Measuring Soil pH

Simple mistakes when measuring soil pH will leave you with results that can ruin your garden. Don't get caught out.

Do not test cold soil. Cold inactivates bacteria, resulting in a false reading. Wait until soil temperature (not air temperature) has been above 60 degrees for at least two weeks, then test.

When measuring soil pH, don't neglect the subsoil, unless you are the lucky owner of a four-foot-deep black prairie loam. We forget that if surface soil is only six to ten inches deep, most roots of many crops will grow through that upper layer and get the majority of their nourishment from the subsoil. In checking soil where deep-rooted trees and shrubs are growing or will be planted, perhaps only subsoil need be considered.

Several years ago I saw a good example of why subsoils should be checked. A friend of mine north of Chicago had some magnificent oaks growing at the foot of a steep hill on his property. Heavy washing rains fell all spring, and suddenly my friend noticed that the oak leaves were beginning to turn yellow. A tree man sprayed them with an iron solution and they turned green for a while but soon reverted to yellow.

Tests of the surface soil around the oaks showed it was fairly high in pH, about 6.0, but low enough so that some iron would stay in solution. When measuring soil pH of the subsoil however, we found it tested 7.5. This we diagnosed as a temporary alkaline condition produced by lime washed out of the upper part of the hill by the heavy rains and carried down the hill, along a gravel layer just under the surface, to the roots of the oaks.

Holes bored around each tree and filled with ferrous ammonium sulfate soon brought about improvement in leaf color and tree growth. Drains to lead rain-wash from above into side channels, away from the oaks, prevented further trouble.

Get soil testing kits from my online store.