Ontario Grain Farmer April/May 2021

area then flows under several fields before discharging at the surface, this intermediate system is indicated by the black arrows. Local flow systems usually occur in the immediate area and are indicated by the green arrows. The travel time here can be less than one year. There may be cases where wet spots show up in a field but are not part of the flow systems just discussed. These wet spots may be caused by perched water tables. A perched water table is where water percolating down through the soil is slowed down by a tighter layer such as clay. The water pools on this layer like filling a bathtub until it overflows along the edges which may be the side of a field as shown in Figure 2. Perched water tables may dry out in the summer as groundwater is always flowing, sometimes very slowly. The subsurface is not all one type of material and the soil at the surface may not be the ONTARIO GRAIN FARMER 19 APRIL/MAY 2021 continued on page 20 The travel time for the water to discharge may be measured in thousands of years. same all the way down to bedrock. Ontario soils were laid down by glaciers and moved around by water and wind after the glaciers melted. It is beyond the scope of this article to describe in detail the subsurface of all Ontario. While this may sound complex, the following methods can be used to understand the composition of the subsoil in your fields.