It also grows on sandy soil on river beds ,wich is light in color.
Sprinkling somme charcoal over the mulch might work.
Charcoal not lighted off course
and not ash ,because that washes away while the charcoal can last for houndreds of years.
The char,because it contains a little ash on it,its slightly alkaline at first ,but you can wash it with somme water ( preferably in the process of making the char ).
You make a fire from twigs and at the end when( when theres no flame) only the red coals remain,you estinguish those with water and you just made yourself somme charcoal.