Monday, 5 December 2016

You can't sterilise soil by digging.

Soil cannot be sterilised by digging.  If this was the case, the danger of such things as onion white rot, club root and parasitic nematodes could be removed by simply turning over the soil.  Even after several years of digging and trenching, these diseases and pests remain in the soil and can only be diminished by strict rotation and removal of infected plants.

Unfortunately slugs cannot be discouraged merely by digging over the garden. 

They couldn't decontaminate Gruinard Island from anthrax just by ploughing the soil.

What irritates me is that I was beginning to be convinced by the statements that digging killed life in the soil.  I was also taken in when Geoff Lawton said that the heat of a compost heap came from dead organisms killed off by the turning process.   Experience and thought experiments must at least give pause for thought.

The compost in the bins in the photograph below has been turned every other day for two weeks, following the Berkley method of composting.  It could be suggested that the mixing of the compost is similar to digging and trenching allotment soil.  The population of worms in the compost does not diminish, in fact there is evidence that it actually increases. 

The species of worm, probably Eisenia fetida,  may be adapted to living in a habitat that is constantly being disturbed by the compost turning process but that is unlikely.  However, it is remarkable that Eisenia fetida seem to prosper in an environment of disturbance in a compost that is regularly turned whilst, it is often said, soil living worms, like Lumbricus terrestris,  have their population drastically reduced by a similar activity when allotment beds are dug over. It is very hard to accept. 

This is the rough compost sieved and returned to the bins.  The black berry and black
currant stems prunings can still be identified but they are very easy to break apart now.
Oh, and lots of worms. 

The evidence of the survey on results, found that vegetable beds had the highest density of total earthworms.  They suggest that this was against their expectations and probably due to the beds being regularly turned over and organic matter added.  However, with the data that they have at the moment there is no significant correlation between the amount of organic matter and the number of worms. 

Previously it was reported that there were far more worms in pasture than in cultivated land.  For example Satchell in 1967 showed that at Rothamsted  there were more worms in Park Grass, a permanent mowing meadow, than of Broadbalk which was under a continuous cultivation of wheat.  I think that Darwin said something similar too.  So it is a surprising result from the earthworm watch experiment.  This may just be due to habitat preference or some other factor.

This says more about monocultures and soil degradation than it does about worms.  If the suggestion that we get more worms with more organic matter is correct,  then maybe the amount or quality of the organic matter in the Broadbalk field may have some bearing on the population of worms. 

There is some support for the idea that compost made from a variety of plant and animal material is better than that made from one source. The hypothesis is that the types of organic matter, their associated ecology and how these are mixed within the compost heap will affect how beneficial it will be for vegetable crops. 

Continuous secretion of the same exudates from a monoculture year after year will lead to a modification of the populations of soil organisms.  

Apart from the concentration of organic matter, other factors may influence the quantity of worms in the soil.  This is already quite evident that worms like Eisenia fetida prefer to live in habitats that are almost entirely decaying plants and animals, whilst others will live in soil which has a much lower percentage of organic matter. 

There is some suggestion that putting lawn mowings into the compost will produce a thick mush of wet, smelly, anaerobic decomposition.  Although turning the grass mowings and mixing them thoroughly with other ingredients usually prevents this from happening,  the number of compost worms associated with matted compost is noticeably greater than the rest of the compost. 

Turning the compost every two days makes it particularly friable.  The low bulk density of this material must allow ease of movement into and through the compost.   Also the introduction of copious amounts of air will allow access of many different invertebrates to populate the heap and this is what is found. 

The comparison with turning the soil over cannot be ignored particularly if copious amounts of organic matter, that is well mixed and from a variety of sources, is added at the same time. 

There are many reasons for not digging the soil but the often stated "It reduces the populations of invertebrates" is not necessarily a valid one.

I would suggest that the reduction in the population of all soil organisms in seriously degraded soil has little to do with cultivation by digging or ploughing but much more to do with the lack of compost, manure or other good quality organic matter being added. 

Soil organisms need a source of carbon to consume.  Without it they will starve and populations will fall.  Digging adds air to the soil and this will increase the speed of decomposition of organic matter leading to its depletion unless it is constantly replenished. 

The production of heat in a compost heap comes from the exothermic reactions of metabolism.  Food (carbon) is burnt (in a very controlled way) to produce energy.  One type of energy that is produced is heat.  To get heat in the compost heap there must be life.  So much life that the compost heats up quite a bit.  The carbon used up to form this exothermic reaction produces energy and is lost as carbon dioxide to the atmosphere.  The amount of carbon in the compost heap is reduced and the concentration of other nutrients is concentrated in what is left.  The nutrients are already there they are not made by the composting process they are just concentrated. 

If the life in the compost were to die, due to turning every two days, there would be no increase in temperature.  The temperature increase is brought about by the life in the compost thriving and multiplying.  If these organisms had been killed off by the turning/digging of the compost there would be no increase in temperature. It would remain cold. 

The heat of a compost heap does not come from nowhere.  Turning or digging the compost adds air to the compost heap and this enables more aerobic decomposition to occur.  Now there are obligate anaerobic bacteria that are poisoned by oxygen and there are obligate aerobic bacteria that need oxygen but there are a significant number of facultative anaerobic bacteria that can live both anaerobically and aerobically.  In fact there is a confusing number of different methods of metabolism in bacteria. Believe me bacteria are ubiquitous and, as we are now finding, they are quite difficult to kill off.  Bacteria are so minute  turning/ digging the compost would be extremely unlikely to affect their populations significantly.

While I can accept that the heat generated by the compost heap will tend to kill off weed seeds, I find it difficult to accept that disease organisms will be overly affected.  Best not to put diseased material on the compost heap. 

Fungi are obligate aerobic heterotrophs.  They are also throughout the compost.

To say that anaerobic bacteria are bad and aerobic bacteria are good is totally meaningless.   Both types of bacteria decompose dead material and mineralise forming plant available nutrients. 

The thing about an aerobic metabolism is that it produces much more energy, due to the citric acid cycle and electron transfer chain,  decomposing and assimilating much more quickly.  This means that mineralisation, producing plant available nutrients, occurs much more speedily. 

So you don't sterilise compost heaps when they are turned and you don't sterilise soil when you dig it.  Use better reasons for not digging. 


  1. Personally although I think that cultivations will effect populations of soil organisms -compared to no dig - for good or bad depending on circumstances for anyone to suggest that cultivations sterilise the soil in any way is completely ridiculous. it is nice to agree with you as I often do!.
    And I also think the idea that cultivations expose pests to the elements is flawed

  2. Thanks Roger I always value your comments. I think that you are right about cultivations exposing pests to the elements and to predators is not correct. Also I find that the common quoted, "leave your soil rough dug in the winter so that the frost will break it down into a lovely tilth." Is completely wrong. It is not the frost that breaks down the concrete like blocks of soil that have cemented themselves together during the winter; it's you and a lot of very hard work in the spring.