Blanket-applied soil wetting agents
Soil wetting agents usually contain surfactants that work by reducing the surface tension of water. Reduced surface tension improves the ability of the water to spread on the soil surface and enter water repellent soil, similar to the way surfactants used in herbicides help the droplets spread across the leaves of plants.
It is important to note that surfactants do not directly reduce, or alter soil water repellence; they simply help water enter the repellent soil more quickly. The formulation of wetting agents can also contain other active components that may help penetration of the wetting agent into the soil, or have a water absorbing and holding function.
Blanket-applied wetting agents are applied using a boom spray to the whole soil surface with the intention of aiding the uniform wetting-up of the soil surface. In an agricultural context this may result in reduced occurrence of dry soil patches resulting in earlier and more even germination of pasture, weeds or sown crops.
Extensive research into blanket wetting agents was carried out in Western Australia in the 1980’s and 1990’s. While wetting agents were often found to be effective in improving crop establishment (see Figure 1) the longevity of the benefits in subsequent years and impact on crop yield were variable.
A deterrent for large areas of application was the cost at the required rate of about 50L/ha. This high cost of blanket application was one of the key reasons wetting agents were reduced to banding at the base of the furrow — lowering the total amount of wetting agent applied.
The economics and efficiency of blanket wetting agent use are improved when they are only applied to relatively small problem areas, especially those which can be origins of weed invasion. This may include, sheep camps, headlands and small areas of deep sand or gravel, for example.
Earlier research discovered the potential for wetting agents and their ability to reduce surface tension to cause greater leaching of mobile nutrients. Wetting agents were thereby made to degrade in the soil after plant establishment. Research trials have also tested wetting agents, with water holding compounds that do not reduce water retention and these are being used in some existing blanket wetting agent products.
Figure 1: Lupin, barley and wheat crop establishment improved with blanket and banded applied soil wetting agents in Western Australia.