A soil drench is a specific technique of applying a diluted fungicide, or even fertilizer to a particular plant or tree, or to a specific group of plants, rather than the entire garden.
Rather than spraying an entire lot of plants, a soil drench is applied directly at the plants roots.
Plants are subject to a number of problems, ranging from infection by fungus to infestation by pests, to the need for additional nutrients and minerals to support healthy growth. It’s also common for just a single plant to experience such challenges, when its neighbors do not. A soil drench can help ensure that treatment is applied to the affected plant, without coming in contact with other plants in the area.
A soil drench is used to apply fungicides and even fertilizers to a specific plant. Unlike other application methods, the diluted liquid is not sprayed onto the plant or tree. This can lead to the liquid being carried by the wind onto other plants, or contaminating other plants due to over spray.
Note that while spray applications can often show immediate or fast results, soil drenches take more time. The chemical applied must make its way into the soil, and then be taken up by the roots, and sent into the trunk or stem, then the branches and leaves.