Application of Herbicides And Pesticides This subject is described in detail in machinery texts. However, certain basic principles related to the covering of the leaf and soil by sprays will be mentioned. The application of liquids and wettable powders by means of sprayers may be adjusted in terms of pressure and nozzle type to provide the required spray rate.
Cone nozzles produce a turbulent spray pattern suitable for fungicide and insecticide use, while fan nozzles produce a flat spray pattern for herbicide application. In periods of active plant growth, fortnightly sprays may be necessary to control pests and diseases on newly expanding foliage. High volume sprayers apply the diluted chemical at rates of 600-1000 1/ha in order to cover the whole leaf surface with droplets of 0.04-0.10 mm diameter. Cover of the underleaf surface with pesticides may be poor if nozzles are not directed horizontally or upwards.
Soil applied chemicals, e.g. herbicides or drenches, may be sprayed at a larger droplet size, 0.25-0.5 mm in diameter, through a selected fan nozzle. The correct height of the sprayer boom above the plant is essential for downward directed nozzles if the spray pattern is to be evenly distributed. Savings can be achieved by band spraying herbicides in narrow strips over the crop to leave the inter-row for mechanical cultivation.Medium volume (200-600 1/ha) and low volume (50-200 1/ha) equipment, such as knapsack sprayers, apply herbicides and pesticides on to the leaf at a lower droplet density, and in tree crops, mist blower equipment creates turbulence, and therefore increased spray travel, by means of a power driven fan. Electrically charged nozzles, which transfer a strong charge to the sprayed droplet, have been developed.
Wettable powders containing extremely small particles of active ingredient and wetting agents form a stable suspension for only a short period of time when diluted in the spray tank. Continuous stirring or shaking of the diluted formulation is thus required. An inert filler of clay-like material is usually present in the formulation to ease the original grinding of particles, and also to help increase the shelf life of the product. It is suggested that this formulation is mixed into a thin paste before pouring through the filter of the sprayer.
This prevents the formation of lumps which may block nozzles.Dusts are applied dry to leaves or soil, and thus require less precision in grinding of the constituent particles, and less wetting dressings protect the seed and seedling against pests and diseases. A low percentage of active ingredient, e.g. captan, applied in an inert clay-like filler or liquid reduces the risk of chemical damage to the delicate germinating seed.