Thursday, November 17, 2016

Marvelous Manure

Manure is nature's best fertiliser.  For as long as farms have had sheep, cows and chooks, manure has been used to fertilise food gardens. But are all manures of equal value? And how should we use manure on our gardens?


Manure adds organic matter to your soil.  It improves in a major way your soil's structure and water holding capacity. It adds nutrients and it improves the uptake of these nutrients by plants.  Manures also encourage microbial activity and this improves plant nutrition.

Some people simply put a layer of manure on top of their soil and then plant crops in it, but that is not the best way to go about it because flies may breed in it and the manure will dry out due to exposure to the sun and lose nutrients. Some manures (e.g. chicken manure) are so rich that they would damage plants.

If you insist on using fresh manure on your garden, mix it into the soil and cover it with soil, so composting is aided by the coolness and moisture of the soil it is under.  Postpone planting for as long as you can, so the manure 'ages'.  Composting is a process that needs nitrogen, so if you add un-composted manure to your soil, nitrogen may be taken from the soil.

The recommended and by far the best way to use manure is not to use it on your garden in its fresh un-composted form, but to age the manure by putting it on a compost heap, mixing it with other compost ingredients, and leaving it there for at least half a year, preferably longer, ideally a year.

By the way, green manure is the confusing term used for growing certain plants and then digging them into the soil to feed the soil. It is a good way of improving soil, but it is not part of this blog post.

Nature's fertiliser
Four reasons for composting manures before using them
  1. fresh manure, especially chicken and horse manure, may burn plants and kill soil microbes, because it may contain a lot of nitrogen
  2. un-composted manure may contain high levels of salts that kill soil microbes
  3. un-composted manures may contain weed seeds.
  4. un-composted manures may contain chemicals.
In the composting process heat and microbes break down salts, seeds and chemicals.  Composting needs nitrogen, so nitrogen-rich manures lose their 'over-dose' of nitrogen.

The message is don't be impatient, plan ahead!  If you know you are going to need a lot of compost next Spring (and which food gardener doesn't), get as much manure as you can get the previous summer, use the manure as a major ingredient in one or more compost heaps, and use the resulting compost the following Spring.

If you like to learn more about composting there is a series of articles on this blog. See this blog's Index page or start reading here.
Bagged manure on its way to the compost heap
About chemicals in manures

For food gardeners it would be ideal if all manures came from animals that had not been treated with chemicals and that live in an environment that is completely chemical-free.

However, in reality  ......
  1. We may have no idea whether manure we get contains chemicals or not because we don't know whose farm the manure came from and therefore can't ask the farmer.
  2. Farmers and land-holders will worm their livestock from time to time, and may use chemicals in other aspects of health management of livestock.  
  3. Some animals may be given supplements to improve performance (race horses) or meat quality (pigs and chickens).
  4. Use of herbicides is wide-spread.
A couple of months ago Food Garden Group member Margaret reported on the Food Garden Group Facebook page that her tomato plants had unhealthy-looking curly leaves and were not growing well. Here is one of the photos that show the problem:

One of Margaret's affected tomato plants
Margaret wanted to know whether anyone knew what caused this curling and what she could do about it. Cameron, another member of the page, then reported that he had a similar problem.

It can be very difficult to find definite answers to queries of this kind. However, in this case, a concerted effort buy a number of members of the page lead us to the cause.

Margaret researched the issue and began to suspect that a chemical in horse manure she had bought half a year ago was to blame. Cameron had made seed-raising mix with 6-12 months old compost containing sheep manure bought from a local scout group and thought it might be a chemical in that.

Daniel, another member of the page, joined the discussion.  He works in a lab and knows people who work for Analytical Services Tasmania (AST).  To make a long story short, AST made a wonderful gesture: they provided a free test of soil samples provided by Margaret and Cameron.

AST found in the soil samples traces of a herbicide that kills broad leaf plants such as blackberries, gorse and bracken in pastures, without affecting grass and grains. Livestock eat the sprayed but unaffected grass and pass the chemical via their manure. The chemical is designed to not easily break down, so its eradication of broad leaf plants is more long-term. This herbicide is one of many examples of an effective short-term solution, with long-term negative environmental consequences the makers chose to ignore.

It will take time for the chemical to break down. Margaret reported that she has started the clean-up process in her garden by adding Letetia's fermented liquid (see here on this blog) to the affected soils, so microbes will accelerate the break down of the chemicals.

The lesson here is NOT to avoid manures. That would be throwing out the baby with the bath water. The lesson is to always compost manures for at least half a year, ideally a year, before using.

