Destroying Soil Structure

I have been busily destroying soil structure; killing microbes; and depleting nitrogen from my allotment soil today.  ( Am I just being facetious?)

Taking out the second spit 

The second spit was taken out of the trench and used to make the path behind the large shed.   The fork was used to break up the bottom of the trench another spit deeper.  I was still removing mare's tail rhizomes even from this depth.

The wooden edging supports had rotted
away so I had to replace them.  

Either there was a hard pan or I had reached the bed rock because there was a hard layer beneath this which I broke up as much as I could with the fork.

Once the trench was big enough, lots of organic matter could be buried.  In this case several branches; lots of rotten processed wooden planks and bits of wood, tops of herbaceous perennials; cardboard and non rhizomal weeds.  This is a little like hugelkultur.

Guaranteed to produce nitrogen deficiency? 

This rough organic matter was covered in
subsoil and leveled out.

Can you add too much organic matter to your soil?  This amount of undecomposed organic material is sure to deplete the soil of nitrogen locking it up in the bodies of microorganisms and fungi.

Partially decomposed woody chippings 

So I added six barrow loads of woody chippings for good measure.  These were leveled and worked into the subsoil. Was this enough organic matter to cause nitrogen depletion?

Adding comfrey liquid to the trench.

A watering can full of neat comfrey liquid was then watered over the woody chippings to add a little nitrogen.

Six inches of top soil were put on top of the chippings and leveled. Then four barrow loads of farmyard manure were added to the trench.  I will cover the farmyard manure with sieved top soil and a little chicken manure tomorrow to a depth of at least 30 centimeters.  Probably enough organic matter now to cause nitrogen depletion.

Have I been causing nitrogen depletion?  Although most of the scientific papers I have read are ambiguous about this, I probably have caused some nitrogen to be locked into the bodies of microorganisms.  However, most of this is happening in the subsoil where vegetable plant roots do not normally penetrate and thus will not affect cropping.  I am adding organic matter during the winter allowing it time to rot down and add some nutrient to the soil  for the summer months.  The nitrogen that is being scavenged by the microorganisms and fungi during decomposition has been mainly leached from the top soil. So this gives me a means of capturing any nitrogen losses that might occur from the top soil.

Therefore I am; adding nutrients locked up in organic matter; capturing leached nitrogen from the top soil; increasing the cation exchange capacity of the soil; increasing the depth of the top soil and increasing the population of soil microorganisms which all lead to increased fertility.

What goes around comes around.  I will continue to add copious amounts of organic matter to the soil by digging.

So, have I destroyed my soil structure?  The soil structure is how sand, silt and clay are arranged in soil particles and how they adhere to each other in aggregates.  It determines the bulk density and the water and air filled porosity of the soil.  (How well it drains and allows oxygen to enter - among other things) This is what some gardeners call friability of the soil. Experience and reading has convinced me that adding organic matter to the soil aids in the forming of useful soil aggregates; increasing water and air filled porosity and reducing bulk density. To destroy the existing soil structure  moisture, clay and organic matter needs to be removed. Wind erosion, takes away the lightest soil components of clay and organic matter.  Mulching prevents this from happening particularly in hot climates.

After a little more reading I have been convinced that soil aggregates are produced by root hairs, fungi mycelium and animal mucilage both alive and dead.  Once organic molecules are removed from soil it looses its coherence and falls apart. 

In temperate countries such as England the ground is seldom dry in January and I am adding copious amounts of organic matter.   I don't think that I am destroying the existing soil structure because I am not loosing clay, organic matter or moisture and soil aggregates will continue to be produced through adding organic material.

However, I am loosing fungal hyphae and root hairs...

Soil compaction occurs when soil particles are squashed together reducing air and water filled pores.  This can occur on allotments when the ground is very wet and walked over.

However, I sieve the soil through an old bread tray which brakes up large sods of soil allowing organic matter to be thoroughly mixed in.  The sieving adds lots of air, and by default oxygen, to the soil which aids in the decomposition of organic matter. Most of the organic matter is being mixed with the subsoil where there is little evidence of any previous organic matter.  When the added organic matter decomposes, it forms a very friable, top soil like material. Thus I am deepening the top soil.

Therefore, I would suggest that I am increasing water and air filled porosity by encouraging the formation of additional soil aggregates.  I am improving the soil structure.

There is some evidence that cultivation reduces the number of soil organisms.  For example, the population of worms is greater in grassy swards than in tilled soil.  This I can understand because, although the proportion of worms killed during digging is very small, there are some that inevitably die because of the operation.  However, there are copious amounts of worms living in the woody shreddings and farmyard manure and these are being added to the soil.  In addition any worms I find in parts of the allotment that are not growing areas - in storage areas, under the hedge and under paving slabs of the path - are put onto the top soil in the trenches to find their own way into the soil.  All these worms may not be species that live permanently in top soil, however a population of some of these species may find a more long lasting habitat because of the amount of organic matter I add to the soil each year.

The difference in population of worms seen in cultivated and grassy areas may be due to a habitat preference by the worms rather than a result of digging. Also the number of worms in compacted soil of paths seems to be much greater than in cultivated soil.   So worms seem to like soil with a relatively high bulk density.

Digging may well reduce the numbers of fungal hyphae in the soil.  However, the amount of fungal hyphae on and in the rotting wood added to the bottom of the trench was prodigious.  Not only have I added a significant number of hyphae, I have also provided a source of nutrition for them to use and reproduce in.

Whenever plants are planted in the allotment, I add mychorrhizal fungi spores to alleviate the possible reduction of fungi due to cultivation.  It is now suggested that there are plenty of mychorrhizal fungi already in the soil and adding spores is unnecessary.

Well you can't have it both ways.  Either I am reducing the amount of fungi in the soil through digging and need to add more spores to ameliorate this or digging has no effect on the soil population of fungi and adding fungi spores is unnecessary.

I will continue to add mychorrhizal spores.

Microbes are far too small to be affected mechanically through digging so they are probably only affected if the soil is allowed to dry.  Even this will only lead to them going into a more resistant dormant form.

Digging does not sterilise the soil.  The reduction of organic matter through erosion; addition of excessive nitrogen; mixing in of oxygen and stimulation of the microbial element leads to sterilisation of the soil.

I am told in a round about way not to increase the population of soil microbes by adding undecomposed organic matter because this leads to nitrogen being locked away in their bodies.   I am also told not to dig because it reduces the numbers of microorganisms.  Therefore it is bad to both increase the number of microorganisms and also to decrease the number of microorganisms. You can't have it both ways.

Some heterotrophic microorganisms are autonomous nitrogen fixers which add nitrogen to the soil. Their energy source is the dead organic matter in the soil.  These are the Azotobacter, Bacillus, Clostridium and Klebsiella. While it is suggested that they don't contribute a great deal of nitrogen to the soil, every little helps.  Adding organic matter to the soil will increase the populations of these anaerobic bacteria.

I will continue to add organic matter.

And I will continue to dig.