| ||One of the chemical characteristics that farmers should be aware of is their pH range of their soils. “Acidic soils have a relatively low soil pH and soils with a high pH are basic or a measure or indicator of how acidic or basic a soil is,” says Dr. Ross McKenzie, agronomy research scientist with Alberta Agriculture and Rural Development. “A soil pH of 7.0 is neutral, soil below a pH of 6.5 is slightly acidic, 6.0 is moderately acidic and below 5.5 is strongly acidic. Soils with a pH of 7.5 is slightly alkaline, 8.0 is moderately alkaline and above 8.5 is strongly alkaline. Soils with a pH greater than 8.5 are often sodic (high in sodium).”
Most of Alberta’s strongly acid soils occur in the Gray Wooded soil zone. This is a result of hundreds of years of acidic organic material from the leaves of deciduous trees, needles from coniferous trees and other acidic organic material being added to the surface of soil. Most of Alberta’s soils that have developed under forest vegetation are acidic.
In southern Alberta, soils on the Milk River Ridge and in the Cypress Hills tend to be acidic as these soils, which were unaffected by the last glaciation, are much older than the rest of the soils Alberta.
“For the past decade we have been noted that the surface soil pH has been declining slightly in cultivated soils across Alberta that are direct seeded. This is primarily the result of acidification caused by nitrogen (N) and sulphur (S) fertilizers over a period of years being added to soil to increase crop yields. Currently, it is not a problem, but it is a situation that must be monitored,” says McKenzie. With conventional tillage, the upper six inches of soil is constantly being disturbed. Therefore, the acidifying effects of N and S fertilizers have been less noticeable.
The effect of strongly acid soils on crops is generally poor growth of a sensitive crop such as legume crops such as alfalfa or pulse crops such as pea. With N fixing crops such as alfalfa or pea, the rhizobium bacteria which live in association with plant roots, do not survive nearly as well in acidic soils or when soil pH declines below 5.5. When soils are strongly acidic (pH <5.5) aluminum (Al) and manganese (Mn) tend to be more soluble and may increase to levels that can be harmful to crop growth.
“The first step in the management of acid soils is to identify the extent and severity of the problem - this can be done by taking soil samples in various locations across a field to determine the size, extent and severity of the problem,” says McKenzie. “While poor yields of acid sensitive crops may indicate an acid soil condition, soil tests are the only sure method of identifying an acidity problem.
“An application of lime is the only way to correct strongly acidic soils. Lime is required to neutralize the acidity and raise the pH of back into the near neutral range of 6.5 to 7. Very careful sampling of fields is required to identify the areas with strongly and moderately acidic soils. A soil testing lab can provide a Lime Requirement Test, to determine the rates of lime required to raise the soil pH back to near neutral.”
For detailing information on soil acidity and liming see the factsheet Liming Acid Soils, which can be viewed or downloaded from Alberta Agriculture and Rural Development’s website.
Ross H. McKenzie PhD, P. Ag.