Stevie Horvath, age 10, of Pittsburgh, Pennsylvania, for his question:
What is meant by soil pH?
This is a symbol for the chemical quality of soil. Chemicals behave in complicated ways and many of them have long, complicated names. It takes pages of ordinary words to explain the details. So chemists use a special shorthand system of letters, numbers and symbols. One of their most useful symbols is pH. When a number is added, it explains a very complex chemical situation in the space it takes to write one small word. This handy little symbol began work in the laboratory and later moved outdoors to work in the garden.
Plants need a balanced diet of chemicals dissolved in their ground water. The soil contains an assortment of suitable and unsuitable chemicals. For example, few plants can survive in soil that is either strongly acid or strongly alkaline. These two chemicals are in conflict. More acid means less alkaline. Reducing the acid increases the alkaline. If the acid alkaline content is equally balanced, the soil is neutral. When we test a sample of soil water, we use the pH symbol to rate its acid or alkaline concentration. If either is too strong, we can correct it to please the plants.
The opposing chemicals are particles of hydrogen and ions of hydroxyl that form when molecules of water break apart in the soil. More hydrogen creates an acid condition, more hydroxyl creates an alkaline condition. You can measure this with a soil testing kit, though you need an elaborate one to do an accurate job. Experts used an electronic meter to figure the hydrogen hydroxyl rating and the answer gives the precise pH factor of the soil sample.
The scale of pH ratings runs from 0 to 14, in whole numbers and decimals. The midway mark is pH7 which is neutral soil. The acid ratings run from 0 to 7 ¬and the strongest acid concentrations have the lowest numbers. The alkaline ratings run from above 7 to 14 and the strongest alkalines have the highest numbers. But watch out, those tricky numbers jump up and down in 10s. For example, pH4 is 10 times more acid than pH5 and 100 times more acid than pH6. While above the neutral mark, pH9 is 10 times more alkaline than pH8 and 100 times stronger than pH7.
Pure water rates a neutral pH7. But rain water usually is slightly acid, with about a pH6.5 rating. Most plants prefer this slightly acid tang to their soil water. However, corn does its best in soil with a pH6 rating, which is 50 times more acid. Other vegetables do well in soil ranging from pH5.5 to pH6.5. Many fruit trees do well when the pH factor of their orchard soil is in the 5 to 6 range.
Lab chemists refer to alkalines as bases and they can supply chemicals to change the pH factor of the soil. Most gardeners use the older word alkaline and many prefer nature’s older methods for correcting the pH factor of their soil. To increase its acid content, they add helpings of compost or bone meal, oak leaves or pine needles. To increase the alkaline, they add dolomite limestone. Increasing the acid reduces the alkaline; increasing the alkaline reduces the acid.