Soil pH

pH is a familiar term. It measures the acidity or alkalinity of a solution. The pH scale runs from 0 to 14, with 7 as the neutral point. The pH decreases from 7 as a solution becomes increasingly acidic. It increases from 7 as the solution becomes increasingly alkaline.

Battery acid is very acidic at a pH of 0.0. Common household vinegar is quite acidic at 2.0. Coffee is around 5.0. Pure water is neutral at 7.0. Household ammonia is alkaline at 10.5. Drain cleaner is extremely alkaline at 14.0.

Soil’s pH is one of the invisible factors that determines plant health. The symptoms of unhealthy pH are serious, but their cause isn’t always obvious.

Soil acts as a reservoir for water and minerals. Water forms a film on the inside surfaces of soil pores. Mineral ions, dissolved in that water, stick to charged sites on soil particles. There, they can be taken up by roots. But the solubility of these ions depends on the soil water’s pH. At acidic pH’s, their solubility decreases, and metal ions like iron and aluminum leach from minerals and take up space that could otherwise be occupied by nutrients like magnesium, calcium, potassium, nitrogen, and phosphorus. Phosphorus is particularly troublesome. Its solubility is low even under ideal conditions, but it’s also a critical nutrient. At over-acidic pH’s, it forms insoluble complexes with iron, which are no use to plants.

For most plants, the ideal pH is slightly acidic. At a slightly acidic pH, most nutrients reach their maximum solubility. This includes key micronutrients like boron, iron, and molybdenum. However, when the pH drops too low, these nutrients (as well as aluminum, which isn’t a plant nutrient) become too soluble, and can rise to toxic levels.

The issues that arise from acidified soil aren’t limited to plants themselves. Soil microbes, which are essential to liberating nutrients from soil and pulling nitrogen from the air—are much less efficient at extreme pH’s, or they can die off altogether.

(It should also be noted that too high a pH (a soil that’s too alkaline) is harmful, too.)

This all leaves two obvious questions: What makes a soil acidic? And what can be done about it?

Soils can acidify naturally. When water passes through them, it carries away some of the mineral ions. And plants consume mineral ions as they grow. Mineral ions tend to be alkaline, so their loss drives the pH down. On top of that, root and microbe metabolism produces carbon dioxide, which is also acidifying.

And, unfortunately, nitrogen fertilizers, useful though they are, tend to acidify soil over the long term, as they decompose into usable nitrate and nitrite. In urban and polluted areas, there’s also the problem of acid rain and acidic runoff.

Some soil problems are difficult to treat. It’s hard, for instance, to remove lead from contaminated soil. It’s labor-intensive to remedy soil compaction. But acidic soil is relatively easy to treat. It’s usually done by liming: applying alkaline minerals like limestone, chalk, dolomite, or slaked lime, to push the pH back up. This has the side-benefit of supplying calcium and magnesium, which are essential plant nutrients. Care must be taken not to over-lime, of course. That requires testing the soil’s pH, which can be done with inexpensive soil pH testers, with test strips, or with test kits.

It’s also important to adjust planting and management practices, where needed. Care must be taken not to over-water, and to limit the application of acidifying nitrogen fertilizers where possible. And, in ornamental plantings, it helps to choose plants that don’t mind acidic soils. Some plants even prefer their soils acidic. These include ferns, azaleas, some pines, oleanders, and blueberries. (For more detailed and specific information, the US Department of Agriculture has a fairly comprehensive database available here.)

pH problems are easily missed, since, without testing, the symptoms are vague and easy to misinterpret. But when the problem is found, it’s also a relatively easy fix.