This lesson is to clarify the qualities of compost. This will give you more confidence in choosing and using it, and also in discussing it with other gardeners, who often mistake compost for fertiliser and don’t understand its biological qualities.
Compost is a little word of many meanings, and you see it used to describe materials of totally different quality. The basic description is decomposed organic matter, with a great variability in life content, colour, consistency, aroma and weed seeds.
Manure
A commonly used compost is old manure, and manure is another confusing word. If you are discussing manure, and to avoid confusion, I suggest to precede the word manure with an adjective to describe its age and origin, such as old/fresh/horse/rabbit. Even describe whether you mean just poo (excretions/faeces), or poo and bedding, such as straw or wood shavings.
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.
Age of compost
Quality changes all the time as heaps age. Compost can reach ripeness or maturity in anything from 2 to 24 months, depending how quickly a heap was filled, the materials used, and the amount of turning. Differences you may observe include:
- Fresh compost and manure is lighter in colour, fibrous and lumpy, and usually contains many weed seeds.
- Half-mature composts are darker in colour, and still have a few recognisable ingredients. They probably have some odour and red worms, and half-ripe compost can be used for mulching around larger plants, but not where you want to sow seeds or plant closely.
- Mature or ripe composts are sweet smelling and crumble into smaller pieces, dark brown or black in colour. Sometimes there are bright red brandling worms, but not always, and compost can be good without worms. There may or may not be weed seeds, depending on the temperatures during the composting.
- A further stage is old compost, say two years or more: the volume has reduced, the texture is dryer and finer, and the colour almost black.
The photos of horse manure below illustrate the first three of these stages (points 1 – 3); the photo above is at the stage described in point 4.
Weed seeds in compost
Homemade compost has a bad reputation for its quantity of weed seeds, as a result of heaps being insufficiently hot to kill the seeds. Mostly they are fast-flowering annual weeds, such as chickweed, groundsel and shepherd’s purse, depending what is native to the soil and climate. Other ‘weed seeds’ may be flowers such as forget-me-nots and calendulas, depending on which flowers you grow, and whether you add them to the compost heap after clearing.
- I was advising a head gardener about conversion to no dig, and recommended spreading their homemade compost on top. She looked hesitant, and confided a fear of coping with the compost’s chickweed seeds. However the garden had been dug every winter, and her worry was based on how chickweed proliferated in that disturbed soil, then becoming difficult to hoe or hand weed. She decided to spread the compost as mulch and the weed levels decreased.
I advise you never to let a fear of weeds stop you from using compost. Rather, adopt strategies for dealing with them. A combination of hoeing tiny weeds, pulling some larger ones, or mulching with cardboard when there is a mass of weeds, solves most of the problem. Then your garden has fewer weeds seeding, resulting in compost with fewer weed seeds the following year.
To check the content of weed seeds in various composts, I filled four trays with different samples – soil, homemade compost, old horse manure and fresh horse poo. It was late autumn and weed growth was slow, even in the greenhouse. Even so, the results bear out my mistrust of disturbed soil, in terms of new weeds. Plus I was surprised to see so few weeds in the fresh horse poo.
- There were only a few weeds in my homemade compost, mainly meadow grass.
- The largest amount of weeds, including plenty of buttercups, had grown in Homeacres soil.
- There were no weeds at all in the old horse manure, taken from my hotbed of the spring – the result of sufficient heat.
- Interestingly there were only a few weeds in the fresh horse poo – perhaps the horses had eaten more grass than hay.
Compost types
Here we look at the different qualities of composts that you can buy and make. The descriptions are not ‘absolute values’, because of the extent to which compost can vary. However they are a starting point, to help you choose from what is available to buy in your area, and to give you an idea of how this compares with composts that you can make.
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’. The compost is sold (and occasionally donated) by municipal or private garden processors. Usually it has been shredded, and you can see a lot of woody pieces.
Decomposing wood reduces the nutrient value of compost, especially when it’s under about nine months old and much of the wood is still decomposing. The fungi and insects involved in wood decomposition need more nitrogen to survive and thrive than is contained in the wood itself. Once decomposition has mostly finished, nitrogen becomes more available – it’s a temporary shortage.
Heaps in commercial facilities are turned frequently, and may reach 80°C (176°F), which assures their guarantee of no weed seeds or pathogens. In the UK, all suppliers of compost adhere to a PAS 100 scheme, one part of which ‘requires that the number of weed seeds and propagules does not exceed zero germinating weed seeds and propagules per litre of compost. Propagules are seeds and other parts plants necessary for their reproduction’.
