Defining compost

Compost is organic matter that has decomposed, including leaves, manure, weeds, kitchen scraps, ash, wood and paper. Compost provides food for soil organisms in a slow and steady manner. Their excretions feed plants, and their activity helps to aerate soil and create structure, such as aggregates of crumbs.

Compost holds nutrients in a stable, soil-enhancing form. Using it as mulch results in healthier plant growth plus better soil quality. A compost heap speeds up nature’s process of decomposition, and allows us to add other wastes to our own.

So what is compost tea?

This term is used to describe two completely different liquids: aerated and anaerobic tea. In gardens you don’t need to use either, though you may choose to. Aerated tea makes sense on farms where there isn’t enough compost for mulching, and also on golf courses, because you can’t lay compost on the greens.

1. Aerated compost tea is made with small amounts of the highest quality compost, aerated for at least 24 hours by an aquarium air pump. Add sugar such as molasses, to feed and help multiply the bacteria and fungi. This tea is all about stimulating soil microbiology; it is not a nutrient feed, but it helps nutrients to become available.

Aerated compost tea gives great results for such a small input of material. You need the kit and time to set it up and run it, then to apply it. However, in a garden with mulches of composted and decomposing materials, I see little benefit from using it, in proportion to the time needed.

2. Anaerobic compost tea is smelly, made by soaking compost in water, and is a weak feed of nutrients. I have used this a few times over the years and have been unimpressed by the difference it has made, in proportion to the time needed and the unpleasant odours. It’s simpler to mulch with compost, or comfrey leaves for that matter, and let nature do the work.

‍

Organic matter is food for soil’s billions of mostly unseen inhabitants. They aggregate soil into crumbs, resulting in structure, drainage and aeration.

• Organic matter is carbon based – more in the soil means less in the atmosphere.

Fresh manure is also organic matter. However, compared to compost, it contains fewer living organisms and its nutrients are more water-soluble. Hence the worries over nitrate leaching from slurry, worries which confusingly and wrongly have become associated with compost.

The nutrients in compost are generally insoluble in water, and do not leach into rainfall. At the same time, compost is about way more than fertiliser and feeding plants. Its biology is more important than its chemistry. Don’t worry about compost’s ‘low’ NPK figures, compared to when you may have used synthetic fertilisers. Compost and no dig serve to unlock food in soil which was previously unavailable.

Compost process

Decomposition happens within a spectrum of possible ways. Garden heaps have a mixture of processes enabling decomposition:

• Bacteria are at one end of the spectrum – heat builds from their proliferation, and the rapid decomposition of organic matter which they bring about.
• Fungi are at the other end – breakdown happens at lower temperatures and more slowly (through mycelial activity), in piles of leaves and woody materials for example.

One stage of maturity is when a heap’s warmth has mostly gone, because the bacterial processing is finished. Then fungi multiply, especially in heaps with woody materials.

Often brandling or tiger worms arrive when heat reduces, and heaps effectively become wormeries for a while. They reduce in quantity and increase in quality.  At Homeacres it takes three to six months before worms appear in my heaps. Before that it is too warm for them, except in winter months.

Contrast this with green waste or 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). This is a result of thermophilic bacteria being encouraged by the regular turning and introduction of air.

I take deliveries of such compost in this so called ‘ready-to-use’ state, and measure temperatures of 60°C (140°F) at the time of delivery, even though the appearance is black and crumbly. I have tried spreading this compost and then planting through it. The resulting growth was poor, compared to when I spread it after four to six months of extra fermentation. Lesson 16 compares and explains the qualities of several different composts.

In climates where the air itself is often damp, as are the materials that you are adding to a heap, watering is rarely necessary. As damp materials decompose, their moisture is free to seep into the heap.

Check your compost’s moisture by squeezing a handful to see how much comes out. A rule of thumb is that more than two droplets of water suggest soggy compost, which therefore needs more air.

If there is enough brown matter in a heap, excess moisture can either drain out or be absorbed by drier materials. Too little brown results in compost becoming soggy and airless, called anaerobic. This slows or halts the process of breakdown. A remedy would be to turn the heap and add paper, soil and other brown ingredients.

Maintaining correct moisture

Moisture levels are hard to assess when adding materials, but during dry summers you probably need to water. For example 2018 was unusually dry here, and when turning heaps made in the summer we found many dry pockets.

This afforded a chance to water with as fine a rose as possible, otherwise water droplets would have run through and away. Plus, in dry conditions, it’s worth firming materials or even walking on the heap. This reduces air pockets, holds moisture and builds warmth.

GREEN compost ingredients are soft, leafy, high in nitrogen, usually moist and low in fibre.

• Kitchen peelings, grass, weeds and food wastes are mostly green. Some green ingredients, such as coffee grounds and horse poo (both 3% nitrogen), look brown.
• Brown ingredients are fibrous, drier, and more woody than leafy.
• Some materials are both green and brown, see the broccoli stems in the photos below.

