What do the p and H in pH stand for? And why is a proper pH level in your soil so crucial to plant growth? The “p” is a mathematical operator like “+” or “x”. In chemical mathematics, “p” means “the negative logarithm of something”. In the case of pH, it would be defined as “the negative logarithm of the concentration of hydrogen (H) ions in the soil.”
Soil pH simply measures how many hydrogen ions are affecting plant roots. 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.
In many parts of the U.S., soil tends to be too acidic and needs to be adjusted with lime. In general, areas with heavy rainfall and deciduous or coniferous forest tend to have more acidic soil. In drier parts of the country, the soil is usually too alkaline. To increase acidity, you can use ferrous sulfate, aluminum sulfate, or garden sulfur. Avoid sulfur applications to lawns during hot weather to minimize the possibility of burning the grass.
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. Dolomitic lime is recommended for gardens with a low pH that are also low in magnesium, since this lime contains small amounts of magnesium.