Looking after underground “Livestock” critical for soil health

May 6, 2009
By: Mark Kesby - Projects Officer, LPLM

Soil health and soil biology in particular is vital to the long term viability of farming (and gardens as well!). So I thought that I would attempt to explain a few of the basics in this column – but it will take a few weeks to do so!

There can be more organisms in a single teaspoon of soil than the total number of people on the planet (6 billion). The variety is staggering and includes millions of different species. Another way to look at it is that in a healthy soil, the total weight of “underground livestock” can be greater than the above ground livestock (sheep or cattle) grazing the pasture.

These “underground livestock” come in an incredibly diverse range of size and type. They can be divided up into the macrofauna (earthworms and beetles), mesofauna(tiny mites), microfauna (nematodes, protozoa and amoebae), fungi and bacteria.

We all know earthworms but know very little about them or how to encourage them. Worms feed on plant debris and soil. Their digestive system concentrates the organic and mineral constituents in the food they eat, so their casts are richer in available nutrients than the soil around them. New Zealand research has shown that worm casts release four times more phosphorous than the surrounding soil. So worm tunnels are nutrient rich preferential paths for roots to penetrate deep into the soil. As well, the tunnels (and the worm casts) improve soil structure and hence water infiltration and drainage.

Research in New Zealand and Tasmania found that earthworms introduced into worm-free perennial pastures produced an initial increase of 70-80% in pasture growth with a long-term
25% increase. Overall, there was a close correlation between pasture productivity and total worm weight – the more worms, the more pasture.

Earthworms don’t like the soil too acid, alkaline, dry, wet, hot or cold or compacted. Consequently, their presence is a good indicator of a healthy soil.

Farmers who first cultivated the rich black soils of the Liverpool Plains often reported that the earthworms were so thick that they blocked their machinery. Those days are long gone – earthworm numbers in many cropping paddocks are now low. Increasing their numbers is a crucial step in improving soil health.

Worms can be encouraged to breed up in our soils by increasing the organic matter that they feed on. Permanent pastures, green manure crops and retaining crop stubbles (direct drill) are all ways of achieving this. A surface cover of litter also reduces the extremes of temperature and maintains soil moisture for longer, further encouraging worms. In the garden, adding a mulch cover will encourage worms as well as all the other benefits.

Some farming (and gardening) practices reduce earthworm numbers. Top of this list is cultivation. Contrary to urban myth, an earthworm cut in two is a dead worm NOT two worms. So cultivation kills worms – especially aggressive tilling of moist soil. All farmers who move to zero or minimum till report a significant increase in the number of worms.

Highly acidifying fertilisers such as ammonium sulphate and some fungicides reduce worm numbers. For example, orchards sprayed with Bordeaux or other copper sprays contain few earthworms. Consequently, they contain few earthworms and have peaty surface mats and poor soil structure.

Anyone looking for more information on earthworms (or the wider topic of soil biology) would do well to start at the NSW DPI website soil biology page. It was the source of much of this article and contains excellent basic information.