Literally, pH stands for “potential of hydrogen” and is the term used by soil scientists to represent the hydrogen ion concentration in a soil sample. Soil testing below 7 is said to be acidic; soil testing above pH 7 is alkaline. Because it is based on logarithms with a base of ten, a soil with a pH of 5 is ten times as acidic as soil of pH 6 while a soil testing pH 4 is one hundred times as acidic as soil testing pH 6. Nutrients such as phosphorous, calcium, potassium, and magnesium are most available to plants when the soil pH is between 6.0 and 7.5. Under highly acidic (low pH) conditions, these nutrients become insoluble and relatively unavailable for uptake by plants. However, some plants such as rhododendrons grow better in acidic soils. High soil pH can also decrease the availability of nutrients. If the soil is more alkaline than pH 8, phosphorous, iron, and many trace elements become insoluble and unavailable for plant uptake. (For more about pH, see the chapter “Basics of Biology.”)