Primary tillage largely dictates the power requirement on an arable farm. As power and
machinery costs continue to rise, it is important to provide a sound management base for the
optimum selection of tractor size. A tractor power selection programme has been developed by
examining a single operation for a single crop, namely, ploughing for cereals.
The tractor power selection programme comprises seven essential sections each of which
an be used separately and independently depending on the type of the output required. The
even major steps are the prediction of soil moisture, soil workability, soil strength,
tractor performance, plough draught and system cost and the selection of a limited number of
suitable tractor plough combinations.
For the prediction of daily fluctuations of soil moisture content, the amount of water
rained by the soil is balanced against the amount of water lost. Water added to the soil in
he form of precipitation and irrigation is lost by means of evapotranspiration, drainage
and surface run -off. Potential evaporation is calculated by means of an empirical equation
sing mean monthly air temperature and converted to actual evaporation from the soil and
transpiration from vegetation where present. Factors are incorporated to correct for the
dryness of the soil, the duration and intensity of precipitation and the stage of the crop
which is covering the soil. Daily values of drainage flux was calculated from the hydraulic
conductivicies and moisture content of the soil at saturation and field capacity and soil
moisture content prior to commencement of drying. Existing empirical procedures were utilised
to calculate run -off.
By analysing the predicted soil moisture contents, each calendar day can be assigned as suitable for farm work (a work day) or unsuitable for a given operation (non -work day). As
oil workability varies from soil to soil, machine to machine and farm manager to farm manager,
the adoption of a unique soil moisture value to differentiate between soil workability and
unworkability is unrealistic. A procedure has therefore been adopted to enable the number
if work days to be calculated at different levels of soil moisture content or workability
criteria. The data is analysed for a number of years (up to 20) and then the cumulated number
years on which a given day was a work day or a non -work day with a given workability
criterion was determined for different probability levels. This data is of direct relevance
lot only to machinery planning but also to irrigation planning and for timeliness penalty
Soil strength in terms of the cone penetrometer resistance or cone index of the soil at a given soil workability criterion level is predicted by an empirical equation containing soil
bulk density. The cone index influences the pull produced by the tractor for a given set of
tyre and deflection data and the tractor power required. It also affects the draught requirements of the plough of given dimensions, tail angle, number of bodies and depth of cut.
The cost of owning a machinery system is calculated in the form of the present annual cost,
taking into account the effect of inflation and interest rate by using discounted cash flows.
the purchase prices of tractors and implements were related to the average price per unit of
power and per unit width of plough, respectively. Crop loss or timeliness penalties through
delayed operations are also determined.
Finally, the various different ploughing systems with different sizes of tractors and
Ploughs and at different operations speeds are examined and a small number of suitable systems
are presented in a form which enables the farmer or farm manager to take into account other
critique management parameters of his particular farm business.