A test for soil pH measures the acidity or alkalinity of a soil. Soil with a pH below 7 is acidic; soil with a soil pH above 7 is alkaline. Alkaline soil can directly influence the growth of plants or turf. Lime and other chemicals added to make alkaline soils more acidic are not fertilizers; they are soil amendments.
Ammonium in the soil contains three atoms of hydrogen to each atom of nitrogen. Soil bacteria convert the ammonium into ammonium nitrate, a form plants can use to make nutrient sugars.
The three hydrogen float in water contained within the soil; they are very reactive and account for the "acid" taste in acidic substances. Alkaline soil literally tastes "sweet" and acidic soil is tart (if you want to taste it).
When soil has too many hydrogen ions in the soil and water, plants have a hard time getting the nutrients they need including nitrogen, phosphorus, boron, copper, iron, manganese and zinc.
Types of Soils
Some soils are more able to cope with the added hydrogen particles. Some soils contain more natural lime (calcium carbonate). Some have more organic matter and clay to bind the hydrogen atoms, taking them temporary out of the soil and water. Other soils have fewer carbonates and a lower capacity to absorb the hydrogen so the hydrogen builds up in the soil. If the soil water tested has more hydrogen particles than oxygen combined with hydrogen, then the soil is acid.
Each pH number is a multiple of 10, so a soil with a pH of 5 has 10 times more hydrogen particles than a soil with a pH of 6.
The floating hydrogen atoms can react with lime to form water and carbon dioxide, so lime is generally used to treat alkaline soils. Some plants like acidic soils; others like alkaline soils. Learn the preferences of your plants to determine whether your soil needs amended to treat alkalinity.
Most pH soil tests give the lime requirement in pounds of calcium carbon per 1,000 sq. ft. or per acre. Most liming products are not pure calcium carbonate. The label on the bag will state the calcium carbonate equivalent as a percentage.
Burned lime and hydrated lime are fast acting but are hazardous and difficult to apply; they can burn your eyes and lungs if you inhale them. Limestone pellets are fast acting, but are expensive. Dolomite limestone and ground limestone are slow acting, but are generally safer to apply.
Peat, compressed fibers of bog masses, can also be used to reduce the alkaline content of soil. Peat holds water so it can be used to amend soils in hot, dry climates where water is scarce. It is not nearly as effective in treating alkaline soils as lime.
Some special-use fertilizers contain sulfur and ammonium sulphate, both of which will lower the pH of alkaline soil.