Many agricultural exploitation systems cause accelerated soil degradation, leading to an imbalance in its physical, chemical, and biological characteristics, consequently affecting its productive potential. For each agricultural operation and soil condition, there is appropriate equipment to use, so that if the operation is not properly planned, it can cause undesirable effects on the soil, such as deterioration of its structure due to compaction, loss of nutrients in deeper layers, and loss of organic matter at greater depths.
Soil preparation can be defined as the physical, chemical, or biological manipulation of the soil. Among the possibilities of this operation, it is capable of performing soil decompaction, weed control, incorporation of seeds and fertilizers, and management of plant residues, optimizing conditions for seed germination and emergence, as well as seedling establishment (Knob, 2012).
Among the implements used in soil preparation operations, light disc harrows are among the most widespread among producers. These are basically intended for secondary soil preparation, which includes activities such as leveling and breaking up clods and soil fragments after primary preparation, aiming for suitable conditions for sowing (Becker et al., 2014).
For these operations, which involve pulling the harrows, agricultural tractors are used, considered the main power sources on farms. However, when used incorrectly, the tractor-implement system can increase production costs, since its sustainability is directly linked to energy efficiency and fuel economy.
Overall energy efficiency is the ratio between the energy transferred from the tractor to the implement and the equivalent energy consumption required to perform the operation. Because it is a complex system, it depends on a range of performance factors such as the engine, power transmission, and the interaction of the tires with the soil. The latter has a definitive influence on the machine's interaction with the soil, standing out as one of the most important variables in overall energy efficiency.
Still considering the relationship between the wheel and the soil, Silveira et al., (2011) state that this traction force is also influenced by soil conditions and cover. This relationship can be seen in Table 1.
Considering these aspects, it can be stated that choosing a suitable and balanced tractor-implement combination can determine the farmer's profitability in their activity, making it essential to understand some operational aspects of the machines in order to support this decision.
Regarding operation, the capacity of a machine system is one of the most important variables, and can be determined by the work rate achieved in the operation and the amount of time the machine is operated. Capacity can be defined as the amount of product (area, weight or volume) that can be handled in a given period of time (Alonço, 2011).
Therefore, the simple multiplication ratio between operating speed (expressed in km/h), nominal width (expressed in meters), and field efficiency (expressed as a decimal), divided by the constant value 10, results in the machine's operational capacity (commonly called "C") and is expressed in hectares/hour.
Field efficiency is the ratio between the machine's theoretical capacity (value referring to the machine's work at its maximum efficiency, for example, width) and its actual field capacity (value that the machine actually achieves, excluding maneuvering times, etc.). The American Society of Agricultural Engineers – ASAE – (1984) presents the reference values for this variable, as shown in Table 2.
The determination of the power available at the tractor drawbar is based on a conversion factor of 0,86 relative to the maximum power developed in the engines. Proposed by Bowers (1978), it can be expressed as:
Regarding soil resistance, according to Bowers (1978), although it is a situation evidenced and determined in practice, there are indicative values of force per meter, varying according to the type of soil, which support the decision-making for the tractor-implement combination (Table 3).
According to Alonço (2011), the power required to pull a soil preparation implement (commonly called “P” and expressed in kW) is a direct multiplication function between the traction force required for the implement (expressed in kN/m), the nominal width of the implement (expressed in meters), and the operating speed (expressed in km/h), all divided by the constant value of 3,6.
According to Silva (2005), the traction force required to pull a harrow varies depending on the type of soil, such that:
The same author also states that, on average, in soil conditions he calls soft, a harrow will consume approximately 2,7 hp per disc. In soils he calls hard, it will consume 3,4 hp per disc.
If we consider a harrow with 32 discs, 22 inches in diameter, spaced 200 mm apart, with a working width of 3,10 m and a weight of 1229 kg, according to the manufacturer the power demand will be between 89 hp and 96 hp.
Using the methodology proposed by Silva (2005), it can be observed that 86,4 hp would be needed in soft soil and 108,8 hp in hard soil:
Using Bowers' (1978) proposal and as presented below, the power required for this leveling harrow would be 53,91 hp in heavy soil and 13,47 hp for light soil, when considering a speed of 8 km/h, these being the usable power values at the bar.
However, if it is soft but heavy soil, it will require 133,25 hp or 33,30 hp:
Thus, the discrepancy between the power demanded by the harrows, as stated by the manufacturer and as evidenced by the literature, becomes evident. Therefore, it is recommended, whenever possible, to carry out a practical verification with the tractor-implement combination, seeking the best conditions for soil preparation.
*Per Eder Dornelles Pinheiro, Airton dos Santos Alonço, Luana Freitas Knierim e Gessieli Possebom (Laserg/UFSM)
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