Manure may smell, but as a source of fertility for your soil, it’s hard to beat. It’s also one of the best compost materials you can use.
Despite that, there are problems with manure that you need to be aware of. You’ll also find that different animal manures have different properties, and need to be treated differently.
Let’s take a deep dive into this valuable resource, and find out how to make the most of it.
- Manure in history
- The value of manure
- Fresh manure v. rotted manure
- How to compost manure
- Types of manure
- Manure problems
- Where to find manure
- How to use manure
Manure in history
People have used manure as a fertiliser for almost 8000 years.
Scientists studied crop remains in 13 early farms, and found they contained high levels of Nitrogen-15 (N-15). N15 is a relatively rare isotope which is found in manure, as well as in vegetables grown with manure compost.
Manure continued to be used throughout history, and by the time of the Greeks and the Romans the ancients were distinguishing between the different types of manure, and which manure should be used with which plant.
It wasn’t just human manure that was used – humanure was far more widely used in the past. Ancient Athenians collected human waste in their sewage system and piped it to farmland. Aztecs collected waste from public toilets and used it for intensive growing on farmland, while in China and Japan ‘night soil’ was collected from houses to spread on the land.
The value of manure
The Greek general Zenephone criticised peasants who did not make use of manure. But he would probably be far more disgusted now.
While energy and scarce resources such as phosphorus are put into artificial fertilisers, the world produces an astonishing 5 billion tons of manure, much of which ends up in the rivers or the sea.
What’s in all that poo? There is nitrogen, phosphorus, potash and calcium. The quantities found vary considerably on the type of manure, but collectively the amounts are huge.
Phosphorus, for example, is currently mined. 80% of it is lost or wasted and the world’s resources are running out. Yet, according to the Rodale Book of Compost, 75% of phosphorus (and 80% of potash, which is also mined) used on fields can be reclaimed and returned to the soil.
Yet manure is rich not just in nutrients. It contains huge quantities of bacteria – possibly up to 30% of its complete weight. These can help rapidly break down both the manure and other materials, speeding up composting.
As it rots down, manure can also provide heat, which can be used to provide heating, and methane, which can be used as a fuel.
Fresh manure v. rotted manure
Fresh manure is different from well-rotted manure.
- Fresh manure contains more water.
- Fresh manure contains more soluble nitrogen, while composted manure contains more fixed nitrogen.
- Fresh manure is more likely to contain pathogens, although some manures are more susceptible to than pathogens than others.
- Fresh manure is likely to smell unpleasant – composted manure has a pleasant earthy smell.
There are advantages to composting manure before using it.
- Soluble nitrogen is more easily washed out of the soil.
- The high concentration of nitrates in some manures can burn plant roots.
- The compost process can kill pathogens. That’s why some countries have strict rules about composting manure before it is used for food production.
- Hot composting can kill weed seeds in manure.
- The fixed nitrogen in composted manure is less likely to be washed out.
- Raw manure may lead to acid soil and an imbalance of nutrients, especially if repeatedly applied.
So in theory, it’s usually better to use rotted manure.
That doesn’t mean you always have to compost it. For example, a friend of mine gets great results in his raised beds in his polytunnels.
He puts a base of fresh manure down in late winter, and covers it with a layer of compost. As the fresh manure decomposes, it generates heat, which helps his plants grow.
The freshness of the manure doesn’t seem to hurt the growth of his tomatoes and chilli peppers – indeed, his tunnel often looks like a jungle in summer. (It may be that, by the time the roots make it down through the compost, the manure has sufficiently rotted.)
That’s not a new concept – as we’ll see later, ‘hotbeds’ have been popular since at least Victorian times, and probably longer.
How to compost manure
In general, manure can be composted in the same way as other materials. It’s worth making some additional notes, though.
Working with high nitrogen levels
Manure’s high nitrogen level means it generates heat easily. Even in raised beds it can generate heat. That makes it great for combining with other materials in compost heaps.
It raises the heat level of the compost heap, encouraging bacterial activity and balancing high carbon materials. If you have a compost thermometer, you can easily see the difference in heat after adding manure to a hot compost bin.
