Morel mycelium is good for testing microbial nutrition; and the optimization requirements would be similar for most microbes. As the mycelium grows over an agar surface, it is in continuous contact with new media which is not altered by depletion or byproducts. The rate of mycelial extension is easy to determine. It will differentiate between different brands of dextrose based on contaminants.

With agar gel, pH indicators can be added showing acid accumulation and residual autolysis of morel mycelium as alkalinity. The morel excretes acid to feed on bacteria, which kills the mycelium under artificial conditions. In the wild, the mycelium spreads so thinly through sandy soil that the mycelium can tolerate more acid then the bacteria.

The most broadly applicable, complete and chemically defined growing medium for microbes is Wickerham's basal medium for yeasts. It can be used for most microbes. The plant physiology medium is similar but contains about twenty times as much iron which is hard to dissolve and manage.

Handling the basal media for quantitative exactness is demanding. Simply heat sterilizing it batched is not quantitative, as precipitation occurs. Plastic containers must be avoided, as plastic is porous and absorbs molecules which accumulate and change upon heating. Complex nutrients such as amino acids must be filter sterilized, because heat sensitivity has usually not been determined and unknown contaminants could react upon heating.

It is therefore not easy to put together quantitatively definable media. Concentrations of components must be as high as possible so they can be mixed together after sterilizing. Generally, the minerals can be heat sterilized together except for phosphate, which can be heat sterilized separately. Presterilized nutrients must be covered with aluminum foil to keep contaminants away from lids.

Optimum nitrogen for morel mycelium is 1.7 g/l ±0.3. It would probably be quite similar for most microbes. The best source of nitrogen is casein hydrolysate, which must be filter sterilized. The best amino acid is glutamine, since it is a hub amino acid for other amino acids. Arginine has a lot of nitrogen; but it is metabolized quite slowly.

Here's a list of percent nitrogen in possible sources:

Three examples:
casein=0.4%, gln=0.2%, urea=0.2% = 0.183% TN (total nitrogen)
urea=0.2%, gln=0.1%, casein=0.2%, Arg=0.1%, NHcl=0.05% = 0.178% TN
urea=0.2%, gln=0.2%, Arg=0.2% = 0.184% TN

Urea and ammonium chloride are marvelous sources of nitrogen being high in energy and easily metabolized, except that they cannot be used in even moderate quantities, because they create endogenous alkalinity and exogenous acidity.

Nitrate is such a poor source of nitrogen that it should not normally be used as a nutrient. It is low in energy and hard to metabolize, requiring three enzymes. Microbes repress those enzymes and only metabolize nitrate when all other sources of nitrogen are used up. Then nitrate utilization leaves a highly alkaline exogenous medium which becomes inhibitory to growth.