Soil Tillage

Generally, tillage is defined by the type of activity carried out (Friedrich, 2000):

• Inversion: this type of tillage turns over the soil in the part that is worked. Surface layers are completely incorporated and deeper soil layers are brought to the surface. The argument that ploughing controls weeds is not valid when done every year, as the same amount of weed seeds is brought to the surface.
• Mixture: this operation mixes all materials more or less homogeneously to a certain soil depth, which is usually around 10 cm.
• Ripping: this type of tillage breaks the soil open, in order to loosen the soil without moving the clods, for instance in soil de-compaction operations (subsoiling).
• Pulverize: this operation crumbles the soil clods in order to form a very fine horizon, i.e. the seedbed. It is executed within a few centimetres from the surface.

Both inversion and aggressive mixing affect the quantity of residues that are left on the soil surface. Ploughs and discs turn over the soil completely, whereas chisel ploughs break and mix the soil and cultivators only mix. Harrows pulverize the soil in order to prepare a seedbed. At least four types of tillage operations can be distinguished in a conventional system (Krause et al., 1984):

• Clearing the land and management of residues, including burning fallow vegetation or residues.
• Primary tillage.
• Secondary tillage.
• Crop management activities, like weeding, ridging, breaking crusts, etc.

But sometimes deep tillage or subsoiling is necessary to break dense and compacted layers deeper in the profile.

In tropical and subtropical areas, where the danger of erosion through rainfall is high, the soils are usually poor and eroded and the temperatures are high and thus decomposition is rapid, tillage systems are usually selected with the objective of preparing the topsoil to create a very fine seedbed. And in only attending this objective, tillage systems are applied that bring certain degradation processes with them (Figure 1) (Vieira, 1996).

The type and number of land preparation operations determine the quantity of residues left on the soil surface. For example, ploughing leaves less than 15% on the surface, and a cultivator leaves between 50 and 70% of the residues intact on the surface.

Table 1 Percentage of residues left on the surface with different land preparation activities

Therefore it is important to choose land preparation practices that protect the natural resources and at the same time improve productivity and reduce production costs. In conservation agriculture systems, land preparation practices are reduced to almost no tillage at all.

Zero tillage or no-till practices are those activities in which the seeds are placed into the soil with the least soil disturbance possible. That means planting and sowing into the residues of previous crops and weeds. Therefore, farmers, extensionists and researchers have been developing not only instruments and equipment to seed into the residues, but also tools and implements to manage the crop residues and fallow vegetation.

These tools and implements have been developed for three levels of power usage:

• manual power (hand tillage)
• animal traction
• motorised equipment.