Hi Valerie,
Composting can open up a world of inquiry. Children might have observed a compost heap, or read an information leaflet about composting (like this one: https://compostingireland.ie/wp-content/uploads/2022/09/Compost-Booklet-Web-Version.pdf)
They then might look at how shredding improves the break down of brown materials, or explore the mix of brown and green materials to see which breaks down the best.
When it comes to composting, the smaller the particle, the faster it will break down. This is because composting works from the surface of materials inwards.
This statement could be explored – how small is small enough…
If they are looking at the breakdown of materials under soil, then they should reference their understanding of the various materials, perhaps sorting them into green or brown materials and non-green/brown (like the marble).