Why differentiate? When you achieve the desired balance of about 50:50, this contributes to having the correct level of moisture, warmth and structure for air. The result is decent aeration and sweet compost.

An excess of green results in high heat, bacterial dominance and a lack of air – making the compost sour and anaerobic, often dark black. A dominance of brown material results in moderate warmth, more fungi and a slower process.

Fresh manure from any animals, especially chickens, is green and so is urine. Both are excellent for speeding decomposition. Should you have large animals, such as a cow or a horse, their manure and bedding would ‘take over’ the compost heap volume-wise, meaning your compost heap would become more of a manure heap.

Beware of adding too much wood-flake bedding, because it’s kiln dried and very slow to decompose. Old manure with a fair balance of poo and bedding becomes great compost, just with different qualities to garden wastes.

BROWN compost includes chipped and shredded materials, which are woody. Their speed of composting depends on size, the type of tree wood, and whether they have been crushed or simply cut – crushed results in more surface area and faster decomposition.

Coniferous wood is better kept for mulching rather than composting because it rots so slowly, thanks to oils in the wood. However, if you are given a pile of chips and it contains, say, a third or less conifer, then that can work for adding to a compost heap when half decomposed and in small pieces. Other brown materials are:

• Shredded and crumpled paper.
• Cardboard in medium pieces, say 20 cm (8 in) diameter, and not shiny cardboard or food packaging with hidden polythene.
• Soil and yard sweepings (only they often contain many weed seeds).
• Tree leaves.
• Wood ash up to, say, 5%.
• Straw, which gives good structure and aeration, and works best when already wet and half rotten.

Summer sees a surplus of green, so I keep a pile of decomposing wood chips near to the compost bays. We add them to any large additions of grass mowings and fresh leaves. They can be anything from six to eighteen months old – I find the older the better. To speed up decomposition, we run a lawnmower over them before adding to heaps

Weeds (green) often include soil (brown) on their roots, which means you can make fine compost from them alone. Shake off as much soil as possible, to create a chance for heat to be created from a sufficient amount of green.

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. Save time by not separating out perennial weeds and  allow them to add value to your compost.

• With no dig you apply compost on the surface. This gives the chance to see and remove anything you don’t want in the finished compost, such as an odd root of bindweed.

This comment is from Stringfellow, in a forum topic about horsetail, June 2018:

‘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 vegetables.’

Tree leaves start green and older leaves become more brown, so autumn’s tree leaves are mostly ‘brown’ in compost terms. They compost better if chopped by a rotary lawnmower. Large amounts may be better left to decompose slowly into valuable leaf mould – in their own heap, through fungi.

Rhubarb leaves and citrus peel are green, and good to compost.

Eggshells are brown and bring structure to a heap but decompose slowly. After spreading the compost they may sit as a mulch on top, which is fine.

Diseased leaves are good to compost, such as mildewed courgette and lettuce, blighted potato and tomato. Also tubers, and fruits with late blight. Blight (and other fungal spores which cause disease on leaves and stems) need living plant tissue in which to survive, hence they die in a compost heap, and likewise in soil.

• On two occasions I have spread compost made with blighted leaves around tomatoes in the polytunnel, with no ensuing problems.
• Because blight spores do not survive in soil, there is no need to empty greenhouses of their soil after blight occurs.

Size/length of materials

It is worthwhile to cut and split longer lengths of stems and wood into pieces of 10 cm (4 in) or less. Otherwise they don’t pack down, and only very slowly convert to compost. As much as a heap needs air, it also needs an even density, and this keeps moisture consistent too.

We use a rotary lawnmower to chop stems of broad bean and pea, and a shredder for woody stems including hedge prunings. Softer wastes which are long and thin can be cut with secateurs or a knife.

‍

In a word, no. Heat is not vital, but it speeds up  the process and kills weed seeds. You can also make excellent compost at temperatures of 30–50°C (86–122°F). To know the temperature, buy a ‘compost thermometer’ like the ones in my photos.

These are some of the differences between compost from cooler and hotter heaps:

• Temperatures below 55°C (131°F) do not kill weed seeds.
• Hotter heaps decay more bacterially, especially when temperatures are over about 50°C (122°F).
• Cooler heaps below about 50°C (122°F) decay in a more fungal manner.
• Cooler heaps result in browner compost, compared to the black colour of hot heaps.
• Cooler heaps take longer to decompose.
• There is a greater range of microbes in cooler heaps.

A bin with plastic or wooden sides keeps materials together, increases warmth and moisture, and allows you to 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 this can result in dry edges.

Homeacres’ heaps with solid wooden sides make great compost, the wood conserving both heat and moisture. I screw them onto the shed posts, then it’s simple to unscrew them when turning and emptying heaps.

Or, if you have heaps made from pallets, line the sides with thick cardboard.

Plastic bins are usually smaller, and this restricts the heat they can maintain. My trial with a Rotol ‘dalek’ bin saw temperatures rarely exceed 45°C (113°F), and weed seeds survived the process. Nonetheless it was good compost, and the bin is easy to lift off when you want to use it.

