Fall is the time for all good gardeners to consider the acidity of their soil. Soil-testing laboratories are not as busy now, so the results will come quicker than in the spring. Also, if your garden needs lime to raise the pH, or ''sweeten the soil,'' the lime should be added to the soil this fall in order to benefit the crops next summer.
The term pH is a shorthand notation for the number of hydrogen ions, or charged atoms, in a solution. Chemists have devised this notation to indicate that a solution with a low pH contains many hydrogen ions, which make it acidic, while a solution with a high pH has few hydrogen ions and is alkaline.
Vinegar, for example, has a low pH (near 3) and is classified as acidic. On the other hand, many soap solutions have pH values near 9 and are termed alkaline. Any pH value between 0 and 7 is acidic; any pH value between 7 and 14 is alkaline. A solution with a pH of 7 is called neutral.
Just as acid will destroy a piece of cloth, so similar reactions take place in the soil when too many hydrogen ions are present. The excess hydrogen ions force some fertilizer elements out of the soil solution and dissolve other potentially toxic elements into the soil solution.
When the soil is too acidic, calcium, magnesium, and phosphorus are forced out of the soil solution or are changed into different forms that plants cannot use. Thus plants may be deficient in these elements and grow poorly.
While calcium, magnesium, and phosphorus may be deficient in acidic soils, other elements may be dissolved into the soil by the acid and become toxic to plant growth. These elements include iron, manganese, and aluminum.
Most vegetable crops grow best when the soil pH is about 6.5, but they generally tolerate pH values down to 5.5. Below this value, plants often become stunted. You may not notice the stunting at first because all of your plants will be growing at the same, slower rate, and you will have no healthy plants for comparison. However, if an acidic soil is maintained for several years, poor crop growth will eventually become conspicuous.
Beans would probably be the first plants to indicate soil acidity. Plants within the row will grow quite unevenly. Upper leaves may crinkle due to manganese toxicity. If the seeds were inoculated to fix nitrogen, the plants may look nitrogen-starved because the nitrogen-fixing bacteria need calcium to function properly.
Spinach and onions are also quite sensitive to acidic soils and will grow slowly and unevenly when the pH is low.
Lime is added to the soil to correct acidic conditions. This should be done in the fall because lime takes several months to counteract the effects of the excess hydrogen ions in acidic soils.
Once you have followed the recommendations of your county agent (listed under your state university's cooperative extension service in the white pages of the telephone book) or soil-testing laboratory on correcting the soil pH, you probably will not have to add lime again for at least two or three years.
For the low price of a bag of lime, this particular soil-fertility practice is one of the least expensive ways to raise old garden plots to their most productive status.