Archive for November, 2012

Because manure is organic and natural it must be good right?

Well not always.  The use of raw animal manures is actually banned to a large extent under most food safety programs (and that includes organic produce) due to the risk of pathogens that can infect humans such as Salmonella, Listeria and E coli.  They are totally banned from applying to any leafy veges and have limitations in terms of days before harvest for other crops.

Despite what most people think, the nutrient content of most animal manures is largely water soluble and in sandy soils doesn’t hang around for long.  Given most people also apply most of the manure at planting time when the plants are smallest and have the least extensive root system, all this results in is ground water pollution!

Animal manures also have quite a low nitrogen to phosphorus ratio which means you end up putting on a heap of manure simply to get enough nitrogen – which means the system is well overloaded with phosphorus – more leaching!  The reason manure gets such a favourable rap in most places is because if you have a large clay component to your soil, some of the phosphorus binds to that clay and becomes unavailable.  That has the effect of bringing the nitrogen ratio up to reasonable levels.  That doesn’t happen in our sands.

The ability of the clay to bind to phosphorus diminishes over time.  Depending on how much manure and fertiliser you apply, eventually those sites are filled up, the clay can’t hold any more and any more phosphorus you apply will leach in the same way as nitrate.  A soil survey done a few years back showed that most gardens in the older suburbs of Perth are in that category and don’t need phosphorus to be applied – probably for the next 10-20 years!

The nitrogen in raw animal manures is mostly ammonium to start with –that is toxic to plants in large amounts – most plants need to convert ammonium to nitrate to take up the nitrogen.  While ammonium is more readily bound to cation exchange sites in soil, in our sands it’s also very readily converted to nitrate which is highly leachable (more groundwater pollution).

Average composition of stored animal manures at 40 to 60% moisture.

One tonne (1000 kg) of manure contains
  Nitrogen (kg) Phosphorus (kg) Potassium (kg)
Range Average Range Average Range Average
Horse 7 to 12 9 5 to 9 7 4 to 13 5
Cow 8 to 11 9 5 to 8 6 4 to 13 7
Sheep 5 to 14 9 4 to 10 8 5 to 7 6
Pig 6 to 12 9 5 to 8 7 4 to 10 7
Fowl 8 to 26 18 6 to 20 13 4 to 12 7
Note: To convert these figures to percentages, divide by 10.

Chicken manure breeds flies – in particular, stable fly which is a biting fly that attacks horses.  These flies will also breed in vegetable residue if it is applied and buried too thickly.  Other types of manure breed flies but to a lesser extent.  The flies are attracted largely by the ammonium but also by other compounds in the manure.

Many manures can bring in in large amounts of weed seeds – weeds that may not exist in your area or your garden until you buy in manure from elsewhere.

So how to use manure responsibly? 

  • Composting is a great start, it gets rid of the harmful pathogens.
  • Apply in amounts that are relative to the size of the plants and only in the drip zone.  Don’t assume that one application will last.
  • Amend your  soil with clay so it can better hang on to nutrients.
  • Dilute manure with other sources of organic matter that are lower in phosphorus such as greenwaste materials (also composted to avoid bringing in disease and weed seeds to your garden.

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