What causes soil acidity? A commonly asked question. There are four major reasons for soils to become acidic: rainfall and leaching, acidic parent material, organic matter decay, and harvest of high-yielding crops. Wet climates have a greater potential for acidic soils.
In time, excessive rainfall leaches the soil profile's basic elements (calcium, magnesium, sodium, and potassium) that prevent soil acidity. Soils that develop from weathered granite are likely to be more acidic than those developed from shale or limestone. Organic matter decay produces hydrogen ions (H+), which are responsible for acidity (an ion is a positively or negatively charged element).
Like that from rainfall, acidic soil development from decaying organic matter is insignificant in the short term. Harvest of high-yielding crops plays the most significant role in increasing soil acidity. During growth, crops absorb basic elements such as calcium, magnesium, and potassium to satisfy their nutritional requirements.
As crop yields increase, more of these limelike nutrients are removed from the field. Compared to the leaf and stem portions of the plant, grain contains minute amounts of these basic nutrients. Therefore, harvesting high-yielding forages such as bermudagrass and alfalfa affects soil acidity more than harvesting grain does.
Nitrogen fertilizer has been blamed for the increase in soil acidity problems throughout the region. Yes, when ammoniacal fertilizer materials are applied to the soil, acidity is produced, but the form of nitrogen removed by the crop is similar to that found in fertilizer. In reality, nitrogen fertilizer increases soil acidity by increasing crop yields, thereby increasing the amount of basic elements being removed.
Raising pH is not a fast process. This is why we recommend you attend to it in the fall. The most common method of raising pH is by adding lime to the soil. There are several kinds of lime. Do not use builders lime or quicklime; they will fry your plants. The preferred limes for garden use are hydrated lime and ground lime.
Hydrated lime will act faster, changing the pH within 2 to 3 months, but can burn roots. It is also caustic, so handle it with gloves. Hydrated lime would be useful in building a new bed (or in a vegetable bed) that won’t be used until spring.
Ground lime is slower, taking up to 6 months to work its magic, but is safe for use around existing plants. There are two types of ground lime, calcitic and dolomitic. Calcitic lime is 70% calcium carbonate and 15% magnesium carbonate; dolomitic lime is 50% calcium carbonate and 40% magnesium carbonate. Dolomitic lime is generally easier to find, and adds both calcium and magnesium to your soil. You might want to look for calcitic lime, however, if you have been applying heavy doses of epsom salt (magnesium sulfate) to your beds. (Normally we recommend one or two applications of epsom salt per season; some people make four or five…a "heavy dose.")
The chart indicates how much lime should be added to different types of soil to change the pH up to a pH of 6.5. A pound of lime is one cup. Do not overdo an application: no more than 6 lbs. per 100 sq ft per application. If your soil needs more, make two applications a couple of months apart.
The following chart shows the number of pounds of ground limestone needed per 100 sq ft to raise soil PH to 6.5. If the amount is large (more than 6 lbs), add in stages.
|Current pH||Sandy Loam||Loam||Clay Loam|
And here's a very good Q and A article on liming...