The pilot of an agricultural airplane reported that he planned to apply an herbicide and test the spray system at maximum pressure at a specified airspeed. He added that a ground tractor was also in the field applying the same chemical. During the flight, he saw the tractor and passed over it between about 50 and 100 ft above ground level (agl). While spraying at 10 ft agl, he noted pressure, output, airspeed, and ground speed. At the end of the field, he climbed and made a left, 180° turn for another pass. The pilot reported that he "lost focus" outside of the airplane while looking at the spray results, and when he noticed the tractor, it was too late; the airplane's wheel collided with the tractor. Examination of the airplane revealed substantial damage to the fuselage. The tractor driver reported that he heard someone call him on the radio and that he answered the pilot on the radio and saw the airplane coming "directly at him, very low." The tractor driver heard and felt the impact and was knocked from his seat by the collision. The tractor driver then called his supervisor for help. The tractor driver added that his supervisor and another ground sprayer had been "buzzed" by the pilot before.