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May/June 2022 | Muedanyi Ramantswana

Focus on purpose-built soil preparation equipment in re-establishment operations

Site Preparation

Soil Preparation

1. Background
Soil preparation is an important requirement prior to planting trees. Good soil preparation enables tree survival and rapid tree growth during the early stages of growth. When conducted manually, soil preparation tends to be a labour-intensive process. Over several decades, various purpose-built soil preparation technologies have been introduced to increase productivity, improve the quality of soil preparation and significantly reduce the ergonomically unfriendly aspects of manually preparing the soil. In fact, globally soil preparation is one of the silvicultural activities which has been highly mechanised. It is important to note that there are differences in the manner in which soil preparation is conducted in different parts of the world (for example, pitting, subsoiling, disc trenching, mounding and scarifying) In this article we will focus on different soil preparation methods and make inferences to equipment used. This writeup is limited to the technologies described, even though several other equipment exist (e.g. ploughing equipment) across the globe.

2. Pitting
Pitting or spot pit cultivation involves the preparation of a planting position by creating a suitable spot for the planting of a trees. Only the spot where the tree will be planted is disturbed. This process is often carried out by excavator-based or wheeled machines with auger type pitting head which drills into the soil to create the pit. The depth and the width of the pith will vary depending on the type of pitting attachment (deeper than 25cm and width >30cm). This method is advantageous because there is minimum soil disturbance and causes little erosion, however when conducted using excavator-based machine the productivity is lower compared to continuous soil preparation methods.

3. Sub soiling
This method is also commonly referred to as ripping. Sub soiling can be performed continuously or intermittently depending on the terrain. On steeper areas subsoiling (downhill) is avoided due to the high risk of soil erosion. In some cases, the subsoiling operation is combined with residue clearing with a blade in front of the carrier machine. This type of method is typically used on dryer soils. The sub soiling shank is dragged in the soil surface and it breaks up the compacted soils which enables better root growth and reduce the soil bulk density (depth can be >60cm). The shank can be mounted on a dozer, excavator and wheeled machines like tractors.

4. Scarification
Scarification is a process where the surface soil layer is disturbed mechanically to clear vegetation or residues on the soil surface. Scarification can be performed intermittently or continuously depending on the objectives and  landowners needs.

4.1 Disc trenching
One of the common ways of performing this soil preparation technique is using disc trenching equipment which can be configured in different forms (two row, three row disc trenchers). The discs are purpose built for soil preparation and the carrier machine can be a skidder or forwarder. The discs can be configured to move the soil to the desired direction required. The planting of seedlings can then be done on the scarified surface.

4.2 Mounding
Mounding involves the inversion of the topsoil layer to facilitate a planting position on top/side of the mound. Like the other methods, mounding can be done intermittently or continuously depending on the prevailing conditions. This type of method is usually effective in poorly drain sites because the seedling is planted at an elevated position. Mounding can be performed by tracked excavators and wheeled machines and the size of mounds can vary (e.g. mounds can 2m wide and 20cm high). When using the equipment, it is important to ensure that the sites do not have thick layers of slash with may hinder the creation of appropriate mounds on the sites.

                                                                                                    Source: Bracke Forest
Soil preparation methods differ across the globe and some methods have evolved over time as research provided results of using different practices. Soil preparation methods applied need to create an environment which supports survival and good early growth. Mechanised soil preparation on its own is not sufficient to yield optimal growth rates, it needs to be combined with timeous tending operations like weeding. Even though soil preparation has highly been mechanised, the are opportunities to further improve certain mechanised methods to cause less site disturbance, especially where complete soil disturbance does not yield any benefits.