The open-cast method of coal mining causes the disturbance of the land surface of the coal-bed and its adjacent territories. The damaged lands and hence destroyed soil cover in the region of Maritsa-Iztok coal basin, South Bulgaria, are about 18000 ha and trend to grow at the final stage of the coal-bed exploitation. The damaged lands are represented by geological material poor in nutrient elements and without humus content. They are easy compacted due to the low water stability of the aggregates. This is shown by high bulk density of the geological spoil at moistening, low total (37-38%) and aeration (6-10%) porosity of the 0-30 cm layer. The presence of 50% silt and clay explains the relatively good water holding capacity and available waterholding capacity of the layer of the geological spoil. Their usage by plants is probably restricted by compact structure. The conclusion is that they have unfavorable physical properties for plant growth. For the melioration of these geological materials different ecological useful methods are experimented utilizing industrial (coal ashes from thermo-electrical power station) and agricultural (sludge from swine-breeding farm) waste products. The variant without waste product is marked as “control”. The five years crop rotation (barley/forage peas/winter wheat/millet/colza) have been realized in all variants. As a meliorative material the coal-ashes penetrate in the cracks of the geological materials and thus diminish susceptibility to compaction of the substrates. The results of single application of coal-ashes (15% per weight) in 0-30 cm layer of geological spoil is effective less than the combined application with organic sludge (15 % per weight). The coal-ashes single application lead to diminishing of the particle density from 2.65 to 2.60, and the bulk density of the substrate by 0.2 g/cm3 versus 0.5 g/cm3 at combined application and increasing the porosity with 6% v/v versus 16% v/v. The combined application of coal-ashes and sludge also significantly diminishes the tendency of the new substrate to form the agronomically unfavorable cloddy structure. In regard to the organic matter content of the arable layer of the meliorated materials it could be said that it was achieved more significant effect – increasing up to 1.3% at the end of the five year investigation. If it is supposed that the new formed humus is a result of humification of the whole organic matter of the crop residues and of the applied sewage sludge, the soil aggregates should become water-stable. Such effect is not found (Figure 1). This is proved by the low values (0.25-0.35) of the ratio (MWDR) of the mean weighed diameters of aggregates after and before wet sieving at the fifth year of the meliorative experiment. The statement in some publications (Vershinin, 1958; Grorbunov, 1957, Giovannini and Sequi, 1976 etc.) that not the whole humus content but mainly the fraction of the humic acids contribute to bounding the soil particles in water-stable aggregates provokes the idea to investigate the fractions of the newly formed humus in the studied variants. The data show that only 20% of the total humus content are humic acids. It could be concluded that the main reason of absence of positive effect of the humus increase on the water stability of the aggregates is the low humification of the organic matter of geological material. It should be reminded that the humification is a slow process of complex transformation of organic residues in compounds with new composition and properties. This suggests that the biological recultivation with application of the waste products – coal ashes and swine sludge is effective, practically accessible, but a slow method for recovery of the water stability of soil structure of lands destroyed by coal mining.

Figure 1. Effect of humic acids content on water stability of soil aggregates expressed by MWDR in studied meliorative variants