Soil preparation is necessary for all types of vegetative slope stabilizing measures. As far as watershed rehabilitation projects are concerned, the following points should be borne in mind when carrying out site preparation, especially for plantation establishment:
- The appropriate soil preparation method should be carefully selected to ensure the establishment of fully stocked, actively growing, uniform stands at minimum cost.
- Incorrect mechanical land preparation techniques may cause serious environmental damage to land and water resources. This can be due to any one of the following:
- churning of hillsides with heavy machinery resulting in loss of topsoil, organic matter and plant nutrients;
- soil compaction leading to restricted growth because of impeded root development, poor water infiltration and percolation, poor aeration, increased run-off and erosion;
- development of undesirable non-wetable soils by slash burning or any other type of burning practice.
The following land preparation methods will be confined to brief descriptions and analysis aimed at achieving better growth and the successful establishment of plantations with minimal environmental damage. Environmental effects in fact are considered the major concern when describing land preparation methods, not only because of the emphasis given to the precautions for avoiding adverse environmental effects but also because favourable ecological changes may take place in many instances. For example, deep plowing prior to planting improves hardwood growth on compacted soils. There are indications that, for cottonwood and other poplars, ploughing to a depth of 40-50 cm improves soils with a bulk density of 1.5 gr/cubic cm or greater (Blackmon, 1977). Experiments showed that this operation increased the aeration and infiltration capacity of soil. Similarly, soil cultivation to a depth of 20-25 cm was shown to have increased the rate of infiltration during a 4-week period. During this time, no run-off or erosion was noted on a 16 percent sloping land (Balci, 1958).
Almost all land preparation techniques include various elements which are closely related to the following major operations:
- Removal of indigenous vegetative ground cover. (This vegetation usually prevents the successful establishment of a new plantation by occupying the required land, thereby creating excessive competition for essential requirements for plant life such as nutrients, moisture, and solar radiation).
- Soil cultivation methods which aim at attaining better survival and growth of the new plantation by improving the following:
- Soil moisture
- Infiltration and percolation
- Soil aeration
- Soil porosity, pore size distribution and bulk density depending on soil texture
- Workability of soil.
Since, afforestation is one of the devices used in watershed rehabilitation, a better plantation means, first of all, the successful establishment of the new vegetative cover and subsequent efficient use of the land's full capabilities. In view of this, land preparation, as a prelude to afforestation, is becoming a widely accepted practice. Such a complex operation needs to be carefully investigated by considering various factors, their interactions and the beneficial and detrimental consequences.
The following factors influencing the nature and scope of land preparation should be taken into account when planning such an operation.
The removal of existing vegetation forms a large portion of the work involved in land clearing and site preparation. This is especially true for tropical rain forests. The selection of methods and machinery depends on the characteristics of the existing vegetation which therefore should be investigated before the removal operation is carried out.
The types of-vegetative cover - degraded high forest, degraded coppice, bush vegetation, grass, weeds, single trees, or stumps - should be studied by a survey on sampling plots. This type of survey will also provide information on the type of trees, tree diameters, number of trees, crowns and roots and/or the density of other plant cover.
The land should be classified into various categories and the various topographic features such as slope classes, elevation exposition, etc. identified. The slope gradient is crucial to the efficiency of land clearing operations and soil cultivation. In mechanical site preparation, operating efficiency begins to deteriorate with gradients upwards of 8 degrees, and 12 to 14 degrees is the limit of operating efficiency for machines working on slopes. If this limit is exceeded, not only does operating efficiency drop further, but the operation also becomes dangerous (Komatsu Ltd., 1980).
Before removing vegetation or carrying out soil cultivation in land preparation
operations, representative soil profiles should be carefully studied. Both top-soil
and sub-soil should be sampled. In addition to the visual inspection of soil
profiles, physical, hydrological and chemical soil properties should be measured,
including: soil texture; bulk density; permeability; porosity and pore size
distribution; soil moisture constants and available water holding capacity;
soil drainage; soil erodibility; soil structure; soil pH; electrical conductance;
exchange capacity; salinity.
In particular, the erosional behaviour of soils on steep terrain must be studied by
measuring any of the erosion indices such as texture, structure and erodibility rate.
These include annual precipitation, its seasonal distribution, its type and frequency. Temperature and humidity levels and the number of rainy days also influence both the growth of established plantations and the selection of site preparation methods. The frequency of rain storms should be the major concern of the watershed project manager when selecting the proper method for land clearing and soil cultivation so as to minimize the subsequent erosion hazard. Data on humidity and temperature, such as averages and extremes, help in assessing and predicting the survival ratio of the plantation.
The past history of land use can provide valuable clues as to the possible environmental consequences of land preparation methods. This knowledge, therefore, can help the land-owner choose the most appropriate method.
Economic considerations play an important role in land preparation activities, particularly in the selection of operational methods. The use of mechanized techniques is increasing rapidly along with a global trend towards more intensive forest-management and afforestation practices. In general, economic considerations come into the picture in situations where the simpler traditional techniques are being replaced by more complex mechanized operations for land clearing and soil ploughing (Rennie, 1971). In land preparation operations, the initial cost of the treatment must be weighed against future returns in the form of material and functional benefits. A treatment which is too intensive may be uneconomical even though the result is an increase in volume (Balmer et al., 1976). From the view of cost-effectiveness, site preparation operations should stick to the guideline: "Do as much as necessary, but as little as possible" (Haines, et al., 1975).
These should be taken into greater consideration when carrying out mechanized land preparation works. This is particularly so, if they are a prelude to plantation for a slope stabilization project. Incorrect mechanized land clearing and ploughing on erodible soils with steep topography may result in accelerated erosion. Loss of top soil reduces site productivity and leads to a deterioration in water yield and quality. Consequently, any operation such as KG-blading, windrowing, or bulldozing should definitely be avoided on susceptible soils if it pushes top soil into windrows or causes accelerated erosion. Another adverse effect of operations involving heavy machinery is soil compaction which restricts growth.
The objectives of land preparation for afforestation may include one or more of the following points:
- Improve early soil moisture conditions by eliminating competing vegetation and excess water depending on the site characteristics.
- Facilitate planting, and establishment and boost root development by soil cultivation.
- Increase survival rate by clearing the site of existing vegetation so as to eliminate competition for light.
- Reduce erosion by proper ploughing and soil and water conservation measures.
- Improve accessibility for fire fighting and logging equipment and reduce fire hazards.
- Improve wildlife food and cover.
- Optimize financial return.
Some of these objectives may be selected and adapted for certain specific site conditions.
Site preparation methods and techniques may be divided into the following main categories of which the first two methods are described in this manual:
- Manual methods
- Mechanized methods
- Methods using draught animals
- Chemical methods.
