PHYSICAL LIMITATIONS TO PLANT-GROWTH ON TAILINGS FROM SANDY ALLUVIUM

It is proposed to rework surface alluvium derived from earlier mining of gold-bearing quartz. After grinding and extraction, tailings would be pumped back into the excavation as a thick slurry. In pot trials of the tailings in winter, it was found that oats failed to establish despite fertilizer addition. To find the reason for this, the particle size distribution, water retention, bulk density, ease of dispersion and hydraulic conductivity of tailings and the starting alluvium were compared. Also tailings were slowly dried in stages and, at each stage, the increase in cone penetrometer resistance was measured. The penetration resistance of tailings was found to be proportional to matric suction and independent of whether the alluvium was ground coarse or fine. The effect of reduced particle size is compensated for by the decrease in bulk density. The suction required to initiate air entry into tailings is greater than the depth of tailings used in the pot trial, so that aeration would have been inadequate. Further, in the case of tailings resulting from fine grinding, when 10% of the volume of the wet tailings has drained to give adequate aeration, the penetration resistance is already 2-5 MPa, too high for root penetration. lt is deduced that none of the water present is available for plant growth. After coarse grinding only, there is a small amount of available water. Possible ways of improving conditions for plant growth on the tailings are briefly discussed.