ARTICLE - Understanding Soil pH

The letters pH stand for "potential Hydrogen" and represents the measure of Hydrogen ions in the soil. The soil pH is measured on a scale of 0 to 14. A reading of 7.0 in the middle represents "neutral". A measurement of below 7 is considered to be acidic while a measurement above 7 is basic or alkaline.

An important thing to know is that each whole number on the pH scale represents a factor of 10. That means that a soil pH of 6 is ten times more acidic than a soil pH of 7. A soil pH of 5 is 100 times more acidic than it would be a 7. It is not surprising that the majority of plants will grow best in a soil pH of 6.0 to 7.0. There are exceptions, however. Experienced gardeners know that it is relatively easy to alter the soil pH for plants that require different levels of acidity or alkalinity.

The more hydrogen ions present in the soil, the more acidic the pH. The pH scale ranges from 1 (extremely acid) to 14 (extremely alkaline). At pH 7 (neutral), hydrogen ions have little effect on the plant's root ability to absorb nutrients. Scientists have found that plants generally need a small amount of acidity in the soil for optimum growth. The roots seek out the major soil nutrients: nitrogen, phosphorus, and potassium absorbing them for healthy flower, leaf and root development. Some ions are dissolved in water making uptake by plant roots simple. Some cling tightly to particles of clay and humus so the plant roots have a harder time absorbing them. Calcium loosens nutrient ions from the soil particles, making fertilizer more available to plant roots.

The ideal soil pH for many ornamental plants is a range of pH 5.5 to 6.5. In this range most nutrients are readily available. Below 5.5, phosphorus, essential for food storage, becomes unavailable. In very acidic soil, some elements become harmful. Below pH 4.0, aluminum and manganese become toxic to plant roots. In highly alkaline soil, manganese and boron are not available for absorption by the plant roots.

If your pH is off, you will get a recommendation for adding either lime, to raise the alkalinity, or sulfur, to lower the soil pH. This is easy enough to do and should be done in stages, so as not to shock the plants. 

Increasing Soil pH (Acidic to Alkaline)

When the soil is too Acidic, it may be necessary to increase the pH. Garden lime is a good material to use.

• Dolomitic lime is recommended for gardens with a low pH that are also low in magnesium, since this lime contains small amounts of magnesium.  Garden lime is available as dolomitic lime and calcitic lime both are sold in powdered or pelletized form. It is best to purchase the ground lime because the pelleted lime leaches from the soil too quickly from the soil.

Note: Limestone is slow to break down and is best when it is roto-tilled into
the soil. When broadcasted over the surface, smaller limestone particles
will work into the soil faster than larger particles. Powdered limestone
is messiest to work with, but will alter pH the fastest.

Decreasing Soil pH (Alkaline to Acidic)

When the soil is too Alkaline, it may be necessary to lower the pH. There are basically two different materials that are used:

• Sulfur - Sulfur is an age old product. The drawback is that it can takes months to affect the pH. It requires the activity of soil microbes to break it down first.
• Aluminum Sulfate - This is the product most professionals use because it works immediately.

You can use the following tables to calculate the application rates for both the aluminum sulfate and the sulfur. The rates are in pounds per 10 square feet for a loamy soil. Reduce the rate by one-third for sandy soils and increase by one-half for clays

Generally it is recommended that you not add more than 5 pounds of lime or sulfur per 100 sq. ft. of existing garden. If you were wise enough to test your soil before putting in a garden or lawn, go ahead and dump in the whole recommended amount.