Wednesday 30 July 2014

Soil and garden compost as a bulky ingredient in seed and potting compost.

Amateur innovation versus professional exactitude

All the plants pictured today are grown in containers of soil that have no extra ingredients other than fertiliser and occasionally lime.

I am playing with fire today when I suggest unsterilized garden soil might be used in a pot! For forty years in my lectures I declared that you must never grow pot plants in soil from the garden! When we moved to Seaton Ross fourteen years ago I found that my soil to be an extremely fine water retentive sand. I tentatively started to use it as a substitute for regular potting compost and liked what I saw. It is very rare now that for other than very special purposes I use anything else.
When I started blogging I wrote a timid little post about this. It was called ‘Pot compost, breaking the rules’. My posts were edited then and I am grateful to Cathi who struggled a long time to make that piece intelligible. I don’t even know if pot compost is a proper name, but I rather like it! I will be more forthright today!
First for my American readers I must explain that strange word ‘compost’. In the UK we use it with two separate meanings. Most of us are besotted with ‘garden compost’ from garden waste that has decayed in a heap, unsightly wooden/wire structures, or in a bin. We also use the term ‘compost’ for mixtures in our seed trays, pots and containers and call them either ‘seed composts’ or ‘potting composts’.
Of course in ancient times garden compost and potting compost were exactly the same thing.
For my purposes today I am treating ‘garden compost’ and ‘soil’ as synonymous terms for these two bulky ingredients in a seed or potting compost. This is not quite correct as both can be separately used and are not the same! Garden compost is bulky material in various stages of late decay and might contain a little soil from the roots of composted plants. The organic matter will eventually decay away to carbon dioxide and water, beneficially releasing its nutrients, but not lasting forever! 
Actual soil can be used and reused many times. Unless garden compost is very well decayed it will be far too rich to be used exclusively as a bulky ingredient. My first choice material is organic-matter-rich fertile garden soil with a suitable texture.
(texture is the technical term that refers to the proportions of sand, silt and clay)

Loam in John Innes composts
Soil in compost has a very long history. John Innes compost has a fine pedigree and was the ascendant compost in horticulture for perhaps sixty years. More than 50% of the volume of this compost is sterilized loam. Loam was defined as a soil with suitable proportions of sand, silt and clay and fibrous material and it was prepared from lifted, stacked and decayed grass swards. This soil was sterilized with steam before making up the compost. Properly prepared John Innes was a wonderful compost. Modern versions are often a pale shadow of its former self although we do have the advantage  today of slow release fertilizers unavailable to its inventors Lawrence and Newall at the John Innes Institute almost a hundred years ago.
Loam for John Innes was sterilized with steam to control pest, disease and weeds. Because it killed almost all soil bacteria, the new micro-organism profile which developed in consequence enhanced the release of soluble nitrogen and this was at the time considered an advantage.
My comments today are about unsterilised soil. Some modern thinking is that an undisturbed micro-organism profile of fungi and bacteria in a natural harmony might be a good thing. Probably misguidedly, many gardeners today add mycorrhiza from a packet! 
The physical structure of specially prepared loam is not usually suitable when used alone for potting compost and the John Innes formula adds sharp sand  (more latterly grit) and granulated peat for water retention and aeration. Both are still superb ingredients for gardeners who make up their own soil based compost.
The slow release nitrogen fertilizer in traditional J.I. compost was hoof and horn meal which as it broke down acidified the soil. In addition with peat as a component, lime was required. This is less relevant now and if the pH of the soil in the ground is satisfactory the same applies when it is used in a pot. In my own all-soil compost if I judge to need lime I add dolomitic limestone.

Problems with amateur composts today
John Innes composts went out of fashion as they were expensive to prepare and soil supplies were limited and variable. Also, with the then available fertilizers, and as a result of the nutrient boost which was a consequence of sterilization, they deteriorated in storage. 
They were superseded by wonderful peat based composts which in particular were superior for propagation. I do not share many folk’s fashionable reservations about peat. I will not explain the reasons for my opinion today but I fully concur with the opinions here. Amateurs and some professionals never properly understood the water management of peat based compost and I do not miss dried out hydrophobic compost when I buy plants from  the garden centre.
There are some really excellent peat free composts nowadays but others are rubbish and present numerous management difficulties. As I never use them I am completely unable to advise which to buy. Many  potting composts today are based on commercially composted green wastes and the product is very variable. Perhaps you get what you pay for. Composted green waste has many legitimate garden uses but only that produced by very well controlled systems is suitable for potting compost.
I do not approve of shipping compost materials half way round the world to find substitutes for peat.

