The pilot reported that the flight was required to conduct an urgent medical evacuation. He was advised that the strip had been recently graded and was acceptable. The strip owner was not contacted to ascertain its current status. The strip appeared satisfactory during a pre-landing inspection. During the latter stage of the landing roll, on runway 20, the nosewheel entered an area of soft dry soil. The additional drag from the soil caused the nosewheel landing gear extension strut to fail in overload and the landing gear to collapse. Although the whole strip had been graded only the first 750 m was suitable for aircraft operations. The remaining 550 m was covered with soft soil that had been washed onto the strip during recent rains. The soil had been levelled but not removed during the grading. There were no markers on the strip to indicate the unserviceable area.