The creation of longitudinal cracks in shrinking soils to enhance seedling emergence. Part I. The effect of soil structure

Stand establishment in crusting soils is one of the most critical stages in the production of crops with delicate seedlings. This becomes more difficult in hard-setting soils of arid lands where dispersion of aggregates due to irrigation results in the formation of a hard layer as the soil dries from the surface downwards and impedes seedling emergence. However, seeds planted near to natural cracks manage to emerge through them. The aim of this study was to improve seedling emergence of irrigated crops in arid and semi-arid conditions by devising methods to create longitudinal cracks in the vicinity of the seed rows during the subsequent drying phase. Laboratory experiments were conducted in soil boxes containing air-dried clay soils to investigate the influence of different mechanical effects on the cracking pattern after flooding. Promising results were achieved by compacting a 7-15-mm wide strip along the seed row. Monitoring the effect of compaction on water release characteristics and tensile strength of soil revealed that the greater water loss of the surrounding non-compacted soil caused a suction gradient towards the points of lower water content resulting in movement of water and particles towards the drier zones. The compacted soil with a greater tensile strength did not permit the flow of water and particles to the loose soil and a discontinuity occurred. A field experiment in central Iran proved the feasibility of the technique in a semi-arid area. This was achieved by lightly compacting a narrow strip of soil by applying 17-35 kg on a 22-mm wide, 558-mm diameter wheel covered by a layer of flexible rubber which ran over the pre-compacted soil.