Lakes and wetlands in Europe and North America are characterized by high levels of calcium, in the form of calcium carbonate. Because calcium levels in Japanese lakes and wetlands are low, they are ill-suited to the cultivation of shells and mollusks. When natural water is used, hardness is a vital factor in the quality of the water; consequently waters with high concentrations of salts such as calcium and magnesium are called "hard," while those with little of these salts are called "soft." Most rivers and lakes in Japan consist of soft water.