Mellow Compost Lab

Different Grades of Compost

Finished compost is sieved to different grades for various purposes (house plants, seed trays, pots, veg plots mix etc..).

We class our finished compost as raw compost or grade III (Gr.III). Finished compost is best sieved when left to dry for a while (or slightly moist). If possible we leave it to dry, either in bags or preferably in a raised bed covered by a corrugated PVC panel. This way it partly dries up (less moisture) and makes it easier to screen.


 Finished compost is partly sieved to two granular sizes: 

  1. Grade II (Gr.II) using a  ½inch (12 mm) sieve

  2. Grade I (Gr.I)  using a 7mm sieve

  3. Sieved Residue (SR) is the leftover material after screening.

Most finished compost is either left as is (Gr.III) or sieved to Gr.II (depending on season and forecast usage). Gr.I is prepared early in the season for seed trays and house plants. I normally filter Gr.I on demand. 

The success in sifting and quality of compost grades depend on:

  • How moist/dry is the compost

  • Distribution of waste sources. The mix of kitchen waste, garden waste, boxwood and grass matters: The more boxwood and shrubs in the initial waste sources, the more small sticks at the end. Some are slow release carbon and may need more than one compost cycle (for Gr.II or better composting); but will be absolutely fine for mulching or compost of medium to large plants or established plants or vegetables. Grass is a great nitrogen rich element and fast releaser; but is quite compact so would need good brown elements (such as bite size sticks & twigs) to get in between.

  • Compost maturity; the longer the compost stays in the mature cycle, the better Gr.I/II compost

  • C:N ratio: If the C:N ratio is right, there will be less leaf mould residue partially decomposed


Sieved compost and grade screeners
Sieved compost and grade screeners
Sieved Residue
Sieved Residue

Purpose & usage

  •  Part of Gr.III is kept and used as soil enhancer around shrubs, (fruit) trees and larger garden plants. It is also used for top-up in raised beds for large veg plants like potatoes, Jerusalem artichokes, broad beans and courgettes (where the plant is either strong or seedlings being transplanted).

  • Gr.II is mainly used for outdoor repotting   and  top-up in raised beds for all sorts of vegetables; in particular where seeds will be planted. Gr.III would be too coarse for seed planting whereas Gr.II would be fine enough for seedlings to break through.

  • Gr.I normally reserved for seed trays or baby home plants. 

  • SR (Sieved Residue) normally contains small twigs, sticks, small wooden parts and also fruit & veg cores (e.g. apricot, peaches, avocados & mangoes) and part of  sea and nut shells. Typical usage of SR:

    • For mulching shrubs and large pots  around our garden (in part to prevent crust and moss developing) and retaining moisture

    •  As the bottom layer of large pots (for drainage).Some plants like Orchids family like soil mix of this nature. 

    • Input Waste air separator: Many items in the SR would naturally require a second cycle to decompose better. So part of SR is re-used in the subsequent compost cycle (also contributing to aerating a compost pile). Fruit & veg cores are further split using sharp pruning clippers; this helps the worms get into them.

    • As part of the mix for Hügelkultur (HK) sublayer of our raised beds 

We sometimes find other bits in our SR that did not decompose, such as  bits of plastic, stones, tea bags, yogourt tops, stones, small cutleries, etc… Despite our best endeavour to have a separate bin in the house for compost, mistakes.

The SR could also have residual leaves (my main brown source); when for example the bin cycle was on a higher brown/green ratio.