Why horse manure earns its place on the smallholding

If you keep horses, or know someone who does, you are sitting on one of the best soil conditioners available to a smallholder. Horse manure is rich in organic matter, reasonably high in nitrogen, and free at the stable gate. What it is not, straight from the fork, is a safe and finished product.

Fresh muck is a raw material. Spread neat on a bed it can scorch young plants, it carries viable weed seeds and worm eggs, and it often smells powerfully for weeks. Composted properly, though, it turns into a dark, crumbly, sweet-smelling material that opens up heavy clay, holds moisture in sandy soil, and feeds worms and soil life for months. The whole trick is stacking, turning and covering so the heap heats through and finishes properly.

Getting the mix right

A compost heap works on a simple balance: carbon, nitrogen, air and water. Manure brings the nitrogen; straw or shavings bedding brings the carbon. Aim for roughly 25 to 30 parts carbon to one part nitrogen, which in practice means muck and bedding together make a decent starting point.

• Straw bedding is ideal. It rots down readily and keeps air in the heap.
• Wood shavings are slower and lock up nitrogen as they break down. Add a little extra green material and allow more maturing time.
• Green material such as grass clippings, comfrey, nettles or vegetable peelings speeds things up. A bucketful per barrow load of muck is about right.
• Water to the consistency of a wrung-out sponge. Too dry and nothing heats; too wet and it goes sour and airless.
• Avoid lime. It drives off nitrogen as ammonia and you lose the best of the muck.

Never tread the heap down. Air is the fuel of the microbial activity that does the work, so keep the material loose and open.

Building the heap

Three pallet bays side by side make the ideal setup: one being filled, one cooking, one maturing. Whatever you build, make it big. A heap under about a cubic metre struggles to hold heat, so aim for roughly 1.2 to 1.5 metres in each direction, built up in one sustained effort over a few days rather than dribbled in barrow by barrow.

Siting matters too. Put the bays on bare soil in a sunny spot so worms, fungi and bacteria can move in from below. Layer the material in 15cm bands, sprinkling a little garden soil or old compost between layers to introduce microbes, and water as you go if it is dry. Build the sides vertically rather than in a cone, so the outer edges insulate the hot core.

Turning, covering and the heat that does the work

A well-built heap should reach 55 to 65°C within three to five days. Push a metal bar into the centre, leave it ten minutes, and it should be uncomfortably warm to hold. This is the stage that matters, because sustained heat is what kills weed seeds, parasites and pathogens.

• Turn when the temperature starts to fall, usually after about five days. Move the outside to the middle and the middle to the outside.
• Repeat every two to three weeks for the first six to eight weeks — three or four turns in total is plenty.
• Cover the heap with a tarpaulin or old carpet once it has heated, to keep rain off and stop nutrients leaching away. Leave it slightly open at the base so air can still move.
• If it will not heat, it is too dry, too wet, too small or short of nitrogen. Turn it, add green material or water, and rebuild.

Weed seeds such as dock, thistle and ragwort need several days above 55°C to be destroyed, along with most worm eggs. Skip the turning and you simply get a slow, cold rot that leaves the seeds alive.

When it is ready, and how to use it

After the hot phase, leave the heap to mature for three to six months. It is ready when it has cooled, shrunk to perhaps half its original volume, and looks like dark crumbly soil with an earthy smell and few recognisable bits of straw. Straw-based heaps take six to twelve months in total; shavings-based heaps can take eighteen.

Use it as a mulch 5 to 8cm deep around borders, fruit bushes and perennials, keeping it clear of stems. Dig it into vegetable beds in autumn or spread it in spring. It is too rich for seed sowing, but makes an excellent potting mixture for hungry crops such as tomatoes, courgettes and brassicas.

Two hazards worth knowing about

First, persistent herbicide residues. Some hay and haylage are treated with products containing aminopyralid or clopyralid, which survive the horse, survive composting, and can twist and cripple tomatoes, beans and potatoes for years. If you are offered unfamiliar hay, ask the supplier, or test a batch by growing broad beans in a sample of the finished compost.

Second, wormers. Residues of ivermectin and moxidectin pass through in the dung and can slow the composting process. Let fresh muck from recently treated horses stand for a few weeks before adding it, or keep it in a separate bay. A little patience in the yard rewards you with barrow loads of the finest soil conditioner you can make.