Compost

My advice on using and making compost

Compost is any organic matter that has decomposed, including leaves, weeds, manure, kitchen scraps, ash, wood and paper. Until the advent of chemical fertilisers, larger amounts of compost and manure were always used for vegetable growing.

Since 1945 especially, more emphasis has been placed on chemical nutrients from synthetic or concentrated sources, but these fertilisers cannot provide soils with food for all its myriad of inhabitants, who are so necessary in growing healthy plants. And some of their nutrients leach away in rainfall, which is both a waste and a pollution.

Why compost, not fertiliser?

I have always felt that using fertiliser is a dangerous short cut in terms of soil health, and human health – see this recent and extensive study. Synthetic fertilisers hurt soil life, and ultimately marine life, as some of them leach away. They short circuit plant growth and are a reason for foods becoming depleted of minerals.

I rely on compost because it’s not a fertiliser in the ‘modern’ sense of the word. Instead it’s a biological stimulant, which feeds soil life and enables soil organisms to help plant roots find food and moisture. Think of it as enabler, more than a primary source of food.

Also, organic matter is carbon, and more in the soil means less in the atmosphere.

Why compost, not just a mulch of undecomposed matter?

Compost feeds soil in a slow and steady manner, allowing soil to feed plants. In gardens, a compost heap speeds up nature’s process of decomposition, resulting in less slugs than from mulches of undecomposed matter, and stronger plants.

Types of compost

Homemade compost

Of all the types of compost, homemade is the most variable and interesting. Every batch is different, thanks to seasonally-changing ingredients and conditions. An added bonus is the range of local microbes in homemade compost, fantastic for human as well as garden health. See ‘Making Compost’ below, for guidance on making your own.

Green waste compost

The name is taken from its main ingredients – garden wastes – even though in composting terms a lot of these wastes are ‘brown’ (see ‘Making Compost’ below for more info on ‘green’ and ‘brown’ ingredients).

It is made in facilities using shredders and turning-machinery, and often sold after two months of composting, perhaps less. It reaches a high temperature which gives the black colour, almost charcoal. It’s often delivered hot and if so is best left in a heap if you have time and space, to finish decomposing. I reckon to order and have it delivered in July, for spreading in November. When it arrives, the heap temperature is often 60°C/140°F, and sometimes the heap needs water as it has dried out by steaming. There are no weed seeds, and nutrient status is low to medium, quite satisfactory.

Spent mushroom compost (SPC)

This is made from straw and some stable manure, with proprietary extras. Some mushroom houses use peat as a capping layer.

The composting process takes less than a month, and the mushroom-growing period is only 20–25 days. After that the mycelia find less food, mushroom growth decreases, and the compost is ‘spent’ for mushrooms, but highly suitable for garden use, with more nutrients than green waste compost.

Again, it is often delivered ‘unripe’ and still warm. Although it can be used at this stage, I prefer to leave it 3-4 months to finish curing.

You may hear that ‘mushroom compost has a high pH’. However, I suggest not to worry about the pH because I have never found it to cause problems, even when I used mushroom compost of pH 9.5 on soil of pH 7.8. Don’t believe everything you read saying that pH has to be ‘correct’ for different crops.

Animal manures

Fresh manure is not compost, but old manure is. Fresh manure is organic matter (so far so good) but compared to compost it contains less living organisms such as fungi, and its nutrients are more water soluble.

Most manure includes animal bedding such as straw, wood shavings and shredded paper. In farming, and most gardening language, the word manure assumes bedding as being part of it.

Dung and slurry are terms for pure cow manure, often fresh and with a strong odour. Avoid confusing these terms with the word compost, because they refer to something completely different in terms of texture, nutrients, leaching, smell, and value to soil and plants.

Wood chip

Wood chip comes in so many shapes and sizes that it’s difficult to offer a simple description of its uses in the garden. As a mulch, it’s best to use wood chip for paths only, and preferably when a few months old, although it can be used when fresh.

A good time to have wood chip delivered is late summer and into autumn. This means you will receive cuttings of new wood which is greener, fresher and faster to decompose. Ideally the pieces of wood won’t all be large, as these can be very slow to decompose.

See this video, ‘5 Ways to Break Down and Use Wood Chips‘, for more detailed information.

Contaminated compost

Unfortunately, horse manure and hay are at a small risk of contamination by aminopyralid weedkiller. See here for more information.

