Yeast Mannitol Agar w/ 1.5% Agar is used for cultivation, isolation and enumeration of soil microorganisms like Rhizobium species. Rhizobium are symbiotic nitrogen fixing soil microorganisms.Yeast extract serves as a good source of readily available amino acids, vitamin B complex and accessory growth factors for Rhizobia. Mannitol is the fermentable sugar alcohol source. Calcium and magnesium provide cations essential for their growth and metabolism.