usually be in the report. Click here for help understanding a Soil Analysis Report.
(The Photo at the right shows one of many home and professional pH meters. Price varies greatly so shop around online for the best price.)
Active pH is the measure of free hydrogen ions that are presently in the root zone when the test is performed. These hydrogen ions bind with other nutrients, making several important nutrients unavailable to plants. Hand held pH meters, including the one above, test for hydrogen ions.
Lime (calcium Carbonate) in the soil neutralizes hydrogen ions by converting the excess hydrogen to water, thus freeing up nutrients.
Where there is an inadequate amount of lime, the test will reveal higher amounts of hydrogen ions. In other words, when less hydrogen ions are found, the more alkaline the soil will be. The more hydrogen ions that are found in the soil, the more acidic the soil will be. The active pH is what is measured when using any of the many types of hand-held home pH soil test kits.
The buffer pH measures the soil's resistance to change and can only be measured in a lab. Based on your soil sample, buffer pH indicates the amount of corrective material that is needed to bring the soil back to the correct pH.
For example, a low pH soil containing generous amounts of organic matter or clay would require more lime to bring it back to neutral than would be needed for a lighter soil. Since all soils are different, it takes differing amounts of corrective material to make the adjustment.
Some labs will not have the words "buffer pH" in the report, but will simply state the amount of lime need to bring the soil back to the proper pH. This amount, however, is determined by what is revealed in the buffer pH test.
Important: It is a common belief that you can indiscriminately apply limestone to your lawn because it "always improves the soil".
This is wrong. Limestone only improves the soil if an improvement is needed. The purpose of limestone is to alter the soil pH and it is possible to apply too much or to over-correct. Apply every year without knowing whether it needs it or not is the best way to ruin your fields or lawn.
You can't remove lime that has been applied but will need to apply sulfur which takes years to work. Never apply lime unless you need it.
Incorrect soil pH can cause several problems. As the pH falls or rises too far from neutral it alters soil chemistry and changes nutrients into forms the plant’s roots cannot take up. Acidic soil mostly affects macronutrients and alkaline soil mostly affects micronutrients.
Tests that reveal pH above 8 where excessive lime was not applied, generally have other problems that push the pH upward. Excessive sodium (salt) or other materials can have that effect on soil pH with the result of binding nutrients and adversely altering soil structure.
In alkaline soils, phosphorus, iron, boron, and zinc become increasingly bound up and unavailable to the plants. Phosphorus begins to bind with calcium making it less available.
All of these nutrients are needed for good plant function. Phosphorus is one of the top three plant nutrients needed for plant development. Organic matter is important for in keeping phosphorus available in higher pH soils.
Iron is the most common micro-nutrient deficiency in alkaline soils and is known as "iron chlorosis".
Low pH soils reduce phosphorus and magnesium to plants and can cause aluminum and manganese toxicity in severe cases. Magnesium is essential for chlorophyll production for producing plant carbohydrates.
The center of the chlorophyll molecule is Magnesium. Chlorophyll gives grass its green color and is a major part of photosynthesis. When lacking magnesium, the grass will be pale green in color.
To make acidic soils more alkaline, we use lime. Lime works by neutralizing hydrogen in the soil.
Below is a description of the different lime products often used on lawns. These products don't work immediately, but must work themselves down into the soil. The response time will be different depending on your soil, how much is needed, and what material you use. Some can be applied using a home fertilizer spreader.
Note 1: Limestone is slow to break down and is best if it can roto-tilled into the soil, when possible. When broadcasted over the grass surface, the smallest limestone particles will work into the soil faster than larger particles. While powdered limestone is much messier to work with, it will alter the pH faster.
Again, one of the best products I have used is a specially manufactured pelletized limestone for easy and clean spreading, but quickly melts down to fine particles when watered in.
Note 2: The purity of the lime product varies with the manufacturer. Products are rated for purity using a Calcium Carbonate Equivalent (CCE) rating. A rating of CCE 100 is pure, but cheaper products may be as low a CCE 60. If you use a cheaper product you will have to use more to get the same results of a CCE 100 product.
Materials for lowering soil pH
When the soil is too alkaline, it may be necessary to lower the pH. There are basically two different materials that are used: Sulfur and Aluminum Sulfate.
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