The use of coal as ameliorant for soils with constraints for plant growth

Increasing global population demands an increased intensity of use of agricultural land, but also to an extension of the use of land previously not suited for agriculture. Likewise, the stabilisation of degraded or reclaimed land e.g. from mining often requires intervention to create conditions which allow soil stabilisation by vegetation.

Mine rehabilitation is often challenged by hard setting or clay rich substrates as the available substrate for shaping the final landform and amelioration is necessary. While chemical amelioration has time limited benefits, long-term physical/mechanical property improvements may lead to better outcomes above all in water limited environments.

In this study we investigated the amelioration of clay substrate with coal with the objective to improve the physical, and potentially also chemical conditions for plant growth. Clay substrate was amended with up to 20%-wt lignite-type coal. The addition of coal occurred in two ways, as fine coal and as crushed coal, still containing aggregates. The clay and clay/coal mixtures were filled into boxes at a height of approximately 0.15m and a total volume of approx. 60 liters. Hydraulic properties were measured following consolidation of the substrate in multiple wetting-drying cycles until the substrate reached a constant height. Samples were extracted to characterise the substrates for their hydraulic and mechanical properties and their biotic activity potential. The test is based on quantifying the production of CO2 through the microbial oxidation of organic carbon compounds in the soil.

The water retention curve tests and analysis (using HYPROP) showed in general an increase of the total pore volume with increase in coal content. The amount of plant available water increased with increase in coal content and was higher with the addition of fine coal, compared to the aggregated coal. The shear strength and cohesive strength decreased with addition of coal. The microbial activity tests showed only small increases in CO₂ production and respiration, along with a low mineralisation rate of the added coal based carbon source. The results show, that the addition of a stable carbon source can be beneficial for the improvement of substrate properties.