The amount of power lost depends mainly on soil conditions, tires, and weight on the tires. A lower tractive efficiency indicates lower drawbar power since axle power remains fairly constant at rated engine speed. The example tractor, if operated correctly, could produce approximately 112 drawbar horsepower on firm soil. The engine burned the fuel needed to produce it, and the transmission applied it to the rear axle, but 32 of the original 144 horse-power was lost at the soil surface. Power was lost to wheel slippage and rolling resistance.
Wheel slippage is simply the tire spinning.
Rolling resistance is the resistance of the tire's movement through the soil. Slippage is reduced by adding weight to the tractor, but you should be careful not to add too much weight. Slippage acts as a safety clutch for the drivetrain, limiting the torque that can be transmitted through it. Excess weight also increases rolling resistance of the tires dramatically which in turn will rob power. Both losses occur anytime a tractor is operated in the field, but they can be reduced by careful operation of the tractor.