REHABILITATION OF OPENCAST MINING SOILS

1. INTRODUCTION

Global agriculture is facing a trend in yield decline for most crops. This is specifically applicable to crops that are practised under a mono-cropping system. It is a well-known scientific fact that monoculture has a negative impact on soil fertility and potential.

With mono cropping and overuse of land, it has become necessary for farmers to resort to more drastic measures to maintain yields. One such practise is to increase N, P and K chemical fertilisers at ever increasing costs, because the perception is that the higher the fertiliser levels the higher the yield.

This same mind-set is prevalent with the rehabilitation of opencast mining areas. The impact of mining operations is just so much amplified as the whole soil profile with all the integrated soil physical, chemical and biological processes is destroyed. This is often the result of a lack of understanding that soil is a living eco-system and that there is a difference between soil fertility and plant nutrition. There is also a difference in understanding the term topsoil from a soil science and mining perspective.

A distinction must be made between restoring soils to previous inherent potential for crop production and sustainable rehabilitation. As previously mentioned soils form over a long period of time with various processes involved. The opencast mining operations totally disturb these process and soil forming factors.

It is not possible to restore the soil potential and initial characteristics to its original state but huge improvements can be made in the methodology of stripping and re-dressing of soil material to ensure sustainability of rehabilitation. Over time these soils can produce proper vegetation and grazing of cattle and arable crop production at lower yields then the initial soil potential.

To achieve this it is necessary to understand the soil forming factors and processes and the difference between soil fertility and plant nutrition.

 

2. DEFINITION OF SOIL

Soil is an open living ecosystem and can therefore be defined as a function of physical, chemical and biological processes.

 

3. SOIL FORMING PROCESSES

The following factors are involved in soil formation:

  • Parent Material (geology, e.g. sedimentary rock (sandstone), acid igneous (granite) or basic rock dolerite) etc.)
  • Topography (slope of landscape)
  • Climate (wind, water, temperature etc.)
  • Microbial Activity and microbial diversity
  • Time (soil formation occurs over a long time period, e.g. 1cm of topsoil is formed over 100yrs)

These factors with different physical, chemical and biological processes combine under specific conditions to form specific soil diagnostic horizons with a unique character and inherent soil fertility.

Photo 1: Avalon soil showing different horizons (Soil classification working group,   (1991)

Photo 1: Avalon soil showing different horizons (Soil classification working group, (1991)

4. FERTILITY / PLANT NUTRITION

Fertility refers to the inherent capacity of a soil to supply nutrients to plants in adequate amounts and in suitable proportions as well as oxygen and moisture to maintain a healthy soil bio-diversity (active micro-biology, immune system). The focus here is soil health.

Plant nutrition refers to the soils ability to supply nutrients to the plant so it can complete its reproductive cycle. The nutrient status of the soil can be manipulated by adding organic and inorganic fertilisers according to the crop’s need. The focus here is on the crop’s needs.

It can now be summarised that different soils have different levels of soil fertility according to the combination of the soil forming factors and soil processes involved under specific conditions. All these factors and processes are interlinked and no single soil type has all these factors in the ideal combination, therefore the yield potential and use of soils varies.

Unfortunately soil fertility and nutrition was relegated to a simple recipe of four elements provided through chemical fertilisers e.g. Nitrogen (N), Phosphorous (P), Potassium (K) and Zinc (Zn) to meet only the crop needs at the expense of soil fertility. Very little attention was given to the important role of bio-diversity and active microbiology in plant nutrition. It is only in the last couple of years that there is a serious interest on this matter.

 

5. THE ROLE OF BIODIVERSITY

 Active and healthy soil microbiology is able to:

  • Mineralise nitrogen, phosphorous and sulphur
  • Suppress nematodes, bacterial and fungal diseases
  • Actively decompose organic material
  • Improve root development with the result of better nutrient and water uptake
  • Recycle and keep nutrients available for plants, especially micro-nutrients
  • Improve soil physical and chemical conditions by increasing the humus content
  • Improve water holding capacity of soil
  • Less KWa power needed for soil tillage

6. MINING PRACTISES THAT CONTRIBUTE TO THE DESTRUCTION OF SOIL FERTILITY AND LOSS OF BIO-DIVERSITY

  • Incorrect stripping of topsoil. Various soil horizons with different properties are stripped together and stockpiled.
  • Stockpiling of proper topsoil with sterile or acidic subsoil (plinthic or grey clay material)
  • Long periods of stockpiling creates anaerobic conditions, resulting in a decline in microbial activity and/ or changes in bio diversity.
  • Soils are nutritionally stripped and have low microbial activity
  • Long fallow periods are as detrimental to soil health as no fallowing.
  • Incorrect soil placement with rehabilitation (plinthic and grey clay material on the soil surface), causes slaking, increasing crust formation, and compaction resulting in poor infiltration, aeration and increased run-off and erosion. These plinthic and grey clay materials are also basically sterile in terms of microbial activity.
  • Poor irrigation practises. Over irrigation causes leaching of nutrients.
  • Decline in water quality in major river systems is causing a gradual build-up of salinity and sodicity.

In most cases poor seed germination or die-back of seeded grass occur because of a combination of these factors mentioned.

The following can be done to improve soil bio-diversity and therefore sustainable rehabilitation:

  • Crop rotation
  • Fallowing and green-manuring
  • If there is not sufficient time to introduce proper fallowing or green-manuring practises compost can be applied to the soil

 

7. ECOMMENDATIONS FOR PROPER REHABILITATION OF SOILS DISTURBED BY OPENCAST MINING OPERATIONS

7.1 STRIPPING

  •  Sequential stripping of soil horizons. In some cases the A and A-Horizons can be stripped together. This has a huge practical, logistics and cost implication, but until such time that it is implemented, no improvement in sustainability of rehabilitation will occur
  • Smaller stockpiles and seeding of stockpiles with grass

7.2 LANDSCAPING AND REPLACEMENT OF SOILS

  •  It is imperative to reshape the landscape as close as possible to its original topographic features (e.g. slope and drainage lines, wetlands). Various surveying and GIS software can be used to achieve this goal
  • Where possible use the “freshest” stripped soils for redressing, as this will alleviate the soils becoming sterile or lose microbial activity
  • Place the plinthic and grey clay material in the sub-soils and the original A and A-Horizon material on top. Create an environment where the topsoil is at least 40-60cm deep for proper aeration, water-holding capacity and drainage, resulting in proper root development

7.3 SEEDING WITH GRASS SPECIES AND LEGUME CROPS

 A three stage approach can be implemented where pioneer species are planted to create a soil environment for sub-climax species. After some time climax species can be introduced. There are many case studies where reseeding is necessary because the sub-climax and climax grass species die back after the first or second season

  • Legume crops like soya, cow peas, Dolichos, or Lucerne can be introduced to improve the soils microbial activity and soil structure.
  • Compost and other organic humic substances can be used to speed up the process of restoring soil biodiversity

 

8. THE ROLE OF COMPOST AND OTHER HUMIC SUBSTANCES IN RESTORING BIODIVERSITY IN DISTURBED SOILS

Many books have been written about the role of compost in improving soil bio-diversity as well as the making of compost. It never became a standard practise in commercial agriculture for the following reasons:

  • It is bulky and transport costs did not make it viable
  • Practical problems with application
  • The value was always measured in terms of N, P and K content and in monetary terms.

Times have changed however and recent research across the world has shown that soil bio-diversity has great value in commercial agriculture and rehabilitation both from fertility as well as a plant nutrition perspective. Compost is a great and fairly quick way in restoring soil fertility although it must be made clear that it is a long term approach that is necessary. Organic and humic products can overcome to some degree the practical and logistical problems posed of importing large volumes of organic matter.

 

9. SUMMARY

  •  There is no quick fix solution to the seriously negative impact of opencast mining on high potential soils
  • Proper stripping and replacement of soils is imperative for any proper redressing and seeding with grass species to take place
  • A holistic long term, staged approach is necessary to restore physical, chemical and biological processes in the growth medium
  • Long term monitoring and relevant adjustments must be made to restore the soils to some sort of arable crop production potential to ensure future food security problems that might loom.
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