SP FAQ - Dust Mulches and Non-wetting Sands
Most dust mulches are deliberately created in arid and semi-arid soils by repeat tillage of a thin layer of surface soil with the aim of creating a thin super-dry dust layer that interrupts the capillary rise of soil water to the surface from deeper (wetter) layers. The objective is to sacrifice a small amount of soil water near the surface in the interests of preserving a greater loss of water from deeper depths over time. The problem is how best to seed into this precious resource of stored water beneath the dust mulch.
The conventional approach has been to use tillage openers that sweep the dust aside, place the seeds in the deeper wet zone and then return the dust over the sown seed, all of which involves considerable soil movement. So long as the dust layer is not too thick, an alternative approach is to use Cross Slot no-tillage openers with long blades that will reach down through the dust and deposit the seed in the wet zone without sweeping the dust aside, even temporarily. There is a limit to how thick a dust layer the Cross Slot blades can reach through before at least some of it will need to be swept aside so as to get the seed into the wet zone. And because there is seldom any residue present, it is unlikely that the dust zone will contain enough vapour-phase soil water on its own to allow seeds to be placed directly in the dust zone and expect germination and emergence to take place. But testing is continuing.
Cross Slot openers are very well suited to non-wetting sands, provided that some surface residues are present. There are a variety of additives that are currently used to break the surface tension of non-wetting sands so as to allow infiltration of rain water or irrigation. It may be necessary to use one of these initially to get a cereal crop established in the first place. But once that crop has grown and been harvested Cross Slot no-tillage should do the rest. The secret is to value the crop residues even more highly than usual since retaining a mulch (or even a partial-mulch) on the soil surface is the best way of gradually (but permanently) reversing the non-wetting properties of these sands.
Over time, the problem will often disappear altogether with continued use of Cross Slot no-tillage and heavy-residue-producing crops. The absence of non-wetting problems under the heavy residues on undisturbed roadside soils illustrates what is possible.