Monday 23 September 2019

The curse of compaction

Soil compaction is widely misunderstood

Do not walk on this!
When in the garden particles of sand, silt or clay are squashed together by mechanical forces the consequences are dire. Free movement of water, air, roots and worms are severely restricted and often completely impeded.

When healthy soil stocked with vegetation settles naturally without being squashed or cultivated when wet with heavy implements it IS NOT COMPACTED. It is firm to walk on, yet when closely examined by for example very light forking it has low density and is permeated with channels and cracks laid down by roots, worms and natural expansion and contraction; all loosely cemented together with soil organic matter such as humus and glomalin. 

With more energetic forking this soil reveals a lovely crumb structure. Such action is the start of its eventual structural destruction!

These are examples of real compaction

1. Walking on really wet loose soil, worse driving over or stacking heavy bricks or equipment
2. Working wet soil. You can of course winter dig your allotment in the rain
3. Rotavating or ploughing at the same level each time (a present from the original farmer)
4. Leaving acid sandy grassed soil undisturbed for decades (or more likely centuries) can create hard iron pans

Compaction often occurs when soil particles are in effect exposed ‘in tooth and claw’ to heavy rain when loosened soil and crumbs wash down and later bake hard. 

I don't care

Certain soil textures are very prone to ‘capping’ if excessively fine seed beds are subjected to rain and a thin layer hardens so much that seedlings fail to push through.

The cure for compaction is usually digging or in the case of very hard pans resorting to a pick axe.
The trouble with regular digging and rotavation is that although it conveniently temporarily separates the particles it also destroys the major soil networks and particularly the soil crumbs. Cultivation sows the seeds for the need of further cultivation. Especially on clay soils.

As a no dig gardener I seek to escape this vicious circle. This does not mean I don’t use my border spade for all manor of processes that do disturb the soil. It’s just that I keep this to a minimum and confined only to where it is needed such as making a (very small) hole to put in my plants.
In a new garden following the horrors of builders I might even dig (once). I did inherit an iron pan in a small corner of my present garden. Although I left most of the patch alone and went ahead with planting after weed elimination, I did double dig a small area to sow a grass path! (Matted dry grass and crusty iron layer just had to go and was broken up and deeply buried).

Gardeners are advised keep off their wet soil. On the contrary I rush out and plant. I explain the logic in this important post.

Does nature correct compaction?

Natures methods can be quite brutal. Cracks in clay soil close in wet weather and reopen in the same place when dry

In the top spit nature will usually correct mild compaction. Traditional Winter digging is promoted to expose clods of clay soil to the expansion and contraction of freezing and thawing, wetting and drying.
With soil severely left alone the same will happen and nature will  have less to do correcting the harm of previous destruction. All Summer the worms will have been in there working away! I find that on my settled undisturbed soil worm casts are particularly prolific in early Spring. Such casts are wonderful mixture of particles and organic matter and are a first step to forming ‘water stable crumbs’
It will take a very long time - if ever - for nature alone to correct hard pans below the level of freezing. Think of puddled clay lining lakes. 

(The cracks going down in the picture will help with root, water and air penetration into otherwise hard subsoil)

What should we do with untidy soil surfaces in our borders that are aesthetically unpleasing?

Just enjoy the liverwort
Might I suggest you just change your mind and regard it as natural and attractive. Brenda’s sister Joyce goes into raptures at the sight off my liverwort and moss. For myself I just cringe if borders are fluffy and loose. Worse if there is a notice telling me not to walk on the soil.

Rowena, I understand that my methods of scattering vegetable debris on the surface is far too untidy for most people’s garden! There must be another way - and of course I did post about Harry’s worm bin.

It is not appropriate to employ a gardener to regularly turn over your borders to correct all those compacted bicycle tire tracks or footprints left by your offspring. They do so much damage on loose wet soil. Don’t tell me you are aerating after you have savaged the roots of your plants.
I know several jobbing gardeners who turn over borders. They have an uncanny ability to fail to turn in the weeds! Perhaps you will call them back sooner?

So what should you do?

Soil surfaces can look very scruffy. You can use or combine various strategies in your ornamental borders and under your fruit and vegetables.

Control weeds often and when they are small. I never remove them but leave them to desiccate and die. They are hardly there two minutes if you do it at the right time. Sometimes when hand weeding I will fling large previously missed weeds out of sight to the back. This is NOT bad hygiene and if it feeds the slugs I approve. They prefer the weeds to my plants.

Regular hoeing is good practice and will do negligible structural damage - especially if you just sever the weed and only the weed at ground level. Do not fluff the whole border as is often recommended.

Readers will know my love of glyphosate and most of my own weeds are sprayed. Again, if weeds from seed are small they will not be unsightly as they die.

I have more recently used my Black and Decker electric strimmer to sever small and medium sized weeds. I set it to the vertical twist. Useless against established perennial weeds such as couch of course - as is hoeing and hand weeding.

In a previous post I identified multiple uses of a spade
Once in a blue moon you might wish to very shallowly skim off liverwort or pearlwort with a skimming action of a small border spade. If moss establishes, wait for dry weather to rake it off - or stick in a label and everyone will admire it.

I find a light plastic lawn scarifier is useful to clear off debris such as untidy cast-off twigs or small branches. You can also use it to restore levels if you have entertained slugs or rabbits.

Let the leaves lie
Where appropriate let Autumn leaves lie

Mulching with well decayed compost helps break the vicious circle of repeated cultivation.

Cover with a mulch and add to all the benefits of no cultivation
More permanent mulching with such as bark chips or gravel gives an entirely different dimension to you garden maintenance and is a perfect antidote to surface compaction

Consider more dense planting and use of ground cover plants

I have said more provocative things than usual today. I have said them all before in more detail in earlier posts of which this is just a precis.
If you insert suitable words or phrases such spade, slug, mulch, compaction, clay soil in my search box at the top (or even the very bottom as you scroll down) you will  find more detail.
I would particularly direct you to these links

I wrote about mulching

I regard mulching with gravel and stone particularly valuable for water conservation

Weed control with glyphosate avoids damaging cultivations

The things people ask about compaction

What I said about compaction in one of my first posts


  1. I recently read in a well known weekly garden magazine that you should double dig your garden this autumn. It is no wonder young people who start of with great enthusiasm to grow their own eventually give up because of the time and work involved. You still have a big education job to do Roger!

    1. I do feel that over our careers we have seen a change in thinking Brian
      You rarely see borders that have been dug over!


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