Summary: The “discovery” of pH has been one of the most important developments for plant growers. It gives a convenient, practical method of measuring acidity, and helped advance our scientific knowledge of nutritional requirements of plants and helped in understanding of the importance of soil acidity to all kinds of crops.
The “invention” of pH ranks among the important developments of modern “plant growing” chemistry. It provides a convenient and practical method of measuring acidity and, since all living organisms are highly sensitive to acidity, it has helped to advance our scientific knowledge of the nutritional and other environmental requirements of both plants and animals. It also has resulted in the reasonably widespread understanding of the importance of soil acidity to all kinds of crops. Countless farmers, nurserymen and gardeners have in fact become familiar with its practical application and today are regular users of testing kits designed to indicate the pH of their soil.
Most of our readers have seen numerous references to pH, and a good many of them have a pretty good understanding of what it’s all about. For the benefit of those who haven’t, or who may have become a little “rusty” on the subject, the practical importance of pH in relation to plant nutrition will be ably discussed in a future article.
The use of the designation pH (the small p and the capital H) in horticultural and botanical literature, however, has always seemed to me a little unfortunate, since it’s never possible to explain precisely what the designation means. To do so would take far more space than is usually available and would involve chemical and mathematical terminology that no one but an advanced chemist or physicist could be expected to understand. And so – for reasons probably good enough – it’s not even customary to state what the letters p and H stand for, but simply to say that in combination with any number between 0.0 and 14.0 they refer to relative acidity or alkalinity. Some people. however, would like to know why p and H are used instead of, say, a and b or Y and z.
For the benefit of those people: The p stands for “Potenz,” the German word for “power,” and the H stands for hydrogen.
But “power” is not used in the sense of “strength” or “force,” but in the mathematical sense, i.e. of exponent or logarithm, as when we say a number is raised to a certain power (e.g. 33 = 3 X 3 X 3 = 27), and “hydrogen” does not refer simply to the gas, but to “hydrogen-ion concentration” (ions being electrified particles, familiar to physicists and chemists but unknowable to most other people). Also, the “power” or logarithm of pH is “negative,” i.e. is preceded by a minus sign. So the pH of a solution is the logarithm of the reciprocal of the hydrogen-ion concentration:
pH = log 1
______
(H+)
If “normal” (another chemical technicality) hydrochloric acid is taken as the acid end of the acidity-alkalinity range and “normal” sodium hydroxide as the alkaline end, then the hydrogen-ion concentration. in grams per litre, ranges from 1.0 to 0.00000000000001, or from 1.0 to 10-14. The exponents of “1.0 to 10-14 it will be noted, are 0 to 14. This “0 to 14” is the pH scale, each successive number from 14 down to 0 representing a tenfold increase of hydrogen-ion concentration, and 0 representing “most acid,” 14 representing “most alkaline” and 7 representing “neutral.”
The foregoing, obviously, does not “explain precisely” what pH means. Nor is it intended to. It’s intended simply to tell what the letters p and H stand for and why they, and not some other letters, are used in combination with numbers to indicate relative acidity.
Learn what all this means to you, as a gardener.