The heat, however, also ‘cooks’ many organisms, resulting in a lack of aggregation in the compost. Such compost can go to sludge when wet, and to powder when dry. I advise three approaches when buying and using it:
- Check that it has been sieved, to a size no wider than 15 mm (0.6 in), in order to reduce the amount of large wood and plastics.
- Buy the compost, or have it delivered, at least two (though preferably four to six) months before you need to use it.
- Spread it thinly on a large area, rather than in depth on one or two beds, so that soil life can colonise it more quickly.
I did a growing test, to see how six-month-old green waste compost would compare with other composts, for seedlings of mustards and salad rocket. The photos say it all in terms of ready nutrients, but it’s still a good compost for mulching. (Also see the trial of different composts in Lesson 14.) A more recent trial showed stronger growth in five-month-old PAS 100 compost.
Spent mushroom compost (SMC)
This is made from straw and some stable manure, with proprietary extras. Some peat is used in mushroom houses 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.
It’s valuable for mulching, even though there are two myths that you may hear. Be informed, and ignore them.
- ‘Mushroom compost has a high pH.’ This was the case in times past, but current formulations in Europe are resulting in a pH of around 6.8. When at Lower Farm, I used the high pH mushroom compost on soils of high pH (7.8), with no problems ensuing over many years of use.
- ‘The high nutrient/salt levels may burn roots.’ This is nonsense, totally contradicted by the experiences of hundreds of thousands of gardeners. I am sure this myth originated from manufacturers of soluble fertilisers, who are afraid of losing sales. Ironically, theirs is the product whose soluble salts may burn roots.
You can use SMC when delivered straight from a mushroom farm, but only as a mulch, and not as the only compost for filling a bed. At this stage you will notice fibres of straw, lumps of peat, white threads of myceliae, and also that its colour is more brown than black.
After a few months it becomes finer and darker, with no weed seeds or other problems. The texture is soft and fibrous, while nutrient status varies. I find it more nutritious than green waste compost and less nutritious than composted manures, as the photos below demonstrate.
Compost trial with Maravilla lettuce in pots in the greenhouse
I sowed lettuce in July, then potted them as four-week-old seedlings into four different composts. Every ten days or so I harvested their outer leaves, giving the results below.
There was a lack of quality in the lettuce leaves from the green waste compost, with some holes and brown tips, while the leaves of lettuce in the other composts were 99% good. By December, and after its slow start, the green waste compost was releasing more nutrients – probably related to the wood content having decomposed, and therefore not taking nutrients ahead of the lettuce.
The West Riding multi-purpose compost was disappointing, although I had expected it to run out of nutrients by November. Both the homemade and cow manure composts were somewhat lumpy, but dark and crumbly too, and growth was good throughout. Put another way, the lumpy composts grew better plants than the very fine ones, perhaps from more air around the roots.
Wood as mulch
The best thing about mulches of wood is their fungal decomposition. Look under wood and you will often see many white mycelial threads near the surface. Plus there are organisms such as millipedes, who eat and breed, excreting food for plants and adding to soil structure.
However there are confusions about wood as soil and plant food, partly from misconceptions about the excellent ‘Back to Eden’ gardens. I understand that Paul Gautschi ages his wood chips before spreading them, often in the hen run where chickens rummage and add their nitrogenous droppings. Plus he is spreading only thin mulches every year.
A thin layer of wood mulch on the surface breaks down reasonably fast, especially if the wood was shredded as well as chopped, into short lengths. Small pieces decompose more quickly than larger chips, thanks to the greater surface area.
Further differences are with green wood, sometimes called ‘ramial’, from branches grown since spring. When used as a mulch, this decomposes faster than brown wood (which takes a year). Another difference is that conifer wood takes longer to decompose than wood from broadleaf trees, because of the oils it contains.
Green wood is an effective mulch for beds, if you are lucky enough to have some. Sometimes it contains green leaves which bring further value, and speeds up decomposition when in a heap.
A summary of wood for garden use:
- Wood chips are usually short lengths of wood, in a range of diameters.
- Shredded wood means that a crushing process has opened a larger surface area, therefore decomposition is faster.
- Green wood describes branches that have grown within the past year – the wood is soft and decomposes more rapidly.
- Brown wood is a year or more old, making it harder and drier than green wood.
- Shavings and sawdust are by-products of working wood, and they decompose quickly thanks to a high surface area, depending on the type of wood.
