New garden bed. Soil fill enough?

[u/Longjumping_Crab_961]

Should I put another bag down or do you think the fill is high enough?

[u/archangel_urea]

People here mention that the soil will settle and compress. That's true but you also lose soil through microbial respiration. Might be a few g per sqm and day but depends on the soil, moisture and temperature. Microorganisms use the carbon in the soil as energy source and turn it into CO2. New garden soil has lots of compost and probably around 5-10% total carbon. This is a lot of food and the respiration is quite high initially but then flattens over the next few years because first the easily degraded carbon sources are eaten up, such as sugars or cellulose. Lignin or humic acid are more stable and will remain more or less inert.

This means you need to regularly top up your garden bed. More so in the first few years and over time you get soil with more stable carbon and more resistant to microbial degradation.

Another option is to add a mulching layer such as grass clippings. This adds carbon and helps the plants grow through reduced transpiration etc.