Soil should be moist for several days before you test it. (Drought affects the pH by killing off large numbers of bacteria, releasing organic acids which result in a false reading.) If the sample of soil used is allowed to dry for an hour or so in a shaded spot, it will give a clearer reading when the liquid is run through.
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.
In making a test, 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, however, we checked the subsoil, we found it tested 7.S. 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.
HOW pH AFFECTS NUTRIENTS
Now that you have a pH reading, the next problem is what it tells you. We want to know how pH affects food elements in soil.
Food elements needed by plants may not be available to them even though present in soil. Phosphorus is an excellent example. Vital to all life, it enters into every phase of plant growth, from beginning to end. It is a major ingredient in cell nuclei, and carries over in chromosomes to the succeeding generation.