Choosing the right method for applying pesticides or fungicides to a tree can have just as much of an impact as the type of chemical you use. A soil drench introduces a chemical into the soil around the root zone of plants. Applying chemicals using a soil drench has several advantages, but this method is not appropriate for every situation.
Defining a Soil Drench
A soil drench applies a chemical mixed with water to the soil around the base of a tree so that its roots can absorb the chemical. Soil drenches are commonly used to apply water-soluble systemic pesticides or fungicides to combat pests or diseases attacking a tree. Soil drenches are most beneficial against boring insects, root rots and other problems that are difficult to treat with chemicals applied to the leaves and stems. Soil drenches are also useful for applying fast-acting water-soluble fertilizers to plants.
Using Soil Drenches
Apply a soil drench when the soil around the plant's base is slightly moist. Temporarily raking back mulch, leaves or other material covering the soil and the uppermost inch of soil within 1 foot of the plant's base limits the impact of evaporation on the chemicals applied to the soil, and it gets the chemical into the tree faster. The amounts of fertilizer and water used in a soil drench are typically calculated based on the concentration of the fertilizer and the area of soil it is used on. Pesticides and other chemicals used on trees are calculated based on the diameter of the tree's trunk and the manufacturer's recommendation for the chemical you are using.
Soil Drench Benefits
Soil drenches introduce fertilizer or pesticides into the circulatory system of plants without the need for special tools. Chemicals used in a soil drench are typically mixed and spread using a bucket. Soil drenches used on trees are less damaging than most methods for injecting chemicals; tree injection methods typically require drilling holes into the trunk around infected areas.
Soil Drench Drawbacks
Pesticides applied with a soil drench are systemic chemicals that indiscriminately target pests in the tree and the soil around it. These chemicals can kill beneficial insects that interact with the tree or micro-organisms in the soil just as easily as the pests they are meant to eliminate. Soil drenches are minimally invasive in trees, but they are relatively slow compared to injections or spraying. A soil drench can take up to four to eight weeks to start producing results, while injection methods typically begin working within two to four weeks. Because soil drenches only work with water-soluble chemicals that the tree's roots can absorb, they are relatively ineffective in wet or compacted soils that prevent the chemical from being absorbed.