Compost and no dig

Adding compost to the surface of no dig beds works so well to feed plants, indirectly.

Compost holds nutrients in a stable, insoluble form, so they do not leach out and are available over a long period. It is indirect fertility, made available to plant roots through a biological process. Soil organisms eat and digest organic matter, with their excretions adding to soil fertility.

Amount needed

The amount of compost needed when creating a new, no dig bed depends on the quantity/type of weeds that are present – see my web page on weeds for guidance on this.

Vegetables are hungry plants and require a soil that is well structured and full of life. First year dressings of organic matter may seem a lot but will repay the effort for years to come.

After the first year, average amounts of compost used annually are about 3 cm/1 in on beds – see this link for a depth calculator of compost. Using less compost is possible but creates more work proportionate to results, through weeds growing, less healthy growth and smaller harvests.

When to spread

Compost can be spread at any time of year. The most practical season is autumn, when soil is moist and still warm, so that worms can access it, and when harvests are finishing and soil is cleared of crop remains.

Some ground may become clear in spring after winter vegetables have finished, and can be composted then if there is no remaining organic matter on the surface.

I find that one composting a year is sufficient for two crops, once soil is in good condition. You can tell this by how healthy and vibrant your vegetables are looking.

Lumps in compost

Sowing small seeds into large lumps of compost and manure is unlikely to succeed, so keep the most crumbly and finest organic matter for your surface layer of mulch.

Break up any larger lumps, aiming for golf-ball size as the largest, then the weather can do the rest.

It also helps, when the surface organic matter is lumpy, if you knock it around in February or March with a rake, just on the surface, to smooth out the lumpy bits. This also disturbs any weed seeds that are just germinating, and they then die on top.

Glomalin

This was only discovered in 1996, by scientist Sara F. Wright while she was working for the USA Agriculture Research Service. She discovered how to extract this sticky material, which binds soil particles together, giving structure and tilth. It accounts for perhaps a quarter or more of soil carbon and exists for decades in undug/untilled soil, unlike most of soil’s short lived, non-mineral constituents.

It transpires that glomalin is almost certainly produced by mycorrhizal fungi, as Sara describes:

“We’ve seen glomalin on the outside of the hyphae, and we believe this is how the hyphae seal themselves so they can carry water and nutrients. It may also be what gives them the rigidity they need to span the air spaces between soil particles.”

During plant growth, as roots extend further into soil, fungi close to the original roots die off at the same time as new fungi colonise and work with the developing root extensions. The decaying fungi shed their glomalin, and it remains in soil as a glue-like sheath around nearby particles.

This raises the intriguing point that plant growth helps build soil organic matter, as long as soil remains undisturbed.

“In a 4-year study at the Henry A. Wallace Beltsville (Maryland) Agricultural Research Center, Wright found that glomalin levels rose each year after no-till was started. No-till refers to a modern conservation practice that uses equipment to plant seeds with no prior plowing*. This practice was developed to protect soil from erosion by keeping fields covered with crop residue.

“Glomalin went from 1.3 milligrams per gram of soil (mg/g) after the first year to 1.7 mg/g after the third. A nearby field that was plowed and planted each year had only 0.7 mg/g. In comparison, the soil under a 15-year-old buffer strip of grass had 2.7 mg/g.”

It’s reckoned that brassicas and beets do not increase glomalin levels, since they do not work with fungal threads in order to grow*. But most of our food crops, including cereals, do cooperate with fungi, and scientists are now looking at fungal encouragement as a way to reduce dependence on phosphate fertilisers.

*I doubt this. On my dig/no dig comparisons, I observe how the no dig brassicas and beetroot consistently outperform the same plantings in dug soil. I remember how in the early eighties I would read that mycorrhizal fungi were used by trees rather than vegetables. The ‘scientific’ view keeps changing because it’s a snapshot of current understandings.

Compost and fungi

The new knowledge about glomalin ties in with older work by Albert Howard ninety years ago, on the value of compost. He taught farmers his recipes developed at Indore Research Station in India, and then he discovered how small applications of compost could transform the soil of tired tea plantations, enabling plants to rediscover their vigour. Howard had trained as a chemist and initially thought of compost in terms of chemical foods such as NPK, that it was recycling nutrients.

