Most can be heard over 500 km away. But operating at high powers increases interference to other stations, so ultimately everyone loses from unlimited competition in transmitter power. As the CCIR points out, improving coverage now that the band is full of interference depends less on power increases and more on careful planning of which frequencies are used where. Energy from distant stations comes down from the sky, but most mediumwave energy reaching listeners within 50 km of a station's antenna gets to them travelling near the ground. Sea-water is the most efficient mediumwave conductor.
Swamps are good, too.
The worse soil types are desert and solid rock. Farm land is between these extremes. In general, the best place to put a mediumwave antenna is in low areas where water tends to collect. The efficiency of the antenna and ground systems also affects the range. The chart on the previous page assumed an antenna gain of 0.