Spent mushroom substrate (SMS) might be worth more or less than you think. According to data from Teagasc Advisory, the value of the available nutrients in SMS is estimated at €14 per ton. This valuation is based on current(2025) nutrient prices: €1.48 per kg for nitrogen (N), €2.69 per kg for phosphorus (P), and €0.95 per kg for potassium (K).
Each ton of SMS contains 8.0 kg of nitrogen, with the majority in an organic form that becomes available slowly over time. In the year of application, 20% of the nitrogen is considered available, providing 1.6 kg(€2.37) of available N per ton.
For phosphorus, SMS contains 1.5 kg per ton(€4.04). On soils with low phosphorus levels (index 1 and 2), 50% of the phosphorus is available.
Potassium content in SMS is 8.0 kg per ton(€7.6), though this can vary between 5 to 8 kg per ton due to potential leaching during storage. Despite this variability, SMS remains a reliable source of potassium.
In addition to these primary nutrients, SMS is a valuable source of micronutrients such as manganese, copper, and zinc. For example, applying 10 tons per hectare of SMS can replace the micronutrients removed by a 7.5-ton crop of spring barley. SMS also provides sulphur, with an estimated 5 to 10% of its sulphur content being plant-available, equating to 0.35 to 0.7 kg per ton.
When compared to other organic fertilizers, Spent Mushroom Compost (SMS) is among the more affordable options. While turkey manure (€69/ton) and poultry manure ( layer manure 55% DM at €43/ton and 30% DM at €26/ton, broiler/deep litter manure at €54/ton ) offer significantly higher nitrogen and phosphorus levels, their cost is also substantially greater. ) provide a balanced nutrient mix but at a higher price point than SMS. Farmyard manure (€11/ton) and dungstead manure (€8/ton) are cheaper but contain lower nutrient concentrations, making SMS a better value in terms of nutrient availability per euro spent. Liquid manures, such as pig slurry (€33/1,000 gal) and cattle slurry (€28/1,000 gal), offer good nutrient content but require different handling and application methods.
While SMS may not have the highest nutrient concentrations compared to other organic manures, it is cost-effective and a balanced nutrient profile. These nutrients are not only immediately available to plants but also in forms that will continue to release over time, creating a nutrient reserve in the soil. SMS also improves soil structure, water retention, and microbial activity, which is not accounted for in just nutrient analysis.
Information sourced from Teagasc Advisory – Mushroom E-Bulletin 1, February 2025.
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