The Long And Short Of Horse Manure Composting

Horse manure can be full of weed seeds. In temperate countries such as the UK, warmer, sunny weather that is necessary for drying grass to make hay tends to come later in the growing year. This means that hay is often made from grass that has already flowered and set seed. The seeds are resilient and manage to come out of the horse's digestive system unscathed, ending up in the most perfect growing medium, manure! One way to cope with this problem is to allow the seeds to germinate and to remove them before putting the manure on your beds. Even then there will probably be a reservoir of seeds that remain in the manure but at least this will help give your plants a head start on the weeds and you will be able to gauge how nasty the weeds that surface are. If you see docks and thistles for instance, the advantages of the manure might not outweigh the extra weeding work.

There are some good arguments for adding the horse manure to your compost rather than directly to the plants. If you are lucky enough to have a compost pile that gets hot most weed seeds will be killed. (For more on the art of creating hot compost see the link below). Traditional folklore had it that fresh horse manure gave many vegetables a harsh flavour so it was composted rather than applied directly to the soil. If horse manure has not been well-rotted before it is put on garden beds it will actually starve the plants before it will be able to feed them. The high nitrogen content in manure is used to make protein to form the bodies of the microorganisms involved in the decomposition process. When a lot of them are produced all at once they need a lot of food. The food comes in the form of carbohydrate, from carbon in the existing humus, which is then depleted. Eventually equilibrium is reached with a stable amount of food for a stable microbe population.

There are two types of manure, known in the past as long and short manure. Long manure is manure mixed with bedding, and short manure that contains no bedding. If short manure is added to the garden directly the same happens as with fresh manure -- there will be an initial surge in plant growth but then the humus (containing stored carbon) will be depleted. Long manure however has a much better balance of nitrogen and carbon in it so it doesn't deplete the existing humus in the same way. Another benefit of long manure is that the bedding straw will soak up urine which is rich in the other main plant foods, phosphorus and potassium. Short manure would be more use added to the compost heap first where it will help heat the heap and speed up compost production.

Grass used as horse hay may come from pasture treated with herbicides that contains a hormonal chemical (ie 2-4D). Like the grass seed, the chemical doesn't break down when eaten which is good for the horse as it doesn't harm it, but very bad for your plants. In the UK a chemical called Aminopyralid was used on pastures as a herbicide and the manure from animals that ate the grass was distributed to allotment holders and others. It was disastrous for that year's crop of vegetables which grew in distorted forms and some established plants were killed.

Sourcing horse manure isn't too difficult but finding well-rotted, long manure from horses fed with non-seeding hay is more tricky. If you have access to fresh manure without bedding the best option is to compost it yourself. Make sure you have plenty of carbon-rich materials (cardboard, toilet rolls, woody clippings etc) to mix in with it and you should end up with wonderful compost for your garden. Manure is one of the best ways to get your compost hot enough to kill those weed seeds too – a win win! Do make sure it's hot enough though or you might be adding a lot of new and very unwanted plants to your beautiful borders.

By: Sarah Cowell

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Want to learn more about the alchemy of composting and how to choose the right system for you? Go to www.CompostEverything.com and sign up for a FREE 10 part mini-course now! Sarah Cowell Dip. Hort. is a gardener and writer on horticulture matters