So what heresies am I advocating?
I cannot promote my own methods carte blanche to use soil in small pots and seed trays. My own soil, almost uniquely, has a satisfactory texture. Most soils, especially clays are completely unsuitable. Where I do part from conventional opinion is for growing plants in large containers, perhaps those greater than ten litres. A lady at the bridge club the other day asked me to recommend a compost for her bourgeoning olive. I was completely at a loss to give her sensible options but after quizzing her closely, discovered she had highly organic well drained garden soil. Eureka!
Visiting my son in Sorrento I noticed all garden ‘planters’ were filled with their native volcanic soil. 
Not all gardeners have suitable soil to use as the sole bulky ingredient in a large container, but in my opinion, very many do.
My comments today are relevant to any gardener who uses unsterilized soil alone as I do, or as an ingredient in a mixture.

The case against my methods

My ferocious cacti need some care when weeding! I use my secateurs as tweezers! The lily seedlings are also in soil.

Unsterilized soil may contain pests, diseases and weed seed.
Most soil is not as water retentive as traditional compost ingredients such as peat.
Many soils will be insufficiently aerated in shallow containers when wet.
Too much clay content will lead to unacceptable hardening, shrinking and cracking when dry.
Soil is highly variable in texture, structure and nutrient content and in no way is suitable for mass production methods that require uniformity.

We have rather exotic pests in Yorkshire. Poppy is nicking the nectar
So why do it?
Because I can!  My own garden soil has properties such as excellent water retention consistent with good aeration.

Where large containers with a deep profile are used the physical properties of soil in a container are similar to those in the ground.

I use huge quantities of compost and my method is almost free!

Soil can be recycled and is not wasted. If you must, you can return the soil to your garden and replace it with some more. Personally I am completely cavalier in reusing refreshed old soil-compost many times over.

If you are using your own soil you are not scouring the environment, driving to the garden centre for sometimes fairly dubious materials transported from all over the world.

Years ago, we used to compare the then new peat composts with loam based John Innes. The prevailing view, which I admit was disputable, was that JI compost was better for long term culture because of the loam. I tend to concur for plants that are to remain undisturbed for many years. Even peat decays and slowly oxidises away and containers need to be topped up. Not so with soil.

Yes I have weeds! I enjoy hand weeding my pots although in the case of large containers of certain plants I can lift up my nozzle and accurately spray them. My own methods in my garden attempt to stop weeds seeding and weedy perennial roots are not present in my soil. I can imagine some soils have so many weed seeds and are wick with wicken (couch grass) that my methods would be completely unsuitable. Plants bought from the garden centre in originally sterile compost are frequently full of weeds and users of ‘proper’ compost have the same problems as I do with liverwort and moss.

It would be foolish to use soil contaminated with for example, club root or white rot of onions - especially to grow onions and brassicas! Subsequent planting would spread these diseases around the garden. I have described how ‘damping off’ is not a problem for me and anyway as a disease of seedlings and not of mature plants, is irrelevant to our discussion today. I personally suffer no more or less with disease, unexplained plant death and sickness than I previously did when I used conventional compost. My plants in pots are just as healthy as those in the ground!

There are even more flowers on my agapanthus this year

There are a number of pests that do not like soil but love peaty and soft organic ingredients. The dreaded mushroom fly and vine weevil, a plague with organic composts, are unknown to me.

Although my soil is hydrophobic, if I water my compost when it has become dry, because of the container’s retaining rim the water soaks in and fully rewets the soil. This is in contrast with hydrophobic materials like peat that shrink when dry and water ‘runs through’.

Heaven forbid that I should neglect the long term nutrition of plants in my containers. Should I be so foolish, my plants, will survive better than those from the garden centre when their slow release fertilizer runs out after very few months.

Using mole soil
never thought I would write this! My fur coated helpers kindly leave neat piles of lovely soil on my lawn. I know my soil is of a suitable sand/silt texture and coming from turf will be high in organic matter. Tongue in cheek, I might add that my grey coated friends have carefully picked out the grubs. I am only too grateful to accept their bounty.
A word of warning, for most gardeners the suitability of mole soil is illusionary. The moles might appear to be magicians when they appear in our gardens from nowhere, but their magic is insufficient to turn clay into sand. It might look nice and crumbly but if the texture is wrong and without divine intervention it will turn rock hard in a pot!

