The pilot reported that while maneuvering during an agricultural spray flight he felt the turbine engine shudder, and then it continued to operate normally. As a precaution, the pilot discontinued dispensing fertilizer and turned the heavily loaded airplane toward the departure airstrip. Suddenly, the engine shuddered again, and the pilot opted to immediately land in a nearby pasture. All engine power was lost as the airplane approached the field. The pilot made a forced landing on the rough ground and the airplane nosed over during rollout. The operator reported that upon inspection of the airplane's fuel filter, debris or some kind of organism was observed that clogged the fuel filter. The same contaminating material was found in the airplane's fuel tanks. The operator also reported that the airplane had been refueled from a tank truck that was from out of the area.