Reclamation Procedures

Methods of reclamation have to be tailored to the specific problems at hand. However, there are some principles that are common to all reclamation procedures. Figure 4.7 shows the steps and the considerations that are necessary in the design of a suitable reclamation scheme. We have to bring back to full life an ecosystem in which normal biological processes are at a standstill. The natural development of such an ecosystem normally takes a long time, and so, the key to improvement is upgrading all of its activities by the input of materials that the soil has in short supply. This can be done directly with nutrients and/or fertilizers, or indirectly with organic matter built up by the growth of plants.

Once the soil is charged adequately, it has the nutrients and the physical properties that allow plants and soil organisms to grow and develop. The cycling of materials can begin and a self-sustaining ecosystem results. The soil structure also improves with the input of organic matter and the concurrent increase in the activities of soil microorganisms.

The most common techniques are topsoil conservation, topsoil application, application of other materials, hydraulic seeding, and revegetation.

Topsoil Conservation

If a land is expected to be disturbed by anthropogenic activities, the fertile topsoil and sometimes the subsoil have to be removed, stored, and replaced later when the disturbance has finished. This technique is suitable mainly for progressive, fast-moving, strip-mining operations taking a relatively thin layer of material, such as coal, bauxite, or sand. Generally, if the procedure is carried out adequately, the topsoil loses very little of its original properties and, provided that care is taken to prevent consolidation, crops and other vegetation can be reestablished immediately [26].

Steps Become Agricultural Engineer
Figure 4.7. Steps involved in the development of a soil reclamation scheme. Source: Modified from [1].

In many sites, however, the depth of work may be considerable and/or the area for storage small. In these cases the topsoil has to be stripped at the outset, stored in large heaps, and replaced later. This procedure tends to cause a deterioration in the topsoil structure and some loss of fertility. Therefore, the topsoil has to be removed and replaced carefully, preferably during dry conditions, without being mixed with other materials if the natural processes are to be restored quickly.

Topsoil Application

Sometimes, it may be possible to remove the top layers of a good soil from the site and spread it over an another area where the soil has been degraded. This procedure may provide a fully developed and fertile soil of good structure and texture. Generally, the establishment and growth of plants when topsoil is used as a covering material are satisfactory, usually with fertilizer addition. However, there always will be a limit to the depth of topsoil that can be applied. The underlying layers will be part of the rooting zone of the plants, and their characteristics, therefore, cannot be disregarded totally [27]. If they are toxic or impermeable, the roots will be confined to the topsoil. Whereas impermeability of the underlying material can be ameliorated by ripping operations, problems of toxicity are more difficult to overcome. Moreover, some wastes, such as chemical refuse, are not only toxic in themselves but can release substances that will poison added soil above. In these cases, a deeper layer of soil will be necessary as a covering.

On the whole, topsoil application is recommended in small sites where high quality and rapid solutions are required and when it is necessary to restore the original ecosystem as faithfully as possible.

Application of Other Materials

This technique is based on the idea of using some waste products to deal with the problems caused by others [1]. As a matter of fact, there are many waste products that are good media for plant growth, such as sewage sludge, mushroom compost, farmyard manure and pig slurry, domestic refuse, fuel ash, and mining and chemical wastes. These materials, notwithstanding their different origins, are similar in the sense of being non-toxic and water-and nutrient-retaining.

Hydraulic Seeding

Hydraulic seeding or hydro-mulching is a technique which consists of spraying seeds and nutrients over the ground in the form of a slurry. Normally the seed is suspended in an aqueous solution that can be sprayed over distances of as much as fifty meters or more, from a machine that does not have to pass over the ground. When the seed reaches the ground surface it is carried into crevices by the liquid. To keep the solution properly mixed, starch derivatives or oil-based emulsions are added to increase the viscosity of the mixture. These substances also play an important role in improving the adhesion of the seeds to the soil when they hit the ground. Adequate nutrients—provided either by fertilizer or by a liquid manure—must also be added to prevent the young plants from failing to establish due to unfavorable soil conditions.

