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Soil pH is a measure of the acidity or alkalinity in the soil.
It is also called soil reaction.
The most common classes of soil pH are:
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.