Mushroom Compost - Peat, foul manure, lime, partially composted wheat straw. The addition of mushroom compost reduces the bulk density of soil and reduces compaction. This helps encourage moisture infiltration and produces a soil surface structure more conducive to plant growth. The liming effect of mushroom compost is also beneficial to plant growth by increasing the availability of nutrients. PH: Mushroom compost is relatively acid. It has a pH of approximately 5.5. This means that it is suitable for use around plants, such as Rhododendrons, and Azaleas. When dug into the soil, mushroom compost is well suited for vegetable gardens.