Appendix II

Electrical Conductivity Mapping

Soil electrical conductivity (EC) has no direct effect on crop growth or yield unless the area is extremely saline (which is not common in Québec or Ontario). EC levels reflect the combination of several soil factors such as water holding capacity, topsoil depth, cation exchange capacity, soil drainage, organic matter level and soil nutrient levels. The usefulness of EC mapping comes from the relationships that frequently exist between EC and a variety of other soil properties that are highly related to crop productivity. In general, the correlation between soil EC and yield will be the greatest when yields are primarily influenced by the soil's water holding capacity B soil texture.

The patterns of soil EC within a field tend to be very stable from season to season and year to year because they are based on soil characteristics that do not change in our lifetime such as soil texture and topsoil depth. Once an EC map has been made, that map will remain accurate unless significant soil moving takes place in a field, such as land leveling. The simplest way to use a soil EC map is to visually compare it to yield or soil survey maps from the same field. This will identify obvious relationships between soil type boundaries and crop yield patterns. However, areas that are low yielding because of sandy soils may remain low yielding because it is difficult to change soil texture.

The conduction of electricity in soils takes place through soil pores that are filled with water. Therefore the EC values will be affected by the following factors which may or may not correlate with yield.

Porosity: The greater a soil's porosity, the more easily electricity is conducted. Soils with high clay content have higher porosity than sandier soils. As well, clays retain more water compared to sands. Compaction normally increases soil EC because the soil pore size decreases, drainage is slower so the moisture stays in the pores longer.

Water content: Dry soils are much lower in conductivity than moist soils.

Salinity level: Increasing concentration of electrolytes (salts) in soil water will increase soil EC. If nitrate or KCl fertilizer has been applied the EC values could be misleading.

Cation exchange capacity (CEC): The higher the CEC the more nutrients that can be held on clay particles or near clays in solution. [The greater the CEC the greater the nutrients that can be held on or in solution near clay particles] These nutrients are in the form of ions (Ca⁺², K⁺¹, Mg⁺² etc) [according to international convention it should be Ca²⁺, K⁺, and Mg²⁺]. The presence of these ions in the moisture-filled soil pores will increase soil EC in the same way that salinity does.

Temperature: As temperature decreases toward the freezing point of water, soil EC decreases slightly. Below freezing, the overall soil EC declines rapidly.