I just googled leonardite shale and it's basically a source of humic acid. I'd look/as for humic acid instead. The garden shops may carry it and just get it from some source other than leonardite shale.

You might check Outside Pride or Nature's Way websites. I know liquid humic acid is a debatable subject on this forum, but it has been well-tested with positive results and I have had excellent results with my severely clay areas of lawn over the past 2-3 years. Liquid forms of HA spread with a hose end sprayer are a fraction of the cost of 50# bags.

Humic acids are formed naturally in soil by the soil bacteria digesting organic matter, get sufficient levels of organic matter into your soil and spending your money on some product that is supposed to supply what Ma Nature will would be unnecessary.