Then the results from using it, coupled with his knowledge that nutrient levels had barely increased because he was adding so few, helped him to see compost as a broad game changer. That was when he acknowledged the role of compost and soil fungi, and the ability of compost to help fungi multiply.

For fungi to grow and multiply in a compost heap, they need fibrous (woody or stemmy) materials, and not too much heat. We see them more around the heap edges when turning, as it’s been too hot in the middle. Then they colonise heaps as cooling occurs.

At the time of Howard’s work in the 1930s, mycorrhizal fungi were being noticed and appreciated by scientists such as Dr Rayner who worked for the Forestry Commission, on Wareham Heath in Dorset.

Which brings us to the value of transforming manure and other wastes, into compost. I notice at Homeacres how crops grow better where the compost applied is fully ripe. It is dark, crumbly and the smell is sweet, not the ammonia or sulphur smells of manure stacked in an airless state.

Then to use your precious compost most effectively, the best method is surface mulching. Soil organisms are waiting, even in mild, winter weather, to eat and excrete surface organic matter, for example as wormcasts. When you give soil organisms high quality compost, the results are wonderful.

Making your own compost

I wish to encourage you to discover the fun and interest of making compost! It is a fascinating hobby and if you have never tried it, do have a go. You will be turning wastes into something valuable – see the following testimonials!

Since visiting Charles Dowding garden and seeing his composting system we have started to see our compost heaps very differently. Previously weeding, trimming, mowing seemed like chores but now these activities have become harvests of food for what we now call ‘The beast’. We gather the ‘browns and greens’ and blend them so as to satisfy the appetite of the beast and enjoy monitoring the process of decay and heating with a long probe thermometer. It’s like having a new pet to care for.

Richard Loader, Facebook – UK Here We Grow, 13.8.18

I have been collecting coffee grinds from petrol stations cafes etc. Waitrose Morrison all give it away for anyone wanting it. I used to throw away grass clippings now I am growing it for compost 😁. Before, mowing was a chore but now I am looking at it in a totally different manner.

I had a lawn of horsetail covering my plot. Being a total beginner back then, and paranoid about horsetail growing through concrete bunkers etc, we mowed the top growth and skipped the lot. Now wish I’d composted it all. Just keep an eye on the heap, you’ll get little if any regrowth – I’ve found they quickly wither and die. It all ends up back on your plot to help grow veg.

Stringfellow – Horsetail forum topic, 16.06.18

A compost heap transforms even persistent perennial weed roots into food for soil organisms and plants. Don’t believe everything you may read about what you can and cannot compost.

Compost quality

Ripeness means that the warmth of a compost heap has mostly gone, because the processing has finished. Often brandling worms arrive at this point, and heaps become wormeries of reduced quantity, but increased quality. It can take up to six months before worms appear in my heaps at Homeacres, which are too warm for worms until that point, except in winter.

Contrast this with municipal compost which looks fine and ‘finished’ after just a few weeks, from being shredded and then turned regularly. However, its blackness is from carbonisation caused by high temperatures, up to 80°C/176°F, because huge numbers of thermophilic bacteria are encouraged by the regular turning and introduction of air.

I take deliveries of such compost and measure temperatures of 60°C/140°F, even though the appearance is like compost, ie black and crumbly. I have tried spreading this compost and then planting through it, with poor results compared to when I spread it after six further months of fermentation.

You can plant/sow into green waste compost once it has cooled down and ripened. Check its heat when delivered – your supplier may have kept it for enough time that it’s ready to use.

In 2016 I invested in a shed for my composting area, to keep the rain off. In the UK, water is often changing aerobic composting to anaerobic, by excluding air. Anaerobic compost is black rather than dark brown, more smelly and less crumbly. Hence a polythene sheet over heaps is worthwhile to keep rain off – to keep air in, not for preventing leaching!

Compost ingredients – the difference between green and brown, and moisture levels

Green ingredients are soft, leafy, high in nitrogen, usually moist and low in fibre. Kitchen peelings and food wastes are mostly green. They lead to high temperatures.

Brown ingredients are fibrous, drier and more woody than leafy.

Some materials are both green and brown, and some green ingredients (such as coffee grounds and horse poo, both 3% nitrogen) look brown.

Why differentiate? When you achieve the desired balance of about 50:50, or a perhaps a little more green than brown, this contributes to a correct level of moisture, warmth and structure/aeration.