- Do not use wood shavings or dust from workshops that cut timber with glues, such as chipboard.
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. Add half-composted wood chip as a brown ingredient for compost heaps, to balance any excess of green (see Lesson 13). You can spread 18 to 24-month-old wood chips on beds, when they’re looking more like compost.
- If your only source of organic matter for beds was woody material, I would wait for it to break down over 18 to 24 months. The only issue with this would be how much space was available for a heap of wood chips. Another option would be to spread fresh wood chips around trees, shrubs, fruit bushes and perennial vegetables.
I suggest you avoid deep mulches of wood on beds, not because of how they would affect soil, but more because of the difficulty of sowing and planting. Seeds and plants need to be in contact with compost or soil, not pieces of wood, hence my advice to use woody mulches as a thin layer only, say 2–3 cm (1 in). Even in this case, transplanting is more viable than sowing direct.
- Don’t ever fill a bed with wood that is not decomposed – see the photo below!
Watch out for woodlice!
Woodlice (also known as chuggypigs, pillbugs, slaters or grammasows) eat certain leaves, and their numbers grow when there is plenty of old wood for food. Plus the babies sometimes eat stems of green plants, when tired of chewing dry wood!
Above all be careful in your propagating area, to keep it clear of decaying wood. Woodlice like seedlings, and tender leaves that are close to the surface, including stems of cucumber and tomato.
A message from Holly Mock on Instagram (peacefulpandemonium), 26 September 2019:
‘I have been gardening in a no till way for five years, but struggled with using wood chip or hay as a mulch, since it causes all sorts of pest problems. So in 2017 I started using compost instead and I have not looked back.’
A compost trial
I was given two proprietary composts to try – Dalefoot wool and Melcourt farmyard manure – and we filled new beds to compare their effects on plant growth. Two other beds were mulched with my own compost (seven months old and less mature than I would have liked) and green waste compost.
This was in a corner of the garden which, a year earlier (2017), had been totally overgrown with brambles and weeds. In June 2017 we had mown the grass and weeds, and then dropped a half-tonne of wood shavings, to decompose and reduce new weed growth. By December it had started to decompose, and we spread it as a mulch on paths.
It would have been interesting to have had more volume of the proprietary composts, to fill the whole depth of each bed. Since I could not do that, each one had a base layer of old cow manure and green waste compost, added in March 2018.
Trials like this are always fascinating: plants tell you things! A notable result here was growth in all four squares being less abundant than I had expected, probably because of the residue of wood shavings under the compost. It was not a thick layer, but enough to deprive plants of nitrogen as the shavings continued to decompose in the rooting zone.
Early potatoes are excellent as a first crop, because after their harvest there is time to grow more vegetables. In mid-June, and straight after the potato harvest, we planted Granat red cabbage as five-week-old plants. In July we harvested the onions, planted a Cabbice cabbage on each bed, and three Treviso chicories.
The trial had to finish early, as a new cabin was being built in this area. The cabbages and chicories did not have time to make firm hearts or heads.
Overall, there was more difference than I had expected:
- The Dalefoot compost did particularly well, all summer.
- The Melcourt compost had what looked like too high a proportion of bedding. It was pale in colour and had many undecomposed fibres – its plants were not impressive.
- The green waste compost showed its low nutrient status, while growing reasonable plants.
- My homemade compost disappointed me a little, though the results weren’t bad. It would have grown bigger harvests if the compost had aged another two to three months. Younger compost is good for mulching, but use older compost for filling beds.
A final thought
Hay and horse manure, in particular, have a small risk of contamination by aminopyralid weedkiller, which is occasionally sprayed on grass intended for making hay. In my experience it’s the only weedkiller which persists, and it is lethal to solanums and legumes, whose growing tips become curled and twisted.
Most horse owners have no idea how their hay was grown, unless it’s from their land. Ask them for as much information as possible, because this poison persists in manure heaps for many years.
- You can check for its presence by growing susceptible plants in pots, such as pea, bean, potato and tomato. The manure can even be quite fresh for a test like this.
- A very few green waste composts contain clopyralid, from lawn weedkillers. It’s an equally strong and persistent poison.
The photos below are to help you identify the negative effects. If you experience any of these, complain to your supplier at least. The mulch would then need either to be scraped off – with the aim of removing the poison, but where to put it? – or left it in place, where soil microbes will break it down. You could still grow brassicas, sweetcorn and umbellifers while this is happening (over the period of a year, or hopefully less), depending on the concentration of aminopyralid. Alternatively, you could sow green manures.
