I am in the process of preparing my garden for the coming season. I topped up the garden with 5 bags of compost. When digging it in I noticed there is a lot of white stuff coming up . Any idea what this might be and is it harmful. 

• The white substance is not harmful to humans or plants and is beneficial to the soil.
• It is soil micro-organisms called mycorrhiza which breaks down the organic matter in the compost (and soil) releasing nutrients into the soil which plants can then use.
• Organic matter is being continually broken down by soil microbes and that is why it is good to replenish the soil when replanting seasonal crops, to replace the organic matter.
• The white fungi disappear over time once exposed to air and moisture and dug into the soil. 
• It is good to be cautious when handling and using compost and potting mix, wither whether store bought or home made as they can contain a bacteria that causes Legionnaires disease. 
• Legionnaires disease has flu-like symptoms that is treated with high dose antibiotics, for healthy people it isn’t usually a concern.
• Always wear gloves and a mask when handling compost and potting mix and open bags in a well-ventilated area with good air circulation.