Soil pH is the measure of the relative amount of soil acidity or alkalinity. A soil’s acidity or alkalinity can influence the availability of elements or mineral nutrients in the soil that support plant life and growth.
Plant roots absorb the molecular forms of soil nutrients such as nitrogen, potassium, phosphorus, magnesium, and boron. These nutrients and others which support plant life are most easily absorbed by plant roots when the soil pH is near neutral (7.0).
Most plants thrive in a pH range anywhere between 6.0 and 7.5 (the acidity-alkalinity scale ranges from 0 to 14; the low end of the scale indicates acid and the high end is alkaline). Nutrient shortages tend to occur when the soil is extremely acid or alkaline.
Do-it-yourself soil pH test kits are commonly available at garden centers or nurseries. You can also have the pH of your soil tested by your state Cooperative Extension service.
Values of pH 7 indicate a neutral soil; a value above pH 7 is alkaline and below pH 7 is acid. Most soils are within a range of highly acidic pH 4 to alkaline a pH 7.5 to 8. Soils that are more alkaline are known as “sweet” and those more acid are called “sour.”
Strongly acid soil reduces the availability of phosphorus, calcium, potassium, magnesium, and molybdenum.
Strongly alkaline soil reduces the accessibility of boron, copper, zinc, manganese, and iron.
Relatively few plants can live in acid soil testing 4 to 5, and few plants can thrive in an alkaline soil over 7.8.
Adjusting Soil pH
You can adjust soil pH to grow plants not suited to the pH of your soil. Or, you can leave the pH as it is and select plants that grow best in the pH that already exists.
You can raise the soil pH or make an acid soil more alkaline by adding lime; dolomitic limestone is preferred because it contains magnesium and calcium, thus fertilizing the soil while neutralizing it.
You can lower soil pH by adding naturally acid organic materials such as peat or oak leaves or by adding sulfur or sulphate. Note: it is difficult to make an alkaline, lime-rich soil into an acid soil.
Adding organic matter to the soil such as well-rotted compost will moderate both excess alkalinity and excess acidity.
Altering soil pH is not an exact science and takes time; you should not expect an immediate change. After adding lime or sulfur to your garden re-test the soil in 40 to 60 days. Expect small changes to the pH—0.5 to 1 unit at most. Work steadily towards achieving the pH that will be ideal for the plants you want to grow.
A good time to begin adjusting soil pH is in the Fall; check the progress again in the Spring. Check the soil pH regularly—twice a year is ideal.