We used to prepare the plain M9 medium as follows:
|900 ml sterile water|
|100 ml M9 salts stock solution|
|1 ml autoclaved 1 M MgSO4|
|0.1 ml autoclaved 1 M CaCl2|
M9 stock solution (10x) is:
|60 g Na2HPO4 x 7 H2O|
|30 g KH2PO4|
|5 g NaCl|
|10 g NH4Cl|
|add water to 1000 ml and autoclave|
Note 1: salts that are added last often tend to precipitate. I didn't bother about it too much as obviously it didn't influence bacterial growth. There might be a way to avoid precipitation (eventually adding magnesium and calcium first) but I didn't investigate this in detail.
Note 2: The M9 stock solution here is essentially as in the M9 salts recipe just that the concentration is about half of that. Nevertheless, bacteria are normally OK with this.