The cheapest way to buy manure is to buy it straight from the farm, but then you have a lot of fresh manure on your hands that you need to age and store properly. There are loads of conflicting theories about ageing and curing manure. Can you use old cow manure that’s been sitting for years? How long do you need to age it for if it’s still fresh?
In this article, we’ll answer these questions, and share some tips and tricks to help you understand your manure heap a little better. Even the manure you buy from garden centres can be too fresh sometimes, but the answer, as we shall explain, is all in the smell.
Does cow manure expire?
Cow manure lasts for years, and, provided it is allowed to breathe, will be a great substrate or compost no matter how long it is stored.
The problems occur with bagged manure when it is left in plastic bags for storage, and stored in humid, muggy conditions where water is allowed to stagnate. If the bags are kept completely dry then horse or cow manure will last, but if any moisture is allowed into the bags or humidity builds they can become stagnant, which will lead to fungal infections. Particularly on young plants.
When properly stored, cow manure is useable forever, eventually becoming a humus-rich compost if turned and mixed with other organic matter.
How to store rotted cow manure
Once manure is sufficiently rotted, or if you’ve bought significant amounts of manure and need to keep it safe, either keep it in the bags you bought it in to protect it from excess rain, or cover your manure heap with a tarpaulin.
While manure is rotting down, its high temperatures help to regulate the water runoff and reduce excess moisture, but when it has fully rotted and no longer creates heat, it needs covering to prevent it from getting soggy.
What is well-rotted manure?
Manure needs to rot down before use in the garden. This reduces the potency of the acids and salts in the manure, which leech out over time, and have a less immediate effect on plants.
But don’t confuse ‘old’ with ‘well-rotted’. Manure is at its best in the garden after 6 months.
How long should manure be left before use?
Different manure takes different times to decompose, but cow manure is typically the fastest ‘hot manure’ to decompose.
Cow manure usually rots to a usable point after around 3-4 months, similar to horse manure, which is slightly slower and can be used in some cases straight away.
Pig manure, where animals have been fed a high carbon diet, concluding wood chip or bones, takes longer to decompose, and should be left for at least 12 months before use.
Think of your classic compost recipe: 50:50 brown (carbon) to green (nitrogen). The same is true of manure. The more carbon in the animal’s diet, the longer manure takes to rot. Nitrogen also helps the heap heat up faster, resulting in a faster decomposition process.
How to tell when manure is rotted enough?
Once the ammonia smell has completely subsided and been replaced with a sickly sweet smell more like rotten grass, your manure is ready to use.
Well-rotted manure should no longer have the distinctive ammonia smell that makes your eye water. Ammonia has many uses in the garden and can act as a really useful nitrogen fertilizer for certain hard-waring vegetables like squashes, but in general, it is too acidic and too concentrated for plants to handle.
How to age manure for the vegetable garden
When it comes to ageing your manure, there is one choice to make: do you have enough space?
The easiest way to age manure is to mix it in with an existing compost pile, meaning you won’t need an extra compost bin, and the aerobic decomposition of the compost will speed up the decomposition of the manure.
Add manure to a 2nd-year compost bin so you can use it the following year.
That said, manure is best aged by itself, as having separate manure and compost means you have more varied mulching materials for your no-dig garden.
Where to age manure
Place manure piles well away from the house, and from areas where children or pets play, as manure does attract flies.
Always put manure piles in full sun. The hotter they get, the faster they decompose. On our allotment, the communal manure pile actually caught fire once, but it’s rare for this to happen, and requires temperatures to reach over 150°C for spontaneous combustion on manure piles.
While the temptation for a manure pile is to place it out of sight, and in a dark corner of the garden, this can lead to rotten (not rotted) manure, which will continue to smell, stagnate and fail to decompose properly.
It’s absolutely fine for manure to get rained on while it’s rotting. The finished product should ideally be dry, but you’ll always get a puddle at the bottom of the manure heap, so when you get to the end, turn over the pile with any new manure to dry out the wet stuff faster.
After a few months of generating its own heat, manure should still be giving off some heat. When it stops decomposing you’ll notice the temperatures drop. At that point, you can dry it out and bag it up. That bagged manure can be kept for up to 6 years as long as it’s kept dry.
Rotting manure needs good airflow. Any decomposition should be properly aerated, as compaction leads to anaerobic decomposition – stagnation. All the goodness will be killed by funguses, which can spread to plants.
This is less of a problem with cow or sheep manure, as they’re fed on grass sprayed with pesticides, but manure heaps should never be wider than 1.5m across as oxygen can’t reach the centre.
Improve your manure’s aeration by turning the pile at least once or twice during the decomposition process, to reactivate some heat and help it break down faster.
Curing manure and ageing manure are slightly different processes. To cure manure, it must be allowed to dry out before adding to a compost pile. The process is done to get rid of the worst of the useless ammonia while retaining the beneficial ammonium nitrate from the liquid ammonia, which is retained as manure dries.
When can you use manure straight away?
In some cases, manure can be used straight away which makes the most of its heat as it decomposes. You need to choose the crops you sow in fresh manure carefully, as some plants thrive on it, but others don’t do so well – check this article for more details.
When we first moved to our new allotment, we dug out the worst of the perennial weeds, covered the ground in wet cardboard, laid wood chip paths, and half-filled raised beds with manure. The top half of the bed was filled with garden compost, and the manure meant we were able to grow things from seed outdoors that usually wouldn’t germinate thanks to the heat.
The next year that manure rotted down more, but still provided some nutrients, along with better drainage.
Beans are incredibly nitrogen hungry plants that appreciate the heat. One traditional allotment hack for growing beans is to dig a trench and fill it with fresh kitchen waste and newspaper. The nitrogen and heat released from rotting are great for beans.
If you fill the trench with manure it works the same way, by keeping manure away from young roots while they establish, and letting the roots grow towards the manure as they mature.
The added benefit of cow manure, which puts it way above horse manure in my books is weed control. Horses have a single stomach, and you’ll notice in any bag of horse muck that there will be full strands of undigested hay, and even full nettles still intact. Cows have four stomachs, meaning everything they eat is fully digested, and there is much less chance of weeds seedling into your vegetable beds from cow manure.
Because dried cow manure drains slightly better than horse manure, it is less likely to retain water while rotting in a compost bin, or in bags, and therefore less hospitable to weeds too.