The pilot reported that he had returned from a prior agricultural application flight. The helicopter chemical tank was loaded with cleaning solution. The pilot stated that he increased engine power and lifted off again. However, immediately after takeoff, the engine and rotor speed began to decrease, and the pilot selected a nearby location for a landing. He flared for touchdown about 10 feet above ground level in order to minimize the forward speed. The helicopter touched down on the rear portion of the landing skids and began to pitch forward. The pilot responded by applying aft cyclic control to prevent the helicopter from nosing over. He subsequently heard a bang and observed debris coming from the aircraft. He shut down the engine and secured the helicopter. A postaccident examination revealed that the main rotor had struck the tailboom resulting in substantial damage to the airframe. The pilot did not report any failures or malfunctions related to the helicopter that occurred prior to the accident. However, the chemical tank was determined to have been loaded with approximately 90 gallons of cleaning solution, instead of the intended 70 gallons. He commented that the aircraft may have been on a slight incline when the solution was loaded, allowing the right side tank to fill more than the left side tank. As a result, he was unaware of the overload condition. The pilot added that closer monitoring of the helicopter and chemical truck incline, and the final amount of solution being loaded onto the helicopter, might have prevented the accident.