Quantities of green and brown are hard to compare. Greens are often voluminous, and browns are dense. So 50:50 means that a layer of, say, 7 cm/3 in green leaves equates in compost making value to 2.5 cm/1 in brown materials, such as old wood chip and cardboard.

In the British climate, air is often damp and so are the materials we add to the compost heap. As they decompose, their moisture becomes free to seep into the heap and if it cannot either drain out or be absorbed by drier materials, the compost becomes soggy and airless, or anaerobic. This slows or halts the process of breakdown – adding paper, soil and other brown ingredients is a remedy.

In contrast, during the dry summer of 2018, I actually watered the compost heaps. Especially when we were turning them and many dry pockets became visible. Moisture levels are hard to assess.

See  this video for more information on balancing green and brown.

• Weeds (green) include some soil (brown) on their roots, so you can make fine compost from them alone. You can compost perennial weeds too – I add roots and leaves of bindweed, docks, nettles, buttercups, dandelions and couch grass. They break down even in winter’s cooler heaps, and regrow only if left exposed to light. You can save much time by not separating out perennial weeds.
• Fresh leaves are green and older leaves become more brown, so autumn tree leaves are mostly brown. Tree leaves take up to two years to compost, or one year when added to green/nitrogenous materials such as grass. They also decompose more quickly if chopped by a rotary lawnmower.
• Diseased leaves are good to compost, such as mildewed courgette and lettuce leaves, rusty garlic and leek leaves, blighted potato and tomato leaves, and also tubers/fruits with late blight. Blight spores, for example, need living plant tissue to survive in, hence they die in a compost heap, and likewise in soil. I spread compost that was made with blighted leaves around tomatoes in the polytunnel, with no ensuing problems.
• Rhubarb leaves and citrus peel are good to compost – I know from experience.
• Eggshells bring structure to a heap but decompose slowly, often ending un-mulched on top.
• Most shredded materials are woody (brown), and their speed of composting depends on size, and whether they have been crushed or simply cut – crushed is best. I keep a pile of shredded branches near to the summer’s compost heaps, for adding to any large additions of grass mowings and fresh leaves.
• Other brown materials are paper (best crumpled), cardboard (which you can add in large pieces), wood ash (in winter my heaps are up to 10% wood ash), soil and straw, which gives good structure and aeration.
• Beware of adding too much wood-flake bedding, often kiln dried and very slow to decompose. Not the end of the world, but your finished compost risks looking woody!
• Fresh manure from any animal is green, and is excellent for speeding decomposition. Should you have large animals such as a cow or horse, their manure and bedding will ‘take over’ the compost heap, volume wise, meaning your compost heap has become more of a manure heap. Old manure is compost, just of a different quality.
• Chicken manure is unusual because of it’s high amount of nitrogen. In small amounts, say you have 6-12 chickens, I suggest adding their droppings to the compost heap, where it helps other wastes to break down. To use it as mulch, there must have been plenty of bedding such as straw, and then it needs to have decomposed for 6-8 months in an aerobic heap.

Storing compost

Choice of bin – solid or open?

A bin with plastic or wooden sides keeps materials together and increases warmth and moisture, plus you can keep rain out if there is a lid or cover. It’s said that wooden bins need slatted sides to allow entry of air but I find this makes little difference. My heaps with plywood sides make great compost – they conserve both heat and moisture. I screw them onto corner posts, then it’s simple to unscrew them when turning and emptying heaps.

Homeacres’ 7-bay system

• The posts are 15 x 15 cm/6 x 6 in of pressure treated softwood, set in 30 cm/12 in of concrete. The posts are 2.4 m/8 ft long, with about 30 cm/1 ft sawn off the back ones to create the roof slope.
• The roof is treated softwood timber with a steel cover. The rafter timber is 4 x 2 in.
• Sides were half inch plywood, but I am now using mostly planks of Douglas Fir.
• We dug the holes, and a builder erected the structure for £3k/$4k.
• Each bay is 1.7 m deep and 1.8 m wide, roughly 6 feet square, and the base is soil. So all materials sit on the base of earth.
• After filling a bay to, say, 1.5 m/5 ft high, the materials sink to half that height within six to eight weeks. Therefore each bay contains about 2.1 cubic metres/2.7 yards of compost, or 1.5 tonnes, depending on moisture content.
• The first bay we fill is no. 2, which we then turn to the right into no. 1. The second bay to fill is no. 3, which we then turn into no. 2, etc etc.

