Abstract: Variously formulated substrates using the discards from mushroom cultivation (MCD) and peat were tested on plug trays to grow seedlings of 3 varieties of Cajanus cajan for the selection of an optimal mixing ratio for the combination medium. It was found that a mixture containing no more than 50% of MCD would promote the increases on the height and girth of the stems, as well as the accumulated dry matters of the seedlings. In the recommended substrate of 1 part of MCD and 3 parts of peat, the Cajanus cajan seedlings sustained significantly higher length and diameter of the stems, as well as dry matter and chlorophyll contents than control, which contained peat and perlite in 3 to 1 ratio.