Hi Westy,

Composted cow manure can range from .25-.15-.25 (from dairy cows) to .70-.30-.40 (from beef cattle) on nutrient values. If we get really loose with our math, that's about a 2-1-2 ratio for cow manure and 2-1-1 for steer manure. The manure from beef cattle may be higher in salts and is more likely to have weed seeds in it. The variations are due to the different types of diets the cattle are fed.

The information I could find on mushroom compost as a fertilizer (which incidentally is usually composted wheat straw, hay, corn cobs, cottonseed hulls, gypsum, and chicken manure before growing mushrooms) listed it with NPK values of 2-1-1 or 1.8-0.6-2.2, and the average pH can range from 5.8 to 7.7. These variations are due to differences in the source materials. Just like many of us have our own preferences for feeding our tomatoes, mushroom growers often have their own favorite formulas for their Substrate.

Either one would be a great fertilizer for your garden. It is slow release, won't burn the plants and adds organic matter to the soil, and as such is not likely to cause rampant foliage growth. One possible advantage to mushroom compost is that it is usually sterilized before being spawned, and sometimes afterwards.

I think the determining factor would depend on your cost and availability.

Bets

Like steer manure, some mushroom compost will have a higher salts content than others, so if you have plants that are not salt tolerant, avoid using it on them. Tomatoes are pretty salt tolerant. If you are purchasing bagged mushroom compost, what you find may be labeled as "Enriched Mushroom Compost" and it has fertilizer added to it.

Bets (again)