Properties of soil wetting agents
Soil wetting agents have combinations of chemicals which can help water enter and be retained in water repellent soils.
Penetrants
Penetrants are surfactants that reduce the surface tension of water and help it spread over soil particles and into the soil. Penetrants also help water drain through the soil and can encourage leaching of water and nutrients. Some penetrants have been designed to breakdown quicker to reduce the leaching risk.
Water retaining compounds
Water retaining compounds, also known as humectants, attach part of their molecule to a soil surface and other parts to water and nutrients, this can reduce soil leaching. Products which have a combination of penetrants and water retaining chemistry are generally better for management of water repellent soils.
Banded wetting agents
This method sprays the wetter into the crop row at seeding typically at cost effective rates of 0.5-2 litres per hectare (L/ha). Usually it is applied to the soil surface at the base of the furrow behind the press wheels to improve the effectiveness of water harvesting and reduce the 'gappiness' of crop establishment. Use of banded wetting agents is technically challenging compared to many other methods of managing repellence.
The following problems can easily occur with wetter applied on-furrow behind the press wheels:
- The band can be broken up and buried by soil disturbed by some designs of presswheels. Flat or square cross section wheels with recessed rims encourage soil throw and rip up the sides of the furrow. Smooth sided and 'V' or 'U' cross-section presswheels are better.
- The spray can be applied too close to the presswheel before the soil at the base of the furrow has stopped moving.
- Wind can blow the spray away from the furrow.
- Wind and rain can fill in the furrow after seeding.
Crop yield responses have ranged from 0-20% and have been negative when water retention chemistry is absent. Best responses are generally with dry sowing with some subsoil moisture and moderate rainfall within a few days or a week of seeding. Banded wetters have proven to be more reliable on repellent forest gravel soils in the south-west of WA.
More recently some soil wetter products have been banded in the furrow near the seed. Typically these products are compatible with other liquid applications including UAN and fungicides and are applied through existing liquid banding kits on the seeder. Research has demonstrated that for certain products this placement can work quite effectively although on deeper sands it can be less reliable than banding on top of the furrow.
Blanket applied wetting agents
This method uses a boom spray to apply soil wetters to the whole soil surface, typically at rates of 20-50L/ha. They are generally applied onto dry soil or stubble from which they are washed into the topsoil with opening rains.
Blanket wetting agents may improve germination of crops, pastures and weeds as they are applied to whole soil surface. Responses in trials and farm evaluations have been very dependent on soil type and season. Most success has been on some forest gravel soils in the south west agricultural area. Some of these products are longer lasting and on responsive soil types can maintain their efficacy for two seasons.
The use of blanket wetting agents needs careful on-farm testing and may be most economic on weed ‘hot spots’, such as sheep camps and gravel hills, where improved weed emergence in water repellent soil may provide an opportunity for better weed control.
Acknowledgement
Soil water repellence research is supported by DPIRD and Grains Research and Development Corporation through DAW00244: 'Delivering enhanced agronomic strategies for improved crop performance on water repellent soils in Western Australia'.