You’ll often see a reference to “acid” or “alkaline” soil in plant descriptions, planting directions, or plant care tips. What this refers to is pH, or a measure of the acidity or alkalinity of the soil. To put it in kitchen terms, vinegar is a weak acid and chlorine bleach is a weak alkaline, or base.
Some plants grow better in an acid soil, and some grow better in an alkaline one. A few plants, such as Hydrangea, will tolerate either; however, their flowers change color depending on whether they’re in acid or alkaline soil.
On the pH scale, a value of 7 is neutral, a pH of less than 7 is acidic, and a pH of more than 7 is alkaline. The soil pH affects how many plants grow because it can change the chemical form of soil minerals and the number and kind of soil microorganisms.
Determining Soil pH
The best way to determine soil pH is through soil samples. Your local Cooperative Extension Service can tell you where to submit soil samples for testing in your area. Or text your own soil with a Soil PH Meter.
Once you get the results of your soil test, you can decide which plants and grasses are best for the soil you have. Most landscape plants and some trees do well on a variety of soils; many lawn grasses do as well. Vegetables gardens are best planted where the pH is 5.8 to 6.3, especially on sandy soils.
If your soil is between 5.5 and 7.0, you probably can grow most common plants without much trouble. If you want to grow acid loving plants such as blueberries, gardenias or azaleas, though, you may need to adjust the pH to make your soil more acid.
Changing Soil pH
Although it’s possible to change soil pH a certain amount, you’re frankly better off to grow plants that do well on the soil you have. That said, if you really want to grow something in particular that needs a soil pH that’s different from what you have, there are some things you can do to adjust soil pH. Keep in mind, though, that this is a temporary remedy, and you’ll have to do it over and over again to keep the soil pH at the level you want.
To raise the pH of an acid soil, you need to add lime in the form of dolomite or calcium carbonate. Be sure to test for lime requirements before you add any lime; this test will tell you how much your soil will resist changes in pH, and give you an idea of the application rate to use and how often to apply lime.
If you’re just establishing a garden or lawn area, mix the lime thoroughly into the top six or eight inches of the soil. If you’re applying lime to an existing garden or landscape, apply it to the surface and water it in, but don’t water so much that you make the soil soggy and waterlogged.
If you need to apply a lot of lime to an already established garden or landscape, you may be better off to divide the amount you need to add into two or three applications. Apply them a week to ten days apart.
Although it’s relatively easy to raise pH by adding lime to the soil, lowering the pH of very alkaline soils can be very difficult or even impossible. If you’re on a soil formed from limestone or another material with a lot of calcium in it, you will not be able to permanently lower the soil’s pH. You really are better off growing plants that are suited to a high pH soil.
By adding elemental sulfur, you can temporarily lower a soil’s pH. Soil microorganisms convert the sulfur into sulfuric acid, which temporary reduces the alkalinity of the soil. This is a temporary change, and is limited to the area where you’ve put the sulfur. If you want to keep the soil pH down, you will need to re-apply sulfur frequently. This can damage your plants, so watch for indications that your plants aren’t happy.
Never apply more than 7 pounds of elemental sulfur to 1000 square feet of lawn or garden at any one time. This will help prevent damaging or burning plants.