Spent Mushroom Compost - A Top Garden Fertiliser and Soil Improver
I recently came across a very interesting scientific study carried out on spent mushroom compost (previously described as SMS "spent mushroom substrate").
This study was carried out to ascertain the nutrient status and particle size distribution of Spent Mushroom Compost
The report is quite technical, so I have tried to identify the "take home message" for gardeners. It does contain some very important discoveries and conclusions.
Spent mushroom compost has previously been thought of as high pH due to the inclusion of lime or gypsum in the mushroom growing process. However this study shows, across 30 samples, that the pH value of spent mushroom compost is about neutral (pH 6.6). This could be explained through many reasons, possibly the fact that only very low levels of lime are applied and that there is also a quantity of straw included, able to "soak" up or "use" the lime during the composting process.
The implications of this are huge. Many companies, including TurfandStuff.com, make blended topsoil using spent mushroom compost as the organic fertiliser in the mix. Previously, although pH samples from the finished stock appeared neutral, there has been the fear that there may be an anomaly in sampling the blended soil.
This study however goes some way to re-enforce the theory that spent mushroom compost/topsoil blends can be used successfully for a wider range of garden flowers, plants and vegetables than previously thought. We have certainly supplied these products for a broad range of uses with nothing but positive feedback
The other great piece of information for gardeners from this study is that we now have some quantities for the nutrients supplied in spent mushroom compost. The numbers, expressed in % fresh/wet weight are as follows:
Nitrogen (N) 1.1%
Phosphate (P) 0.7%
Potash (K) 1.3%
Sulphur 0.9%
This gives an equivalent NPK ratio of 11:7:13. You can now see that this product is excellently balanced, being very good for root vegetables and flowers (elevated K content). Because much of the Nitrogen is in the organic form, it has great slow release properties. This means your plants are fed evenly over a much longer period.
Spent Mushroom Compost also contains excellent quantities of many micro and trace nutrients including: Ca, Mg, Na, Fe, Mn, Cu and Zn. When added to poor or depleted garden soil, these trace elements can be very important and are often limiting on plant growth.
The Application rate would be recommended at 1” for general fertilising. This means that 1 cubic metre will feed 40 square metres
The one final conclusion, of interest to gardeners, is related to the Carbon:Nitrogen ratio. This is important as to whether an application of this product will remove nutrients from the soil to create balance. The answer is no. Spent mushroom compost has an almost ideal C:N ratio of 13:1 and will not strip nitrogen from you soil during the breakdown process.
So in conclusion, this report fully backs using spent mushroom compost as a nutrient source and soil improver, especially on soils with low organic matter status, especially sandy soils.