Normally, not more than 200 kg/ha of a compound fertilizer, containing about 30 kg/ha of nitrogen, phosphorus and potassium, can be added to the mixture containing

Table 4.6. Hydroseeding indicative application rates of materials suitable for the establishment of grass—legume cover in temperate climates

Material

Application Rate

Grass-seed mixture appropriate for situation Wild white clover or other inoculated legumes Mulch: wood fiber, chopped straw, or glass wool Stabilizer: alginate, PVA, or latex Fertilizer: complete 15:15:15

Depends on stabilizer 200 kg/ha, followed by 300 kg/ha after 8 weeks 500 kg/ha 0-5,000 kg/ha

Dry organic manure Lime

the seed. Table 4.6 gives indicative application rates of some materials suitable for the establishment of grass/legume cover in temperate climates. This technique is quite expensive and not yet totally reliable for all situations. On the whole, it is cost effective mainly on steep, less accessible slopes or where there is a serious erosion problem requiring the use of a soil-binding material [28].

Re-vegetation

One of the most common components of all reclamation schemes is the establishment of vegetation. Soils drastically disturbed by anthropogenic activities are often difficult to revegetate because the soil layers have been compacted to the extent that plant roots are unable to penetrate them. Essential plant nutrients may not be present in an available form or in properly balanced proportions. Nitrogen is invariably deficient and phosphorus is generally in short supply. Moreover, the sites may be too dry or too wet. In spite of these problems, considerable success has been attained in vegetating degraded soils on mining dumps and air fields, along highways and pipelines, or around buildings. The first step is normally the establishment of a grass cover, usually containing legumes. Legumes, in fact, are of paramount importance in almost all grass mixtures because they provide and maintain an adequate supply of nitrogen and ensure the build up of an suitable amount of organic matter in the newly forming soil. The choice of the proper legumes depends on soil conditions and on climate. Generally, the most valuable legumes are those which are used in agriculture since they have high rates of nitrogen fixation. A list of these plants on a world-wide basis is given in Table 4.7.

An alternative to sowing grass and legumes is to establish trees or shrubs. These can be planted on ground where agriculture is impossible, such as steep-sided heaps and mining loads. However, they are less effective than grasses in stabilizing soils against erosion. To overcome this problem, because trees and shrubs are established at low density and a lot of surface is left unprotected, a grassy ground cover (grass-legume mixture or pure legume) may play an important role.

In any case, the establishment of either perennial vegetation or cultivated crops represents the final step in the development of any successful derelict soil reclamation procedure.

Table 4.7. Perennial legumes suitable for derelict soils

Soil

Climate

Legume Species

Preference^

Preference

Amorpha fruticosa (indigo bush)

NC

W

Centrosemapubescens (centro)

AN

W

Coronilla varia (crown vetch)

AN

CW

Desmodium uncinatum (silver leaf desmodium)

AN

W

Lathyrus sylvestris (mat peavine)

NC

W

Lespedeza bicolor (lespedeza)

AN

W

Lespedeza cuneata (sericea lespedeza)

AN

W

Lespedeza japonica (Japanese lespedeza)

AN

W

Lotus corniculatus (birdsfoot trefoil)

NC

CW

Lupinus arboreus (tree lupin)

ANC

CW

Medicago sativa (lucerne [alfalfa])

NC

CW

Melilotus alba (sweet clover [white]

ANC

CW

Melilotus officinalis (sweet clover [yellow]

ANC

W

Phaseolus atropurpureus (siratro)

ANC

W

Stylosanthes humilis (Townsville stylo)

AN

W

Trifolium hybridum (alsike clover)

ANC

C

Trifolium pratense (red clover)

NC

C

Trifolium repens (white clover)

NC

CW

Ulex europaeus (gorse)

ANC

C

b a A — acid, C — calcareous, N — neutral. b C — cool, W — warm. Source: [1].

a A — acid, C — calcareous, N — neutral. b C — cool, W — warm. Source: [1].

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