And finally, a story!
My dear friend and colleague, Tony Thompson, would every year build, as part of his teaching, several loam stacks with the students for future use in making up John Innes compost. The method was to stack sods of turf in layers with manure. The principle was that after decaying for a year or so the decayed turf could be chopped up to a wonderful fibrous loam.
The lovely man who was in charge of compost could tell you the year each stack was constructed and we all accepted that when he lovingly described each vintage it was like ‘putting down’ wine.
This of course completely misses the point!  After all those years the benefits of root fibre and manure  would be completely lost.

If you have laboured this far today you will sympathise with my students when I tell you that it used to take me three forty-five minute lectures to cover compost!

You can read more about how I grow the plants in the pictures on these links


  1. I love reading your posts which makes me question my own methods. I think gardening, like life, is trial and error and if the experiments do not cause fatalities, it is worth having a go.

  2. I see you blog from north Yorkshire L. Perhaps we have met, or you might have witnessed my successful and not so successful efforts? Was it your club at Paxton who visited my garden last year?

    1. It must have been another group from Paxton (I'm with Friday Forum at Paxton) but it's possible our paths may cross.

  3. If you have seen Martyn's blog today you will see why we couldn't use our soil for potting. We can't even sow seeds direct and have to sow in a shallow compose drill or plant young plants in a trench of compost, At the moment our soil is rock hard.

    1. I have had a look at Martyn's blog, 'A gardeners Weather Diary' and loved his pictures of his clods. You will just have to start no-dig! After about three years even when it is as dry as it is now it will break up much more easily or you will at least be able to plant into it direct without special compost.
      I suspect that your clayey soil will be completely unsuitable for my potting methods although generously mixed with your garden compost you might have fewer potting compost problems when you use large containers I seem to remember you blogged about some rubbish compost you bought or am I getting my wires crossed with your manure problem (with which I have complete sympathy having similarly suffered in the past)
      Actually your clods don't look that bad with plenty of worm action

  4. Very interesting. The other reason I use potting mix instead of soil is that it is lighter for when I have to move containers around. At the Chicago Botanic Garden we are instructed that we can use potting mix over and over as long as we refresh it with composted manure or something similar. I suppose that means over time it becomes all compost.

    1. You raise some interesting points Jason.
      Light loamless composts are more suitable for hanging baskets for example!
      On the other hand the weight of soil gives stability and tall plants are less likely to blow over.
      As to moving big pots around as we get older we tend to buy lighter plastic containers - I know yuc - but many now are moulded in very natural shapes and sober colours. This compensates for our heavy soil compost as some of our hardware containers are extremely heavy As mentioned our soil is sandy and I find it quite ironic that the books describe it as light! (sandy soils are described as 'light' because they are easy to cultivate)
      Interesting and gratifying that Chicago botanics recycle their compost!

  5. This year for the first time I have used my own potting mix which contains about 30% soil in my hanging baskets, fed with a little yara milla at the start of the season; they have done very well and have needed less water than the usual commercial compost - probably partly because less water overflows during watering. They have good strong brackets! One advantage of growing in a soil based mixture is that the transition to the garden soil is really easy.

    1. Interesting about loam in hanging baskets and the ease of wetting up. Strong brackets indeed!
      Yes if a plant in loam-less potting compost is not properly watered in when planting or plants are planted and not rewatered after drying out, roots do have difficulty in getting a foothold in mineral soil.

  6. You will get no arguments from me Roger, I have never seen the point of using sterilised compost in anything other than the heated glasshouse, however my soil, even with added compost, peat and grit just doesn't work. I buy the cheapest multi-purpose compost available and enhance it unless it is for what few summer bedding plants I grow in which case I use it "neat". Most of this is put on the garden when the pots are emptied but I will re-use some of it to help fill deep containers if appropriate. I was delighted to see that you have included the link I gave you regarding peat, I will be using it again in my next post as a well known TV pundit has crossed the line!

    1. I should have thanked you for the link Rick.
      I used to do exactly as you in my previous home - I used to buy loam less peat based compost and enhance it with slow release fertiliser where a richer compost was required. I used it for seed sowing, cuttings and everything else.
      As to economy, I think that came from buying it in 80 litre bags.
      I would not like to buy the cheapest multi purpose none peat compost- it might be somewhat variable!
      Funny that 'peat free' is now a selling point. If I still used loam less I would be searching out those based on peat!


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