See this video in which I demonstrate my method, and the photos below showing the construction in July 2016.

Plastic bins

The ones from the council are small, which restricts the heat they can maintain.

My trial with a Rotol ‘dalek’ bin saw temperatures rarely exceed 45°C/113°F, and many weed seeds survived the process. Nonetheless it was good compost, and the sides are easy to lift off when you want it.

Soil is best as a base, for drainage, and for organisms to enter from below as heat subsides, or before it happens.

Making a heap enclosure with pallets

This can be simple, quick and cheap. The pallets do not need anchoring to the ground and we simply wire them together at the corners, top and bottom, so only two wires on each corner. My preference is to knock the bottoms of each pallet so that you just have the top frame which is lighter and easier to handle. The photos explain it, and the cardboard you see is only around the edges not underneath. It’s to prevent weeds growing in from the sides at ground level.

Building a heap

Add your garden waste as it happens, in level layers rather than a mound in the middle, to have uniform spreads of different materials as you add them. Sometimes you need ‘balancing materials’ in terms of green and brown.

In much of the growing season there is a surplus of green, especially when adding grass mowings, so keep a pile or some sacks of brown, eg paper, autumn leaves, cardboard and twiggy materials. In winter there is more brown, and some fresh manure or coffee grounds make for a good balance of green.

The photos below show a heap at Homeacres in October 2018, the year’s fifth heap.

When to stop adding more material

• Small gardens generate less material and may struggle to fill a bin, even over a whole year. Use the smallest bin you can find because a fuller, small bin makes better compost than a half-empty larger one. After perhaps a year of filling, lift off the bin to an adjacent spot and fork the undecomposed, top part into it, then use the compost in the bottom part.
• In large gardens, heaps may rise to four or five feet high within a month. Continue filling even after this for another 2–4 weeks as the heap will keep sinking. Then cover with straw/carpet/polythene, preferably polythene to keep rain out, while you make a new heap. For best results, turn the finished heap after 1–3 months and then leave for another 2–4 months.

Turning compost – is it necessary?

Turning is worthwhile for larger scale compost-makers with several heaps, to mix and aerate, and speed decomposition. At Homeacres we turn every heap once, to the right as you look at the bays. You need an empty space or bin next to the heap you are turning. The compost being finer and more even will repay the time taken to turn it.

Use a manure fork with long prongs, and be sure to shake out any dense lumps.Turning involves mixing and shaking, and also allows you to check a compost’s quality. If you discover many dry lumps, add a little water, or conversely add some dry paper if it’s soggy.

For a small heap that perhaps barely fills up in a whole year, turning is not worthwhile.

The law of diminishing returns applies to compost turning. I never do a second turn as gains are marginal, compared to one turn.

Finished compost

Within a year you should find a crumbly texture of variable quality. If there are large lumps they need breaking up with a fork while loading your wheelbarrow. A dark brown colour is better than black, which would suggest some lack of air and too much wetness.

Sieving compost before use is not worth the effort and time needed. Simply pull out larger pieces of undecomposed materials, including roots of perennial weeds which are white and noticeable. There is nothing to fear from such roots because even if you missed them while spreading, you have another chance later when you see them start to regrow. Such visibility and easy removal are advantages of no dig, with compost on the surface instead of incorporated.

• A quality of mature/ripe compost is that carbon/organic matters has been transformed into humus, now known as glomalin (see above).

Vermicompost

During 2022 I developed this method of compost-making at Homeacres. It's a wormery on the ground, which reduces the time needed to keep these worms busy and in top condition. They work best at temperatures between about 10 to 25°C/50 to 77°F. If the temperature falls lower than this, or rises higher, the worms simply go down into soil and you do not need to be moving boxes around.

I always keep the heap covered with black polythene, to retain moisture and protect worms from birds. Often under the polythene we see slow worms, Anguis fragilis.

Apparently they eat red worms but they also eat slugs! This makes a great home for them in the garden.

Once a year, we empty the excretions of all these worms, also called wormcasts. I use this in my potting mix because it has a fantastic effect on seedling and plant growth.

It's much more valuable to me in the potting mix, than simply spread on a bed.

For further advice and information, have a look at my YouTube playlist, Compost Making, and my Compost knowledge pack.

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The information on this page is available to download as a PDF.