Topsoil will always contain weed seeds, it is reported that the number could be as many as 300,000 seeds per cubic metre of soil. Always prepare for weed growth, if the soil grows few weeds it will be a pleasant surprise.
Topsoil varies depending on the supply sites, Loaders always endeavours to produce the best topsoil, often blending heavy soils with light ones to produce a superior loam.
During the production of the mushroom compost lime is added to raise the pH of the new compost due to its acidic constituents. The pH of the spent compost is somewhere between 6 and 7. This is a great pH range for most plants however acid loving plants such as Camellias, Azaleas and Rhododendrons will not like it.
After the mushrooms have been harvested the spent compost is super heated to 70°C to kill fungal spores and anything else that may be present, this makes mushroom compost an excellent weed free mulch.
Mushroom compost is best used as mulch on top of the garden letting the worms break it down because freshly dug in, it can rob the soil of nitrogen and the plants can perform poorly. If spent compost is to be dug in it is recommended that a high nitrogen source such as urea is included.