The pilot reported that he noticed that a spray nozzle was leaking on the agricultural spray airplane. Subsequently, he performed multiple test flights over a nearby field while the head of the ground crew remained on the ground to monitor the leak. He added that he landed the airplane on the grass airstrip, adjusted the nozzle, departed for another visual check, and repeated as necessary. Following the tests and adjustments, the ground crewman told the pilot that the nozzle was not leaking, and the pilot returned to land. The pilot reported that, during the landing roll, he did not see the ground crewman visually check the runway before crossing the runway in front of the airplane in an all-terrain vehicle (ATV). He added that he immediately veered the airplane to the right and sped up to avoid striking the ground crewman with the propeller. However, the left wing struck the ATV and the ground crewman. The airplane sustained substantial damage to the left wing leading edge, and the ground crewman sustained minor injuries. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation.