After an open fall and particularly after a dry season I get many questions about organic matter additions to the soil. Growers have driven around the neighbourhood all summer and noticed how some fields fared better in the dry conditions than others.
Most often, this can be related to a field history of added organic matter and a good crop rotation.
There are several different organic matter sources available to growers. One that is often overlooked is spent mushroom substrate. There are always lots of questions surrounding the use of this product.
Let's start with - What is it?
It is a by-product of mushroom production and is produced in large quantities in Ontario year round.
Mushrooms are grown on a composted and pasteurized mixture of horse manure, straw, chicken manure and gypsum that is laid out in beds in a mushroom house. This substrate is then inoculated with the mushroom fungus and covered in a mixture of soil, peat moss and lime. The environmental conditions in the growth rooms are then manipulated to encourage fungal mycelium growth and produce mushrooms. After several pickings, the substrate is exhausted or "spent" of its potential to produce mushrooms. The substrate is then subjected to a steam treatment and removed from the growing houses.
What to look for?
It can be available fresh from the mushroom farm, or weathered by further composting. Both types of spent mushroom substrate should have an earthy odour and be free of an ammonia or rotten egg sulfur smell. The particle size should be uniform and it should be fairly homogenous and resemble soil.
Moisture contents between 30 and 50 % are easiest to evenly field apply and incorporate.
All SMS should arrive at your farm with an analysis. If not have an analysis done. Ask for a "Compost Analysis Test" from the labs, not a manure analysis. This gives you macronutrient content, carbon to nitrogen ratios, dry matter, pH, and total salts. You may want to add an additional test for nitrates.
What is the nutrient content/or value of Spent Mushroom Substrate?
The nutrient content varies with different sources.
A typical analysis has:
Dry Matter between 40-60
% pH between 6-8
Carbon to nitrogen ratio that is generally below 30:1.
Which means it will give up nitrogen rather than tie up available nitrogen.
A 10 ton/ac fall application typically gives 80 lb/ac of nitrogen, 105 lbs/ac of Phosphorus and 175 lbs/ac of potash.
The nitrogen is about the same total value as dairy manure; only more of the nitrogen is in the organic form and is more slowly released over time. Studies in vegetables in the US have shown that there is less nitrogen leaching from spent mushroom substrate than from commercial fertilizers. As for the phosphorus and potash values, they are actually higher per ton than raw dairy manure. This is not unexpected since the composting process tends to concentrate mineral nutrients.
Not to be overlooked are the micronutrients found in spent mushroom substrate. It contains appreciable amounts of iron, manganese, zinc and boron.
Now, What About Those Salts?
The compost analysis will also give you a measure of the total salts in spent mushroom substrate. Studies in Ontario by Dr. Calvin Chong have shown that a 50:50 mix of spent mushroom substrate and bark chips that is properly managed will have no appreciable salts left 1 week after transplanting nursery stock. When added to the entire soil and incorporated the salts are not high enough to cause root damage. Banding spent mushroom substrate with transplants is probably not a good idea. If there is concern over the salts then a fall application is ideal. It won't take much rain to reduce the salt load in the root zone.
Spent mushroom substrate is a high nutrient, low odour, and weed free way to incorporate organic matter into your soil, reduce some nitrogen leaching potential in vulnerable areas and provide both macro and micronutrients to your crop.