250 Litres of Screened Compost from a Blackwall Converter and a Riddle Sieve

Fill a Blackwall Compost Converter to its 220-litre neckline in September and the base will seldom look even by the following autumn. The top third is usually still recognisable kitchen and garden waste. The middle band turns dark and matted. The bottom 60 to 80 litres is the part that has broken down to something close to finished compost.

Running the batch through a riddle sieve separates the lawn-ready fraction from the coarse material that needs another cycle.

Screening the converter

A riddle sieve is a framed wire mesh, with the opening size controlling the finished texture. A 6mm mesh produces a fine screening for overseeding and lawn levelling, where larger lumps would cover young grass. A 10mm mesh lets more material through each pass and is better suited to general soil improvement and mulching around beds.

Most garden riddles sold in the UK come with an 8mm to 12mm mesh as standard. The traditional round beechwood-framed Burgon and Ball riddle uses 9mm galvanised mesh, which sits between fine lawn work and faster general screening.

Screening a full 250-litre converter through a hand-held riddle takes most of an afternoon. The likely yield is around 90 to 140 litres of screened compost, while the rest comes out as woody pieces, undecomposed stalks, and the occasional intact avocado stone.

That reject fraction still has value. Put back at the top of the converter with a fresh layer of green material, it carries active fungal and bacterial populations into the next batch. The new heap then starts with less lag before it heats.

Compost at field capacity, damp without dripping, drops through the mesh cleanly. Waterlogged compost blocks the openings after three or four riddlefuls and has to be scraped clear by hand. Spreading the batch on a tarpaulin for a dry afternoon before sieving solves the clogging problem without extra equipment. The aim is crumbly material that breaks apart as it is rubbed through the mesh, with enough moisture left for weight and structure.

The 9mm middle ground

A 9mm sieve gives enough fineness for most domestic topdressing while avoiding the slow pace of a 6mm screen. It will still hold back woody fragments and stalks. For a converter that is being emptied once a year, that compromise matters more than a perfectly uniform sample.

Where the screened compost goes

Topdressing a lawn with screened compost is normally a spring or early autumn job. Apply 3 to 5 litres per square metre, then work it into the sward with the back of a rake or a stiff broom. A 40 square metre back lawn needs 120 to 200 litres at that rate, so one well-managed converter can cover a typical domestic lawn once a year and still leave a little material for seed drills.

In turf, the compost adds organic matter and a slow, low-concentration feed that will not scorch the grass. On sandy soils it improves water retention. On clay soils it improves drainage. Bagged synthetic lawn feed is measured in kilograms of nitrogen per hundred square metres, while screened compost releases nutrients over months as soil biology processes it. That makes compost a weak choice for a fast green-up before an event, and a strong one for the long structural health of the soil profile.

Broadcast the compost, then drag a levelling lute or a length of scaffold board across the surface. Low spots fill first and the grass blades should finish standing proud. Compost left on the leaf surface blocks light and can yellow the sward within a week.

Getting the compost below the surface

Compacted lawns do not absorb topdressing well. Compost stays on a sealed surface and washes away in the first heavy rain. Hollow tine coring, done with a manual corer or a powered aerator, removes plugs of soil 8 to 10cm deep and about 12mm across, leaving open channels for screened compost.

The standard spacing is one plug every 10 to 15cm across the whole lawn. On a 40 square metre lawn, that means somewhere north of 1,800 cores. A manual two-tine or four-tine hollow corer turns the job into heavy physical work, usually spread across a couple of sessions. Powered units from hire outfits such as HSS Hire or Speedy Services finish the coring in under an hour, though they leave the lawn covered in soil cores that either have to be collected or crumbled back into the grass.

Brush screened compost into the holes as soon as the coring is finished. The compost fills the channels and improves the rooting zone at depth, while plugs left on the surface break down and return their soil to the sward. Early autumn is the useful window because the soil is still warm, rain is more dependable, and grass roots have weeks to colonise the opened structure before winter dormancy.

Thatch back into the converter

Thatch is the layer of dead and living organic matter between the green leaf and the soil surface. Once it reaches beyond about 12mm, it stops water and feed reaching the roots. A spring-tine rake dragged hard across the lawn, or a powered scarifier set to just nick the surface, tears the layer out. A single scarification pass on a neglected 40 square metre lawn can lift 40 to 60 litres of matted brown thatch, and the lawn can look scalped afterwards.

Scarifying strips thatch off the surface. Coring pulls plugs from deeper down and relieves compaction. A lawn given both in the same autumn is opened up top and bottom, and topdressing afterwards has somewhere to settle. Reverse the order and much of the fresh compost is raked straight back out during scarification.

The lifted thatch belongs in the Blackwall converter as a brown, carbon-rich layer. Alternated with green material such as grass clippings and kitchen waste, it helps balance a heap that could otherwise turn into wet green sludge. A reliable composting mix is roughly two parts brown to one part green by volume. Heavy scarification supplies a useful slug of that brown material in spring, just as grass clippings start arriving in quantity.

Mulch mowing changes the supply again. A mower with the rear discharge blocked and a mulching blade fitted chops clippings finely enough for them to fall back into the sward, decompose within days, and return nitrogen to the soil. A lawn mown this way all season sends far less green material to the converter than one that is boxed off.

A worked volume for one household

Start with a Blackwall converter filled to 250 litres over autumn and winter. By the following September, the contents may have slumped to about 120 litres as the material decomposes and settles. Finished compost occupies less than half the volume of the fresh waste that made it.

Riddle those 120 litres through a 9mm mesh and the likely recovery is 75 to 90 litres of screened topdressing. Another 30 to 45 litres of coarse reject goes back into the converter to begin the next cycle. At 4 litres per square metre, 80 litres of screened compost topdresses 20 square metre of lawn properly, or covers the whole 40 square metre lawn at a lighter 2 litre rate.

To get 250 litres of settled material in the converter, the household has to feed in closer to 600 litres of raw waste across the year. Green kitchen and garden waste is mostly water, and much of that mass disappears as the heap breaks down. A single wheelie-bin-sized converter is comfortably fed by one household’s vegetable trimmings, grass clippings, and autumn leaf fall, though not by much more.

What the arithmetic does not settle is what to do in the years when the lawn needs less topdressing than the converter produces. One heap feeds one lawn well; where the surplus goes when the borders are already full is the question a single converter leaves open.