What makes plants thrive? The obvious answer is light, water and ‘good soil’. Light is not likely to be a limiting factor on most mine sites, as there is little growing that could provide too much shade. However, water can be an issue and that brings us to the concept of ‘good soil’.
So what is a ‘good soil’? It contains a mixture of humus (more than 5 %) and mineral particles (ca. 45 %) of different sizes, so that it has a good water retention capacity without making it water-logged, it is rich in major nutrients (nitrate, phosphate, potassium) and contains micro-nutrients within a concentration range that is somewhere between deficient and toxic, i.e. just right. Its acidity is modest and into its texture, plants can anchor their roots. Soil also contains air (ca. 25 %) and water.
A key component of fertile soil is humus that gives surface soils their dark colour and also can be found in well-tended compost heaps. Humus is defined by Oxford Dictionaries as ‘The organic component of soil, formed by the decomposition of leaves and other plant material by soil micro-organisms’.
Humus provides minerals and nutrients that are readily available for plants and is important for retaining water and nutrients in soils. It also improves the physical structure of soil, which, without organic matter, would be composed solely of mineral particles of different sizes: sand, silt and clay, products of rock weathering containing minerals that are vital for plant growth, such as calcium, magnesium, iron, copper, zinc, potassium and sulfur. A diverse population of micro-organisms is important for both, the production of humus and the breaking down of rocks and making minerals available for plants to use.
Among the particles found in a good soil, clays have interesting properties: they help to retain moisture in the soil and have negatively charged surfaces, on which a lively exchange of positively charged ions (cations) takes place. Among these cations are micro-nutrients (such as manganese and copper) and protons (hydrogen ions). Protons make up soil acidity, and if that is too high, they swamp the clay particles and micro-nutrients will be leached from the soil and washed away into ground water and rivers, leaving behind a poorer soil.
Now, let us turn to the waste heaps of the metal mining industry of the 19th and early 20th century in Southwest England. There are two main types: coarse sandy material and fine silty clay. The main difference between the two with respect to plant growth is water retention, whereby the former drains too readily, the latter tends to become water-logged. Waste heaps often have exposed, relatively steep slopes and the surface material is lose and prone to erosion by wind, rain, ice, walkers and mountain bikers, so that plant roots cannot get an easy hold.
Surface trial pit in metal mine waste heaps at Devon Great Consols mine, Devon, UK. Left image: Coarse grained (sandy) mine waste; Right image: Fine grained (silty clay) tailings. Note ruler and spade blade, respectively, for scale. Photos: C Braungardt, 2010
Typically, there are low concentrations of nutrients in mine waste and the nitrate that arrives naturally through atmospheric deposition is rapidly leached away. The content of organic matter and plant material is commonly very low and as a consequence, natural soil micro-organisms and other ‘helpers’, such as earthworms, have little on which to survive. The micro-organisms that do thrive on mine waste are somewhat extreme: some of them relish the high acidity of the waste material, tolerate the high concentrations of toxic elements (such as arsenic, lead and cadmium) and gain their energy from breaking down (oxidising) minerals that formed millions of years ago under the exclusion of oxygen (metal sulfides, such as arsenopyrite and sphalerite). This process releases toxic elements at high concentrations, essential elements at toxic concentrations, as well as acid, creating an environment, in which only specialised, tolerant plants can survive.
To summarise the challenges for plants on mine spoil:
- low water retention capacity (coarse waste) or water-logging (fine waste)
- low organic matter concentration
- low nutrient concentration and nutrient retention capacity
- high acidity
- toxic concentrations of metals and other elements
- lose surface material and steep, exposed slopes prone to erosion
Yet: plants do establish themselves naturally on these sites…watch this space!
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