Degradation of soil structure due to coalescence of aggregates in no-till,no-traffic beds in irrigated crops

Poor soil structure remains a major restriction to achieving potential yiel ds from crops under zero tillage. Even water-stable, untrafficked soils in which plants experience no limitations due to nutrients, water, or drainage almost inevitably harden within 2-3 months after the initial cultivation. Most agricultural scientists have not recognised the importance of this com mon yet distinct form of soil hardening, which we name coalescence. We identify coalescence as a slow increase in soil hardness which develops during cycles of wetting and drying. The structure of a well-prepared bed o f soil that is water-stable and not trafficked changes to one that is hard, although perforated with biopores. These pores facilitate the infiltration of water, drainage, and some growth of roots, but the hard matrix causes r oot growth and activity to be substantially reduced compared with roots in loose soil and this reduces the productivity of the crop. We suggest that c oalescence is an important cause of poor responses in productivity to zero and minimum tillage systems of soil management. We have found isolated examples of soils in the field that remain soft, loo se, and porous, after more than 2 years since cultivation. This suggests th at it might be possible to prevent coalescence. These coalescence-stable so ils, in common with virgin soils, have properties that enable them to resis t coalescing. Although we do not know what these properties are, high organ ic matter (>4% w/w total C content) is closely related to zero coalescence. We do not understand why we observe low coalescence in some field situatio ns and we have been unable to control coalescence in the field.