Page 115 - Nutrient Management Planning Guide

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Practical Use of Soil Analysis Results

Estimating CEC from Soil Texture

A theoretical soil from the Alberta Peace region contains 40 percent clay and two percent organic matter. Using average values of 80 cmol/kg for clay (i.e., montmorillonite; Table 3.4.8) and 200 cmol/kg for organic matter (Tables 3.4.7 and 3.4.8), the estimated CEC for this soil would be:

CEC contribution by clay = percent clay ÷ 100 x CEC of clay (cmol/kg) = 40 ÷ 100 x 80 cmol/kg = 32 cmol/kg

CEC contribution by organic matter (OM) = percent OM ÷ 100 x CEC of OM (cmol/kg) = 2 ÷ 100 x 200 cmol/kg = 4 cmol/kg

Total CEC = CEC contribution by clay + CEC contribution by OM = 32 cmol/kg + 4 cmol/kg = 36 cmol/kg

Base saturation (BS) is a measure of the proportion of the total CEC in soil occupied by Na + , K + , Ca 2+ , and Mg 2+ expressed in percent. While there is no ideal percent BS, these values are sometimes used to make recommendations for K, Ca, or Mg amendments to soils. This approach fails to consider the cost and economics of such an application, nor does it take into account excessively high levels of cations.

Soil Organic Matter

Soil organic matter is a measurement of the amount of plant and animal residue in the soil. It has several important implications for soil fertility. Organic matter acts as a revolving nutrient bank account, which releases

crop available nutrients over an extended period. As discussed in the previous section, it also has an important impact, together with clay content, on CEC of the soil. Soil structure, tilth, and water infiltration are also improved by building soil organic matter.

Organic matter content is the distinguishing

characteristic of Alberta’s soil zones (Figure 3.4.3). The Brown soil zone has the least organic matter having developed beneath a drier, short grass prairie.