Using bedding material
Manure also works well when composted with bedding such as straw. The straw bedding contains carbon which offsets the high nitrogen level of manure and urine.
The use of straw, or other bulking material such as semi-decomposed wood chips, also helps to trap oxygen in the pile. This helps the bacteria to decompose the manure (see Compost Science) and may reduce or remove the need to turn the manure.
Manure can be composted in piles or in bins. Bins are best for keeping the manure in a neat heap. As with regular composting, a three bin system can be useful here. One bin can be used for adding fresh manure, a second for the process to finish and a third for use.
A larger size will get hotter, though. Sizable bins – at least three foot by three foot – are ideal for hot composting, as are insulated bins.
Manure loses nutrients both to rain and to air. One study in Kenya found that over 6 months manure lost significant amounts of carbon, nitrogen, phosphorus and potassium. To maximise the value of manure, it’s a good idea to start composting it while it is still fresh.
The Rodale Book of Composting also advises taking care to avoid nutrients being leached out of manure, covering the pile with plastic or a tarpaulin. Alternatively, you can cover with a material like straw around the sides and over the top of the manure, which also helps maintain temperatures around the outside of the manure.
When the manure has finished heating, it is a good idea to leave the pile to mature. This allows the ph levels to stabilise, and worms will also turn more of the manure into worm castings, further improving the quality of the finished compost. In general, the longer you can leave the pile the better – a few months is good, a year is better!
Types of manure
Manure is not all the same. Some manures are considered ‘hot’ because of their high nitrogen levels. It is particularly important to compost these manures as the high nitrogen content can damage plants.
Other manures are drier, and are more likely to need watering in dry weather.
There are many types of manure, ranging from rabbit droppings to zoo poo! Both On Farm Composting Handbook and Modern Farmer carries carbon:nitrogen ratios and moisture levels for several manures. Do note, though, that different sources do sometimes give different C:N ratios for manure, but you can still get a general idea of whether a manure is ‘hot’ or ‘cold’.
Here are some of the most common types of manure:
High in nitrogen, and often considered the hottest of the manures. It is important to compost chicken manure both because of its high nitrogen content and because it is more likely to carry diseases such as E.Coli.
A single horse can provide over 9 tonnes of manure a year. This is another hot manure, but it is usually mixed with straw or wood chips which provide a high source of carbon. It is a relatively dry manure.
Some sources call sheep manure a cold manure. However, with a C:N ratio of 15:1 sheep manure is higher in nitrogen than horse or cow manure. It is also a dry manure with low moisture levels.
Another hot manure – in fact, it can be hotter than chicken manure. It is high in both phosphates and nitrogen. Because it breaks down quickly, some gardeners use it directly on the soil despite the high nitrogen content.
Cow manure is cooler than many of the other manures mentioned here. It has a C:N ratio of 25:1, which makes it ideal for composting, and also means odour is less likely to be a problem.
Guano (bat manure)
Guano is such a highly prized fertiliser that two wars were fought over it in the South Americas – the second leaving 18000 dead in its wake! It is high in nitrogen, potassium and phosphate.
Humanure refers to our own poo and pee! The Chinese referred to it as night soil – a more pleasant word which has seen some adoption in the West. Humanure has a C:N ratio of 10:1. It is usually collected by specially designed toilets. It should only be used in hot composting, and is best for experienced composters.
For those interested exploring Humanure further, the Humanure Handbook provides both a guide and an excellent justification for the use of humanure.
Manure from carnivores
Manure from carnivores such as cats and dogs can also be composted. However, due to the pathogens these animals carry extra care needs to be taken, and it is recommended for experienced composters only who can be sure of achieving thermophilic temperatures.
There are also specialised Bokashi systems which are designed to pre-compost dog and cat manure.
Green manure is worth a quick note here – if only to avoid confusion! Green manure is not animal excrement. Instead, it refers to plants grown specifically to benefit the soil.
Some green manures, such as legumes, work with bacteria to fix nitrogen from the air. Others, such as Alfalfa, are grown because they have deep roots which can extract minerals and nutrients from deep within the soil, or to bulk up the soil with organic matter. Some gardeners may also grow green manure in order to avoid leaving the soil bare between crops or over winter.
