More on mushroom compost

I received a good question about mushroom compost and thought I'd go into a little more detail about it.

The question: "I've heard from other people that most of the nitrogen has been taken out by the mushrooms, so it doesn't make very good compost. Is there any truth to this?"

Thank you for your question.

First of all, before I get started, please know that I'm not an agronomist or soil specialist. I'm just a home gardener and I'm sharing what has worked for us.

I've also heard this rumor that mushroom compost is basically "dead" and that it doesn't have any nutrients left. I did some research and found out the nutrient breakdown of mushroom compost.

Mushroom compost contains, on average:
1.12 % nitrogen
0.67 % phosphate (phosphorous)
1.24 % potash (potassium)
2.29 % calcium
0.35 % magnesium
1.07 % iron

From my research (including nutrient analysis tests from area soil conservationists and the USDA), it looks like mushroom compost has plenty of nitrogen, though not as much as other things like chicken manure, soybean meal, bat guano, or commercial synthetic fertilizers. For the home gardener, mushroom compost has sufficient amounts of the necessary nutrients.

So, how much mushroom compost do you need for your garden?

The main three things you need to worry about, as a gardener, is nitrogen, phosphorous, and potassium.

Then, you need to consider how much of the nutrients your vegetables will demand.

Most vegetables are "medium feeders" and require approximately 2.2 pounds of nitrogen per 1000 square feet.

A few vegetables (like purple hull peas and other southern peas) actually "fix" nitrogen into the soil and don't require that much - only .8 pounds per 1000 square feet.

Other vegetables (like cabbage, lettuce, onions, potatoes, and tomatoes) are "heavy feeders" and require 3 pounds of nitrogen per 1000 square feet.

Mushroom compost is a fantastic way to add these nutrients to your soil.

For heavy feeders, you'll need to add 268 pounds of mushroom compost per 1000 square feet of garden area - or 26.8 lbs per 100 sq. ft. Till or mix it in to your existing garden soil.

For medium feeders (most vegetables), you'll need to add 197 pounds of mushroom compost per 1000 square feet of garden area - or 19.7 lbs. per 100 sq. ft.

For light feeders, you'll need to add 72 pounds of mushroom compost per 1000 square feet of garden area - or 7.2 lbs. per 100 sq. ft.

The average pH of mushroom compost is 6.6 (6.0 to 7.0 is an ideal range for most plants). The amount of carbon relative to nitrogen is an important indicator of nitrogen availability for plant growth, and an ideal compost should have a ratio of 30:1 or lower. Mushroom compost has an excellent 13:1 ratio, indicating outstanding nutrient availability and mature and stable organic compost.

A lot of people are also saying that mushroom compost has too much salt.

Excessive amounts of soluble salts (for example, calcium, magnesium, potassium and others) in the soil can cause injury to plants. However, research at Penn State shows that good quality mushroom compost does not contain soluble salt concentrations high enough to impede seed germination or cause damage to existing plants. Also, when mushroom compost is tilled or incorporated into the soil, the salt concentration is diluted greatly, and irrigation or natural rainfall will further reduce salt concentrations by leaching those salts from the root zone.

I know I've thrown a lot of numbers and scientific terms at you in this answer, but I've found that science and cold, hard facts tend to disprove myths. And that's just what the story about the uselessness of mushroom compost is - a myth.

Mushroom compost is just fine for your garden... nutritionally and economically.

Plus, if you buy it locally, you're doing your part to be a locavore and to help promote local business and stimulate the local economy.

I hope this helps answer your question.

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