pH and Limestone

The uptake of nutrients into plants depends on the acidity of the soil (pH). This is more important to the well-being of a planted crop than most people think. If your plants are not growing within the optimum pH range, nutrients will be inefficiently taken up by the plants, having a detrimental effect on not only the health of your plants, but the size and quality of the harvested produce.

What is pH?

pH is a numeric scale that refers to the activity of the hydrogen ion – in this case, in the “soil solution”, or the moisture that surrounds particles of soil. This numeric range is 0 to 14, with low values being the most acidic, in which the soil solution has the highest concentrations of hydrogen ions. High values are the least acidic (ie; they are basic), and in this case the soil solution has the lowest concentrations of hydrogen ions. The middle, 7, is neutral, as is the case for pure water. The scale is logarithmic; each unit of change is ten times different from the next value. A pH of 5 is thus ten times more acidic than pH 6, and a pH of 4 is one hundred times more acidic than pH 6.

What is Acidity?

An acid is defined as a substance that tends to give up hydrogen ions when dissolved. Thus, if a soil solution has a pH 4, there is a high concentration of hydrogen ions, and this soil is very acidic. Soil with a pH of 8 has a lower concentration of hydrogen ions, and therefore the soil is not very acid. It is basic, or alkaline. If your soil is too alkaline or too acidic, some plant nutrients won’t dissolve easily, and thus will not be as available for plant uptake compared to soil with pH closer to the middle of the range; or neutral. See Figure 1 for a graphic demonstration of this. It shows clearly that unless the pH of your soil is not in a suitable range for your crop type, some of the money spent on fertilizer may be wasted.

Soil that has never been disturbed are called “native” soils. The pH of native soils will be stable at a value that is related to the geological origins, the climate and the native plant cover. In the Fraser Valley, this native plant cover in a high-rainfall climate was largely a coniferous forest with soil pH likely between 4 and 5. The pH of your soil can be modified to put the acidity in a suitable range for your crop type.

Limestone

When pH must be modified, it can be raised or lowered depending on the requirement. Highbush blueberries, a major small fruit crop in the Fraser Valley, do well in a pH range of 4.5 – 5.5. If pH is too high for blueberries, it can be reduced by applications of elemental sulphur (see our blogpost here). Crops such as forage grass, corn, potatoes and raspberries do better in a pH range of 5.5 – 6.5. For these crops, the least expensive method of raising pH has been application of calcium carbonate limestone. In lawns and gardens, and where soil levels of magnesium are low, dolomitic limestone is commonly used. Dolomite contains both calcium carbonate and magnesium carbonate in roughly equal proportions.

Start with a soil test, which can be done quickly and conveniently at TerraLink’s Plant Science Lab at our Abbotsford location. The test results will help pinpoint how much limestone is required. When applying limestone to raise pH, keep in mind the stage of the crop; in perennial crops where only topdressing of inputs is the only option, usually no more than one tonne per acre of limestone can reasonably be applied. Prior to a new crop, if the soil will be cultivated, the opportunity exists to apply two or more tonnes per acre and mix it in.

Fall is by far the best time to apply limestone. Typically, several months pass before the greatest change in pH takes place. To raise pH to the optimum target range by the time you apply nutrients in the spring, the best time to apply limestone is in the fall. It is even slower if the limestone is surface-broadcast rather than incorporated.

For more information on pH and limestone, call the TerraLink Sales Desk at 800-661-4559 or email us.