Hard coal-mining spoil forms the parent material for soil genesis on five permanent plots. Differences among these sites are mainly due to varying gravel and stone (skeleton) contents. With its strong compacted layers of brick debris and a mixture of building rubble, the plot SA differs greatly from all others.
The soil classified as “skeleton-humus soil” on the pioneer sites PA and PZ consists of coarse mining spoil over loess with initial humus accumulation in cavities. A humus topsoil has not developed yet. Mining spoil of younger fills is often compacted. Shallow stagnic soils develop which have temporary wet topsoils. This is a characteristic feature of site PA.
On the shrub sites, a Syrosem-Pararendzina from building rubble over loess was found at site SA and a Syrosem-Regosol from hard coal-mining spoil over loess was found at site SR. “Syrosem” indicates an incipient soil development of raw soils. Due to the rapid soil development by humus accumulation in the humid climate, the soils are already in transition stages to Pararendzina and Regosol from material containing or lacking carbonates respectively. Different thicknesses of the humus horizons (Ah) have been recorded for the two areas.
The two woodland locations show Syrosem and Regosol of hard coalmining spoil over loess (WR) or a Regosol of hard coal-mining spoil over loess (WZ). The two sites differ mainly in their stage of soil development. The soil development has not yet proceeded on WR due to erosion on its steep slopes. The development of a humus topsoil has been greatly hindered by this. Only small areas of vegetation provide for local soil accumulation and therefore conditions for humus formation. The site WZ, in contrast, is located in a flat area and 20-cm-thick organic layers (L, Of, Oh) and topsoil horizons (Ah) have developed in places. Thick, raw humus layers indicate an acid soil. Old fills of hard coal-mining spoil, in contrast to the younger ones, are usually loosely packed. Site WZ provides a good example of uncompacted soils developing from hard coal-mining spoil.
The investigated soils are at the beginning of their development. They are very gravelly and stony (skeletic). Down to a depth of more than 1 m, the parent material frequently consists of more than 80% skeleton. Therefore the initial stages of soil development on hard coal-mining spoil
strongly restrict root growth. Weathering of skeleton to fine-earth particles is in an initial stage. Therefore the fine-earth content is low. Fine earth supplies plants with nutrients and water. Due to the limited fine earth content only small amounts of nutrients and water can be stored and are, therefore, less available for plants. The nutrient status must be classified as poor with regard to magnesium, phosphate and potassium, except for site WZ. The low fine-earth content also reduces the cation exchange capacity, which is usually very low. The pH values (Fig. 2) on old hard coal-mining spoil are mostly very low (less than pH 4.0; Burghardt 1989). Hard coalmining spoil that is only a few years old and building rubble material, however, have a moderate alkaline pH value of 7.1-9.0.
Salt washouts can be traced in the decline of electrical conductivity of the soil-water suspension after 1 year at site PA. In 1999, we measured approximately 290-330 pS/cm in the upper 10 cm layer; 1 year later the electrical conductivity dropped to approximately 250-260 pS/cm. This is the “advance notice” of acidification, which usually starts immediately after the washout of the salts.
The heavy-metal concentrations of the fine-earth fraction are mostly slightly elevated. In the raw humus layer at site WZ, 116 mg of copper per kg were measured. Levels of zinc above 150 mg/kg can often be measured in the topsoil. Lead content frequently lies between 76 and 552 mg/kg in the fine-earth material. Nickel and cadmium, however, do not show elevated results. It is striking that plant-available and water-soluble (ammonium-nitrate extract) lead occurs at slightly elevated levels of more than 0.1 mg/kg.