How Much Compost for a No Dig Bed?

How Much Compost for a No Dig Bed?

A 4 by 8 foot no dig bed needs 16 cubic feet of compost for a 6 inch first layer, about 0.59 cubic yards or 8 bags of 2 cubic feet. A 4 inch layer needs 10.67 cubic feet. After that, add about 1 inch a year, which is 2.67 cubic feet for the same bed.

Below, see the first layer at 4 inches and 6 inches, the yearly top up, the bag counts and the weight to move. Use the Compost Calculator to run your exact bed size and depth.

What a no dig bed is

A no dig bed leaves the soil below undisturbed. You do not turn the soil over. Instead, you feed the surface with compost, and soil life, worms and roots do the mixing for you over time.

To start on grass or weeds, you lay cardboard on the ground to block light, then spread compost on top and plant into it. The RHS describes the same method: a double layer of cardboard, overlapped so weeds cannot push through, then 10 to 15 cm of organic matter on top. If the bed is already clear, skip the cardboard.

How much compost for the first layer

Use the calculator formula: area = length x width x areas; volume = area x depth; bags = ceil(volume / bag size); weight approx = volume x 0.6 tonnes per cubic metre. Divide inches by 12 to get feet: 6 inches is 0.5 feet, 4 inches is 0.33 feet, and 1 inch is 0.083 feet.

The RHS first layer is 10 to 15 cm, about 4 to 6 inches. Use 6 inches on lawn or weedy ground, or where the soil below is poor. Use 4 inches on clear, healthy ground. Do not go below 4 inches on grass, because thin compost lets light through and weeds push up fast.

Bed size	4 inch layer	6 inch layer
4 by 4 feet	5.33 cubic feet, 151 litres, 4 bags of 50 litres	8 cubic feet, 227 litres, 5 bags of 50 litres
4 by 8 feet	10.67 cubic feet, 302 litres, 7 bags of 50 litres	16 cubic feet, 453 litres, 10 bags of 50 litres
10 by 10 feet	33.33 cubic feet, 944 litres, 19 bags of 50 litres	50 cubic feet, 1416 litres, 29 bags of 50 litres

Bag counts round up. With 2 cubic foot bags, a 4 by 8 foot bed at 6 inches needs 8 bags, and at 4 inches it needs 6 bags. A 10 by 10 foot bed at 6 inches needs 25 bags.

Worked example: a 4 by 8 foot bed at 6 inches

Area is 4 feet times 8 feet, which is 32 square feet. Volume is 32 times 0.5 feet, which is 16 cubic feet. That is 16 divided by 27, or 0.59 cubic yards, and 16 times 28.32, or 453 litres. In cubic metres that is 0.45 cubic metres.

Bags are 453 divided by 50, which is 9.06, so buy 10 bags of 50 litres. With 2 cubic foot bags, 16 divided by 2 is 8 bags.

Weight is 0.45 times 0.6, which is about 270 kg. Finished compost usually weighs 500 to 700 kg per cubic metre, so this load is about 225 to 315 kg, and wet compost can be heavier.

In metric, a 3 by 1.2 metre bed is 3.6 square metres. At 15 cm deep, volume is 0.54 cubic metres, or 540 litres, which is 11 bags of 50 litres. The site example checks the same way: 20 square metres at 5 cm is 1 cubic metre, or 20 bags of 50 litres, weighing about 600 kg.

How much compost each year after that

After the first year, the bed needs far less: about 1 inch a year for a no dig bed, and 1 to 2 inches a year for vegetable beds.

A 1 inch top up feeds the bed and keeps the surface covered. A 2 inch top up suits hungry vegetable beds, or a bed that has dropped a lot over winter. Do not add a full 6 inch layer every year. That would waste compost and build up excess nutrients such as phosphorus.

Bed size	1 inch top up	2 inch top up
4 by 4 feet	1.33 cubic feet, 38 litres, 1 bag of 50 litres	2.67 cubic feet, 76 litres, 2 bags of 50 litres
4 by 8 feet	2.67 cubic feet, 76 litres, 2 bags of 50 litres	5.33 cubic feet, 151 litres, 4 bags of 50 litres
10 by 10 feet	8.33 cubic feet, 236 litres, 5 bags of 50 litres	16.67 cubic feet, 472 litres, 10 bags of 50 litres

Oregon State University Extension gives the same yearly range: a quarter inch to 1 inch of compost on the surface each year for an existing bed. Add the yearly layer in late autumn or early spring, spread it on the surface and leave it there.

