The fundamental understanding of soil pH is simple. It is simply a way to measure a condition, not so different from using a ruler. Acidity and alkalinity are measured as pH (parts Hydrogen), expressed in a logarithmic scale from zero to 14. Specifically, acidity is associated with an increase in hydrogen ions, and alkalinity is with an increase of hydroxyl ions. The differences affect how molecules (or potential plant nutrients) will interact in the soil.
A pH of 7.0, like that of pure water, is neutral, because the concentrations of hydrogen ions and hydroxyl ions are equal. As the pH number goes lower than 7, the hydrogen ions increase and the soil becomes more acidic; as the pH number goes higher than 7, the concentration of hydroxyl ions increases and the soil becomes more alkaline.
Most garden plants grow best in a slightly acidic soil, with a pH between 6.0 and 7.0. Going from one pH number to the next makes a big difference, because the numbers change by factors of 10. A pH of 6 is 10 times more acidic than a pH of 7 and pH of 5 is 100 times more acidic than a pH of 7.
What is the significance of soil pH?
Availability of Nutrients
Soil pH influences the solubility of nutrients. It also affects the activity of micro-organisms responsible for breaking down organic matter and most chemical transformations in the soil. Soil pH thus affects the availability of several plant nutrients.
A pH range of 6 to 7 is generally most favorable for plant growth because most plant nutrients are readily available in this range. However, some plants have soil pH requirements above or below this range.
Soils that have a pH below 5.5 generally have a low availability of calcium, magnesium, and phosphorus. At these low pH’s, the solubility of aluminum, iron, and boron is high; and low for molybdenum. At pH 7.8 or more, calcium and magnesium are abundant. Molybdenum is also available if it is present in the soil minerals. High pH soils may have an inadequate availability of iron, manganese, copper, zinc, and especially of phosphorus and boron.
Soil pH affects many micro-organisms. The type and population densities change with pH. A pH of 6.6 to 7.3 is favorable for microbial activities that contribute to the availability of nitrogen, sulfur, and phosphorus in soils.
Most pesticides are labeled for specific soil conditions. If soils have a pH outside the allowed range, the pesticides may become ineffective, changed to an undesirable form, or may not degrade as expected, which results in problems for the next crop period.
Mobility of heavy metals
Many heavy metals become more water soluble under acid conditions and can move downward with water through the soil, and in some cases move to aquifers, surface streams, or lakes.
Soil pH is one of several properties used as a general indicator of soil corrosivity. Generally, soils that are either highly alkaline or highly acid are likely to be corrosive to steel. Soils that have pH of 5.5 or lower are likely to be highly corrosive to concrete.