The most common classes of soil pH are:
- Extremely acid 3.5 - 4.4
- Very strongly acid 4.5 - 5.0
- Strongly acid 5.1 - 5.5
- Moderately acid 5.6 - 6.0
- Slightly acid 6.1 - 6.5
- Neutral 6.6 - 7.3
- Slightly alkaline 7.4 - 7.8
- Moderately alkaline 7.9 - 8.4
- Strongly alkaline 8.5 - 9.0
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.
 
Source: Soil Quality Indicators: pH. 1998. USDA Natural Resources Conservation Service. Available at http://soils.usda.gov/sqi/files/indicate.pdf.