New compost is formed including crushed lime bought in bulk prior to each new batch of mushrooms to be cropped. The lime particles vary in size. The particles remain in the compost throughout each new batch use. The lime is part of the composting process, the compost bulk is straw and the mushroom mycelium requires warmth to activate its initial growth after sowing into the new compost batch for even mycelium distribution. After use the commercial mushroom compost becomes and industrial waste product and is sold as a soil compost and conditioner.

Regular applications of new purchased compost cause the pH level of the soil to climb for instance a neutral pH level of 7.0 in a clay soil might be raised to an unacceptable level limiting the varieties of plant capable of growth in that soil.

In Bath, Somerset, a commercial nursery which has used regular applications of spent compost for years found the level had risen to over 8.0. This resulted in most plants being unable to grow. Since soil removal and replacement was uneconomic the business closed.

Some clay soils have a acid pH level because a high water table flushes out the lime. Land with a high water table is limited in what can be grown irrespective of the lime content.

If regular applications of mushroom compost are contemplated a yearly soil test is advised. Leave one area without compost application to act as a "control" area so as to have an original pre application figure for comparison purposes.

Soil pH levels:
High (alkaline) 7.5, 8.0.
Neutral             7.0.
Low (acidic)     4.0-6.5.