‍

I am increasingly asked for the plan of my composting system here. However there was no plan, only a series of rough sketches as I firmed up ideas.

Below are the specifications, which will enable you to build something similar. Alter the specifications to fit your site. Seven bays could be three for an allotment or five for a large garden. I fill six at most in a year, sometimes five. The empty bays are useful storage space, and the roof means a dry and ventilated area, on the heaps, to dry onions.

• The main posts are 6x6in (15cm) pressure treated softwood, and set in 12in/30cm concrete. The posts are 8ft/2.4m long, about 1ft/30cm sawn off the back ones to create the roof slope.
• All the roof is treated softwood timber, and many sides were half inch/12cm plywood. But now I am moving to planks of Douglas Fir.
• Everything else is what you see. Steel roof.
• We dug the holes, builder erected the structure for £3k/$4k.
• Each bay is 1.7m deep and 1.8m wide, roughly 6 feet square, and the base is soil. So all materials sit on the base of earth.
• After filling to say 1.5m/5ft high, the materials sink to half that 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, then turn to right into number 1. Second bay to fill is no. 3, etc.

In 2016 I designed my seven bay system at Homeacres, including a roof to keep rain off the  heaps. I had wanted a roof for years, because too much rain entering heaps can change aerobic composting to anaerobic, by excluding air.

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

• In the growing season keep a reserve of brown: paper, small wood of any kind, autumn leaves, cardboard, soil and twiggy materials.
• In winter you need extra green, such as fresh manure, coffee grounds and spent hops.

Richard Loader on Facebook – UK Here We Grow, 13 August 2018:

‘Since visiting Charles Dowding’s garden and seeing his composting system we have started to see our compost heaps very differently. Previously weeding, trimming, mowing seemed like chores but these activities have transformed into 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.’

The photos below were all taken on the same day, after I had added several barrowfuls of material to finish a heap which was already four weeks old.

Small gardens generate less material and may struggle to fill a bin, even over a whole year. Use the smallest bin you can because a small, full bin makes better compost than a larger, half-empty one.

After perhaps a year of filling, lift up the bin (it will be light!) and place it close to the now exposed heap. Then you can fork out the top, undecomposed part and put it at the bottom of the empty bin. Finally, help yourself to compost from the bottom part.

• In large gardens, pallet sized heaps may fill to a height of 1.5 m (5 ft) within perhaps a month.
• Even at this point, continue filling for another 2–4 weeks, until the heap is hardly sinking any more.
• Unless you have a roof, cover with straw/carpet/cardboard/woodchip/polythene, while filling a new heap.
• Optional for a faster finish: turn the heap 1–2 months after the final additions.

Turning is worthwhile for larger scale compost makers with several heaps, in order to mix and aerate, and to create a more even compost in a shorter time. For most of us, just one turn of a larger heap makes a worthwhile difference. Two hours of turning is sufficient for a Homeacres heap of about a tonne.

You need an empty space or bin next to the heap you are turning. Use a manure fork with long prongs – good for shaking out dense lumps to introduce air. Turning is about mixing, aerating and checking the quality. If you discover dry portions, add a little water; or conversely, add some dry paper if it’s soggy.

• For a small heap that perhaps barely fills during a whole year, turning is not worthwhile.
• A simpler way to ‘turn’ is by occasionally using a proprietary compost aerator, to lift and mix the materials.
• The law of diminishing returns applies to compost turning. Gains are marginal from a second turn.

Within a year, or perhaps within months in the summer, your compost will have a crumbly texture of variable quality. A dark brown colour is better than black, which suggests a lack of air and too much moisture.

Any large lumps need breaking up with a fork while loading a wheelbarrow. Sieving compost before use is not worth the effort and time needed, unless you want it for propagation. If there are large pieces of undecomposed material, simply pull them out while loading a wheelbarrow, to add to your current heap.

You may find the odd root of perennial weeds, which are white and therefore noticeable. There is nothing to fear from such roots because, even if you miss them while spreading, you will have another chance to remove them later when you notice regrowth. This visibility, and the easy removal, are both advantages of no dig – compost being on the surface rather than incorporated.

‍

I have described the principles of easily composting mixed wastes (the variety that many of us have) without too much effort.

You can also make compost more rapidly, in as little as 14 days. I have tried this a few times, but find it inefficient in terms of the time and energy needed, say three turns. Also I preferred the quality of my slower, once turned heaps.

Another variation is explained in this video of the Johnson-Su bioreactor, which contains only wood ingredients, in a fully aerated and moistened pile. The intention is for there to be fungal rather than bacterial decomposition, and without any turning. I made one of these in spring 2020. I like that it’s low-tech, uncomplicated, and uses common wastes of chipped or shredded wood. It took six hours to make the frame and add a tonne of wood chip.

Microbial richness is a key asset of good compost, as much as are the nutrients. Lesson 14 now considers definitions of fertility, and the values of compost for soil and plants.