What is pH?

pH is a measure of the level of hydrogen in the soil. The more hydrogen in the soil, the more acidic the soil is. Soil pH has an enormous impact on the availability of soil nutrients to the plant and also greatly affects the organisms living in the soil.
Being a logarithmic scale, a pH reading fo 5 is 10 times more acidic than a pH reading of 6. A pH reading of 4 is 1000 times more acidic than a pH reading of 6.

In chemistry,  pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at 25 °C (77 °F). Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline.

Effects of pH on nutrient availabilty

In the following chart, the width of the bar represents the relative availability of each nutrient.
Generally, high (alkaline) or low (acidic) pH conditions reduce the availability of trace elements.