We Compost plants not soil
One of our site Reception bins today had a pile of wet soli containing weeds left to be composted. No doubt someone was keen to get it off their plot with the minimum of effort on their part. Unfortunately, a mass of soil added to a compost bin turns it from a compost heap to a soil heap.
Soil is seldom added to the modern cold composting systems in any quantity. While adding some soil on the roots of weeds etc.is acceptable. Adding large clods or clumps should be avoided as too much soil added in one go can become compacted and reduce airflow within the heap. Another potential problem is that whatever is in that soil will go into the compost. Even if it is not compostable e.g., stones, plastic so sieving before adding it to the bin might be advisable and is not possible if the soil is attached to the roots of weeds.
It is best to work on the general rule that when the material is in the wheelbarrow it has the appearance of a mass of plants with some soil attached it is acceptable but if it appears to be a barrow of soil containing some weeds the soil needs knocking off the roots, which might involve drying it, or it may be more suited to a weed pile
However, it is the chance for a reminder[RW1] that a layer of soil, or compost, in the base of a new compost bin will help kick start the composting process; it will contain additional microbes bacteria necessary for the compost process .
If hot composting using the Indore method a thin layer (1/8 -1/4 inch thick ) of soil in a compost bin is an effective way of introducing millions of microbes will give the composting process a boost using other methods layers 3-5 inches thick can be used. Although, in both cases mature compost from an existing bin is probably more effective. Soil layers in the heap will also help absorb any fermentation products and restricts their loss to the atmosphere. Soil added as the top layer of a bin will reduce heat loss and water evaporation.
Adding larger volumes of soil to compost would offer no great benefit with some sources suggesting that the volume of soil should not exceed 5% of the volume of the heap