What is Soil pH?: On/Off Switch for Plant Nutrients

The letters “pH” actually refer to a logarithmic formula which indicates whether a soil is acid or alkaline. The formula is based on a scale of 0 to 14, with 0 being the most acidic and 14.0 being the most alkaline.

Most plants take in the maximum amount of nutrients from the soil when it has a pH level of 6.0 to 7.0 – slightly acidic to neutral soil. The majority of food crops prefer this type of soil.

However, different plant species have varying abilities to absorb nutrients. And different plant species can do well in soils with higher alkaline qualities, while others do well when the pH tips in the opposite direction.


Plants That Love Acidic and Alkaline Soils

Plants which love highly acidic soils (5.0 – 5.5 pH) include azaleas and rhododendrons, blueberries, potatoes, hydrangeas, magnolias, conifers and dozens more. These plants actually thrive on highly acidic soil. Some plants do not necessarily thrive on highly acidic soils, but they have adapted to survive in relatively low pH soils.

On the other end of the spectrum, salvias, dianthus, Russian sage, artemesia, cyclamen, maples and many more can thrive in alkaline soils. Some plants have also adapted to survive in soils with higher pH values.

Soil Bacteria and pH

Soil bacteria love a pH level of 6.3 – 6.8 and are responsible for much of the breakdown of nutrients in the soil, making the nutrients soluble and available to plant roots to absorb. Levels outside of that range are not good for soil bacteria.

Many plant diseases are caused by pH extremes. Additionally, a too-high or too-low pH can cause mineral deficiencies or even poisoning of plants.

Changing Soil pH

It’s very difficult to change the pH of soil. Adding agricultural lime (which is ground limestone) or quicklime can help raise the pH level (make the soil more alkaline). Wood ashes have also been used for small areas. In fact, the word “alkaline” comes from the Arabic “al qaliy,” which means “ashes.”

Adding compost is by far the best way to lower the pH levels of the soil. Almost as good are stable manure and decayed vegetable matter.

Get Soil Tested

Many land-grant universities and colleges in the United States have soil-testing capabilities. For a relatively small fee, the laboratory will test the soil pH, and also run tests on the amounts of various nutrients present in soil. There are also many private testing facilities which will conduct soil tests. Additionally, there are soil test kits available from nurseries and garden supply outlets.

Does Water Affect pH?

The water in any given area usually has the same pH level as the soil. Using ground or surface water to “dilute” the pH is pointless. Rainwater, however, has a neutral pH level, but the amount required to raise or lower pH levels is generally prohibitive.

Fertilizers, Pollutants and Soil pH

Commercial fertilizers can affect the pH of a soil and great care should be taken when choosing a fertilizer. Usually a good soil test will indicate what types of fertilizer should be used.

Pollutants can and do change the pH levels of soil. Chemicals should be disposed of properly (preferably at a recycling center).

“Anthropogenic” is a fancy way of saying “caused by humans.” It refers to things like vehicle traffic and foot traffic compacting soil. Soil compaction can and does change the pH levels of the soil.