The Carbon Cycle:-

The amount of organic matter that will need to be added to your garden soil to maintain the organic matter content will vary. For heavy clay soils in cool wet parts of the UK 5lbs of farmyard manure per square metre is required each year (for sandy soils in dry areas this increases to 8lbs a square metre). Cows eat their food 4 times so there are no weed seeds left, so there winter deep litter is an easier option than horse manure, but if nothing else; then use a 1/2 inch depth of grassmowings on the beds each fortnight.

Unfortunately for most gardens that I see, the beds may have been weeded and nothing returned as a mulch or dug in to replace this carbon loss, which is like cutting your hair to look good and then trying to live with no food and surviving on rainfall.