Green manure is usually dug into the soil, but it can also be cut down and left as a cover or removed and added to the compost heap.
Herbicides in manure
Herbicides such as Aminopyralid or Clopyralid are being used on food grown for fodder. These herbicides do not break down while being digested or while being composted.
If you’ve obtained manure, it’s well worth growing a few test seeds before using for the first time. See Manure in Compost: A Cautionary Tale for details.
Antibiotics in manure
A huge amount of antibiotics are used in farming. For example, in 2009 in the United States 80% of antibiotics were used on farm animals, with over 130,000 tonnes used in animal food.
One study found that manure, at least when combined with chemical fertilizer, can lead to an increase in antibiotic resistance genes in soil and affect the mico-organisms in the soil.
Fortunately, the use of antibiotics in farming has been decreasing in recent years in the USA and the EU. That’s partly down to regulations, but some farmers are exploring innovative new ways to manage livestock without using antibiotics.
Hot composting can help to destroy many (but not all) of the antibiotics in manure.
Odour may be less of a problem than you fear when it comes to herbivores. Fresh manure does smell, however hot composting can ensure that any unpleasant smells are rapidly removed.
Some bins, such as the Hot Bin, also come with a charcoal filter to further reduce smell.
By the time manure has fully matured, it will have a pleasant, fresh earth smell.
Where to find manure
Horse riding stables produce huge amounts of manure, and are often happy for you to come and take it away. Farms are also a good source of manure and many will bring trailer loads to you at a nominal cost. As a source of fertility, it’s incredible value.
As we’ve seen, the problem with these sources in the modern day is herbicides. Farmers are not supposed to sell manure which has been generated from fodder treated with herbicide, but unfortunately it does happen.
The best possible source of manure is a farmer or who grows his own fodder, and does not use herbicides.
If you have a large garden, you may also want to consider keeping chickens. While chickens may not produce huge quantities of manure, the manure carries a lot of nitrogen for its weight, and is fantastic as a compost activator. Chickens also devour kitchen scraps and provide you with free range eggs too!
Manure can also be bought online, both in bulk and in small bags.
How to use manure
As we’ve seen, most manure is best composted. This reduces the number of pathogens, stabilises ph levels and stops excess nitrogen from burning plants. Hot composting can also kill weed seeds found in manure.
Some manures are more suitable than others for straight use on the soil. Cow manure, for example, has a lower nitrogen level while rabbit droppings decompose very quickly.
If adding manure directly to the soil, it is worth considering what plants you are growing. Some plants are more amenable than others to the high nitrogen levels found in manure. (Unfortunately, nettles seems to be one of them!)
When composted, manure can be used in the same way as regular compost. However, manure can also be used in some other interesting ways…
Hot beds use the heat generated from manure both to grow tender plants which need more heat and for growing in winter. They were popular in Victorian times when modern sources of heating were not available. They are also starting to make a resurgence as gardeners try to avoid using fossil fuels to generate heat.
Hot beds can be used to heat greenhouses, but they can also be used outside. One friend I have uses them to grow melons – he constructed a frame on the side of his house, filled it with manure and planted his melons in them. Despite being in a climate which is on the cool side for melons, the crop was fantastic.
The heat from hot beds doesn’t last forever. The Victorians used to deal with this by working more manure into the bed to keep the heat going for longer.
You can also make manure into a nitrogen rich feed. Simply place one or two shovelfuls into a permeable bag. Tie it closed and place it in a large container filled with water. Leave it submerged for a week to steep. The liquid can then be used as a plant feed. (Depending on the strength, you may wish to dilute it.)
Another friend has an intriguing method for fertilising courgettes which his family has been using for decades. You cut the bottom off a plastic bottle, and also the very top of the bottle, leaving the thin neck. You invert the bottle and insert it into the ground. You then fill the bottle with manure.
When you water the courgette plant, you pour the water into the bottle and through the manure. The water absorbs the nutrients from the manure and passes into the plant roots. Soluble nitrogen or not, the results are always excellent.
Manure is a fantastic addition to the garden, full of nutrients that are naturally produced. Just be careful where you source it from, and consider composting it before using it on the soil.