Bags or bulk, and the weight to move

For a single 4 by 8 foot bed, bags are usually simpler: 10 bags of 50 litres, or 8 bags of 2 cubic feet, for a 6 inch first layer.

Bulk becomes worth pricing once the total passes about 1 cubic yard, as with one 10 by 10 foot bed at 6 inches, which needs 1.85 cubic yards. Bulk needs somewhere to be tipped, and time to move it the same day.

Weight matters for transport. One cubic metre is about 600 kg, and one 10 by 10 foot bed at 6 inches is 1.42 cubic metres, or about 850 kg. A car boot can carry a few bags, not a cubic metre, so check trailer limits before you order bulk.

How to build the bed

Mow grass short and remove deep rooted perennial weeds, such as brambles, thistles, docks and nettles, because cardboard alone may not stop them.

Lay plain brown cardboard, with tape and staples removed. Overlap the sheets by at least 15 cm, double the layer where weeds are thick, and wet the cardboard well.

Spread the first layer evenly, to 6 inches on lawn or weedy ground, or 4 inches on clear ground. Level it with the flat side of a rake, water it well, and do not pack it hard, because that squeezes out air. Plant straight away. For seedlings, make a small hole and set the plant in at the same depth it was growing before. For seeds, sow into the compost surface and water gently. Roots will grow down through the compost, through the softened cardboard, and into the soil below over the first season.

What compost to use, and what to avoid

Use mature compost. It is dark, crumbly, smells like soil, and has no recognisable food or plant pieces left in it. Home made compost, council green waste compost, spent mushroom compost, or well rotted farmyard manure all work.

Avoid fresh compost that is still hot, above 35 to 40 degrees C, because that heat can damage young roots in a thick first layer. Leave it to cool for at least a month. Avoid compost with weed seeds, diseased material, or treated wood, and avoid glossy or waxed cardboard.

Common mistakes

Too thin a first layer. A 2 inch first layer on grass lets light through and weeds push up. Use 4 inches as a minimum, and 6 inches on lawn.

Gaps in the cardboard. Weeds find the smallest gap. Overlap every sheet and wet the cardboard so it stays down.

Digging the compost in. Digging breaks the soil structure that no dig is meant to protect. Spread the compost on top and leave it there.

A deep layer every year. A 6 inch layer is for the first year only. After that, 1 inch a year is enough for most beds.

Unfinished compost. Hot or fresh compost can harm seedlings. Use mature compost for the first layer.

Forgetting the weight. A 10 by 10 foot bed at 6 inches is about 850 kg of compost. That is not a car boot load.

Planting too deep. Set seedlings at the same depth they were growing before, or the stem can rot where the compost stays wet.

FAQs

Q: How much compost do I need for a 4 by 8 no dig bed?

A: For a 6 inch first layer, 16 cubic feet, about 453 litres, which is 10 bags of 50 litres or 8 bags of 2 cubic feet. For a 4 inch layer, 10.67 cubic feet, or 7 bags of 50 litres. After that, 1 inch a year is 2.67 cubic feet, or 2 bags.

Q: How deep should compost be for a no dig bed?

A: Use 10 to 15 cm, about 4 to 6 inches, for the first layer, as the RHS advises. Use 6 inches on lawn or weedy ground, and 4 inches on clear ground. After that, add about 1 inch a year.

Q: Can I plant straight into compost on cardboard?

A: Yes. Wet the cardboard, spread the compost to 4 to 6 inches, level it and water it, then plant seedlings at the same depth they were growing before, or sow seeds into the surface.

Q: How much compost do I need per square foot?

A: At 1 inch deep, 1 square foot needs 0.083 cubic feet, or about 2.36 litres. At 6 inches deep, it needs 0.5 cubic feet, or about 14.16 litres. A 1 cubic foot bag covers 12 square feet at 1 inch deep.

Q: How many bags of compost for a 10 by 10 foot bed?

A: For a 6 inch first layer, 50 cubic feet, about 1416 litres, which is 29 bags of 50 litres or 25 bags of 2 cubic feet. For a 1 inch top up, 8.33 cubic feet, or 5 bags of 50 litres.

Q: How heavy is a cubic yard of compost?

A: A cubic yard is 0.76 cubic metres. At 600 kg per cubic metre, that is about 460 kg. Finished compost usually weighs 500 to 700 kg per cubic metre, so a cubic yard weighs about 380 to 535 kg.

Get the first layer right at 4 to 6 inches, with cardboard below it where the ground is weedy. After that, 1 inch a year keeps it fed without waste. Work out your exact bed with the Compost Calculator, and browse the garden guides for more help.