Herbicides are applied directly either to the soil (soil application) or to the foliage (foliar application).
Soil Surface Application
Herbicides are sprayed on the soil surface to form a uniform herbicides layer. Weeds germinating in the top layer are killed due to incidental absorption of herbicides. The herbicides for surface soil application have to be soil active, less soluble and less volatile.
When the herbicides are applied to the soil surface, they are lost by volatilization. Volatile herbicides are incorporated into the soil to reduce losses. Generally, these herbicides are applied before planting as it is difficult to incorporate the herbicides after sowing.
The application of herbicides in a concentrated band, about 7-10 cm below the soil surface for controlling perennial weeds. e.g. Carbamate herbicides to control Cyperus rotundus.
Herbicides can be applied as narrow bands over or along the crop row. The weeds in between the rows can be controlled by intercultivation or by non- selective herbicides. This method is useful where labour is expensive and intercultivation is possible.
Application of volatile chemicals in to confined spaces or in to the soil to produce gas that will destroy weed seeds is called fumigation. Herbicides used for fumigation are known as fumigants. These are good for killing perennial weeds as well as for eliminating weed seeds. e.g. Methyl bromide, Metham.
Application of herbicides with irrigation water both by surface and sprinkler systems. In India farmers apply fluchloralin for chilies and tomato.