Thank you Margaret, Cameron, Daniel and AST for the valuable lesson we have learnt here.

If this story has made you doubtful whether you should use manure in your garden: I have used manure from many sources for a decade and a half, and have never had any problems with chemicals affecting my crops.

A few wonderful members of our group make manure available free of charge if people come and pick it up.  I hope to get info from them re their use of chemicals (if any) and will pass this on.

Comparing manures

Any animal with a vegetarian diet will produce manure that is suitable and very beneficial for use in the food garden after composting.

NEVER EVER use manures in your food garden that come from meat-eating animals.   Obvious examples are dogs, cats, and yes, humans. There is a risk of parasites and disease organisms that is not worth taking!  Chickens are sometimes fed blood and bone (this happens on some chicken farms that farm chickens for meat).  Animals are sometimes fed pellets that contain meat products.

Ideally you know what the animals have been fed because you know and trust the farmer or owner of the animals.

The value of manure can vary greatly depending on the type of animal it comes from, what it has been fed, the age of the animal, the quantity of bedding (straw and other materials) that is mixed with the manure, and how long the manure has been exposed to the environment.

The only useful comparison of animal manures I found was in Rodale's All-new Encyclopedia of Organic Gardening, An Illustrated Guide to Organic Gardening (Sunset Publishing). It compares some commonly used manures on their nitrogen, phosphorus and potassium content, which is a traditional way of measuring the value of fertiliser.  I simplified the numbers I saw in this publication and the table below is the result:

As mentioned above, manure is valuable for many reasons, not just as a way of adding nitrogen, phosphorus and potassium to your soil, so don't think 'the higher, the better' when you look at the table above.  Don't conclude that cow manure is not worth using, and that we should all try to get rabbit manure.  The table does make it clear that rabbit and chicken manure in particular should always be composted thoroughly before use in the garden.

  • Wear gloves when handling (bags of) manure and wash your hands afterwards.  You may also want to put the clothes you wore in the washing machine.
  • Bagged manures are often dusty.  Wear a simple face mask to avoid inhaling the dust.
  • Some people pulverise manure by running it through a mulcher or going over it with a lawn mower.  Great idea, but wear a face mask and gloves.
A look at some manures available in Tasmania

Alpaca manure
Alpaca poo is comparable to sheep manure, both in the way it looks and its benefits. It is a great soil conditioner. It is easy to handle. It will not burn plants and can therefore be used in unlimited quantities.  Alpaca poo is easy to collect because the animals do their poos in a part of the paddock they decide is their toilet area.

Cow manure
Cow manure is low-nutrient, but great for improving soil structure. Because it is low nutrient it safe to use in unlimited quantities. Ideally, the cow manure you buy has been pulverised.  If not, you may like to chop it up with a spade.

Horse manure
Horse manure is richer in Nitrogen and Potassium than cow manure.  This has the advantage that you do need less horse manure than cow manure to achieve the same effect.  Most horses eat grass and weeds like cows do, but because their digestive system is less efficient than that of cows, horse manure generally contains more weed seeds than cow manure does.  Horse manure is sometimes mixed with bedding straw, which is fine, but it will compost and thereby take Nitrogen out of the horse manure. Race horses may be fed supplements.  Their manure should be composted for up to a year.

Poultry manure
Is a strong fertiliser that always needs to be composted first. High-nitrogen chicken manure is also high in phosphorous and is generally free of weed seeds. It is ideal to add to compost as both an accelerator and a nutrient booster. Chicken manure is often very high in calcium because of the feed given to laying hens, so it can be useful when trying to break up clay soils.

Sheep manure
Manure from sheep fed on hay and grain is more potent than that of most Tasmanian sheep that live in paddocks.  Sheep poo is a great soil conditioner. It is easy to handle. It will not burn plants and can therefore be used in unlimited quantities.

How much manure should I use?

This depends entirely on what your soil is like, and what you are going to grow.
If you are going to grow cabbages, broccoli or cauliflower in a sandy soil, mix in as much well composted manure as you can lay your hands on and hope for the best.
If you are going to grow carrots, don't add any manure, or your carrots will fork.
For more info on crop rotation see here on this blog.


Manure is great for food gardens.  There are at least four reasons, all compelling, why it is best to always compost manure before putting it on food gardens.  Before using it on your garden, compost manure for at least 6 months, ideally a year.  Try to get manure from a farmer or property owner who does not use long-lasting herbicides.

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