Whether your soil is acid or very alkaline has an important bearing on how well your plants will grow. But how acid and how alkaline must a soil be? How does a soil become one or the other? A soil may become acid in several ways. Because of certain minerals some soils are naturally acid, just as others are naturally alkaline. Large amounts of organic matter of the kind which rot down slowly will induce acidity chiefly because the bacteria associated with this rotting down generate acids in the process. When acids accumulate, the soil will pass through various degrees of acidity.

It may even return, or become neutral. There is no static phase in any soil, it is always changing. Temperature, moisture, cultivation, manures, fertilizers, outdoor fountains, garden statuary, even large waterfalls, applied to it will change its reaction one way or another. It's largely a matter of degree. You can test the soil for acidity or alkalinity. Any of the simple test kits sold will give you a reaction. Just how accurate it will be depends upon the technique and the freshness of the chemicals used.

Your State Experiment Station or Agricultural College will do it for you. In color tests, acidity is indicated by a gradual changing from pink to red; the brighter the color the greater the degree of acidity. Alkalinity is indicated by a change from pale to dark blue, the changes indicating the intensity of alkalinity. A contrivance of the scientist called the pH scale measures the degrees of acidity or alkalinity. This scale may be compared to a thermometer. The dividing line between gradual increases in warm and cold temperature is 32°F.

On the pH scale, the dividing or neutral line is pH 7. Below this, acidity increases above pH 7 and alkalinity increases. At pH 8, the soil is extremely alkaline; at pH 3, extremely acid. Plants have difficulty growing in either case unless they are plants adaptable to such conditions. Nor is it a matter of increasing acidity by just 1 degree from 5 to 4 or alkalinity 7 to 8. The jump is tenfold. For example, pH 6 is ten times more acid than pH 7. But pH 5 is 100 times more acid than pH 7.

The question arises as to what is the best range at which plants will grow. For acid-soil plants such as rhododendron, azalea and the like, the range is considered to be around pH 5 to pH 6. For blueberries the range is between pH 4 to 5.1. This is certainly pretty acidic. So the term acid soil, as generally used, only tells half the story. Fortunately, the great majority of ornamental, fruit, and food plants grow best in a neutral soil, that is, a soil neither one thing nor another. Many like it slightly on the acid side. Most garden soils are just within this range.

If you are planting acid soil plants and a test reveals that your soil is alkaline or if you are not sure, there are certain materials you may apply to change the reaction. The safest for the beginner is the use of acid peat moss, or leaf mold; sawdust, too, will help. These materials will not affect any potential outdoor fountains, garden water features, or outdoor waterfalls either. Any of these packed around the roots and then spread on the ground surface over the roots after planting, will assure sufficient acidity for growth. Chemicals are suggested in various ways.

One of these is aluminum sulphate. This is not a fertilizer. It is a soil reagent. Enough will change a soil to the acid side. How much you should use to get a specific reaction is hard to say. Only by a careful test before and after application can you tell how much you need. Too much applied at once will cause trouble. It is suggested by many authorities that more than 1 lb. per 100 square feet at one application is dangerous. If more is needed, it is applied over a period of months to avoid a too sudden change. The other chemical suggested is sulphur. This is less sudden in action.