What is the hardness of potassium on moh's hardness scale?

There are a number of hardness scales used in modern science. The Moh's hardness scale measures the ability of one sample to scratch another. This is still often used to identify minerals especially by amateurs. Moh's suffers from some some inadequacies, for example one crystal face may be harder than another in a given sample. Metals such as potassium are more often rated on other scales, for example Rockwell or Brinell hardness tests which measure the ability of a diamond point to indent the sample. The Wikipedia page for Potassium gives the moh's hardness as 0.4 however.