Sunday 4 October 2015

Seed and Potting compost: Part 3

In Part 1 of this series I discussed composts made from green waste. Some are of high standard but others are inferior because of lack of quality control in the organic components. Although there is an issue of pollutants most of the problems arise from erratic levels of soluble content.

In Part 2, I made a case for the use of peat. Deeply unfashionable I believe the case against it is flawed.

In a much earlier post I discussed soil as a compost component when for example it is used as an ingredient of John Innes compost. I have also written of my own penchant for using my own coarse silt/fine sand garden soil fortified with slow release fertilizer as a complete compost. This is inappropriate for most gardeners because their soil has the wrong texture. Never the less I think that where gardeners are growing plants in tubs or large containers, perhaps greater than ten litres if they have high quality garden soil it is far superior to garden centre compost.

More about soil as a compost
Garden soil does contain weed seed and perhaps certain pests and diseases but this is no worse than when it is still in the ground. It does last indefinitely and does not decay away like organic composts. If recycled it does need refreshing with slow release fertilizer or when used to grow more permanent plantings such as containerised shrubs and small trees it needs regularly top dressing with compound fertilizer or liquid feeding.
This cut leaf maple has been in its soil for four years. It is top dressed with yaramila compound fertilizer about every three months
In some ways stripping soil from the land in environmental terms is worse than removing peat from wetlands. Much soil is available that comes from land stripped for building such as new roads or motorways. If it comes from such sources and is of appropriate texture such as soil from my own local area it might be very suitable. Unfortunately if you buy garden centre John Innes compost the soil component won't be prepared from turf as in the compost's original specification! No matter, neither is my own.

I have no environmental concern when I make compost from soil from my own garden because I either reuse it or return it to my ground.
My sandy/silt soil
Over the last ten years I have been genuinely surprised how good my soil garden compost is. The fine sand/coarse silty nature makes it very water retentive and I need to water it less frequently than normal organic composts. Better, the cut off between enough water and severe wilting is much more gradual than the sudden severe wilting of organics. Nutrition is excellent with slow release fertilizer and my plants are generally very healthy.
I do of course get pearlwort and liverwort on the unsterilized soil surface. It is easily removed.
Now going dormant my marsh helleborine is weedy with pearlwort
Liverwort is spread by spores. Both unsterile and  sterilized composts are equally vulnerable
Liverwort and pearlwort can be scraped off and put into the base of a planting hole or when potting-up recycled into the pot bottom. My weed seems very little worse than the gumpf growing on the top of typical garden centre plants.
(You don’t get surface weed from plant sales at the superstore, they kill their plants too soon!)
I am not proud of the scruffiness of this two year old daphne seedling that has been growing-on in my nursery all summer
I took all of five minutes to scrape off the surface and repot all seven fine plants
Peter Williams my scientist friend has been doing my watering this year when we have been away. Although he is scathing about my general irrigation arrangements I think he has been quite impressed with my soil pot-grown tomatoes. To my amazement he even mused about using soil himself for his own tomatoes – it is the same texture as my own.
He even said that some of the fertilizer he might use for his tomatoes in their final ten litre pots might include general purpose fertilizer! Although I recommend top dressing with general fertiliser that is a practice I have never dared admit to in writing this ‘plog’!

Making your own potting compost
Many gardeners prepare compost from garden compost! A contradiction in terms. In the UK we have the different meanings of the term 'compost'. It might mean mixes for sowing and planting or stuff from a heap. I expect in the old days the meaning was the same when compost from a compost heap was used to grow plants in containers. No doubt other organic materials such as leaf mould were used too.
Indeed I remember when as a garden apprentice the old foreman added leaf mould to his new fangled John Innes mix. Perhaps he was ahead of the game as we now recognise the value of mycorrhiza. Readers might recall that I think adding mycorrhiza from a packet is a complete waste of time.
My friend Peter makes the best of both worlds. He has huge amounts of leaves from his trees and mowings from his lawns. He composts them together and when they are well decayed he sieves them and adds fertilizer and lime to make his own mixes.
I remember an old colleague who in the early days of grow bags made up his own in a very similar way. Personally I don't like grow bags and when in the past I have used them for tomatoes I have tipped their content into ten litre plastic pots. I don't think long flat containers have as suitable properties of wetting and drying as do deeper containers.

Nutrients for composts

I am into my fourth year with my 25kg bag of coated slow release fertilizer
Whatever mix of bulky ingredients are used in making your own compost the amounts of fertilizer needed will be very much the same. Peat compost may need a little more lime or if you are strengthening a multi purpose compost a little less slow release fertilizer.

Dolomitic limestone is our favourite lime. It contains both calcium and magnesium
My own methods of measuring fertilizer might be best described as intuitive.
More credible are Peter Williams' ways. He has various measuring methods as short cuts to his target but in principal his formulae is very simple. He uses ‘coated’ slow release fertilizer at between one and three grams per litre of bulky content depending on his judgement of plant requirements. He uses lime for his peat composts over the same range of between 1.25 and 3 grams per litre.

Interestingly he actually uses lime at 1.25 gm/litre even for his rhododendrons that he grows in fertilized peat. This is perhaps because of the benefit of the calcium it contains. Unless a plant is known to need calcareous conditions he uses lime at only 1.25 gram per litre when he mixes his no-peat composts.
The slow release fertilizer we both use is one of the many controlled release coated fertilizers readily available these days.
The form of lime - if any is required - is ground limestone or ground chalk. Peter’s and my own preference is ground dolomitic limestone marketed as dolodust.

Other bulky ingredients that might be used as part of a mixture
A special coarse compost is needed for orchids
Some gardeners and commercial growers use bark. Use the type sold for potting rather than that used for mulching unless you are growing a large plant that prefers a very coarse mix.

Never again
Both vermiculite and perlite are natural minerals which in manufacturer are heated and exfoliate to light fluffy materials. I am personally not enamoured with either but some very good gardeners swear by them!
Perlite has a closed structure and does exactly the same as grit when used to create air spaces in a compost. It holds no water or nutrients and is completely inert. It's light weight might endear it for hanging baskets but in plastic pots tall plants are liable to topple over. I tried it recently in a mix for some special dicentra seed I had collected and was disturbed that my rather heavy handed watering almost washed my seed away. I can never find my watering rose!

I understand some gardeners get very good results sowing seed in pure vermiculite. Vermiculite has an open structure and will absorb water and does naturally contain some nutrients. It seems to me to be a fairly messy material and Peter too curled his lip at the thought of using it!
(You might have noticed I sometimes make dogmatic statements. Do not hesitate to contradict me in ‘comments’).
My soil compost was excellent for bedding these florist cyclamen from Aldi
Now what shall I do with last year’s cyclamen that are now fully established in soil?
This red saxiifage has made a fine root system and is ready to plant in Cathi’s garden
This bog plant likes my soil 
Grown in a six pack of loamless compost these rather chlorotic Harlow Car hybrid primulas were given to me last week. They will soon green up in my soil.

I love my soil composts
Some of you will know of my interest in terra preta and charcoal and biochar. I had intended to include biochar in this post as an ingredient in composts. I now find I want to say too much and will write about it in my next post.


  1. If I don't have a weight issue, then I use a mixture of 50% Peat based MPC, plus 50% sieved garden soil. I have noticed that my soil benefits from an addition of Sulphur and Calcium, so a barrow load gets a 'spoonful' of Epsom Salts (Magnesium Sulphate) and a fist full of ground limestone. I don't add any more fertiliser than that already present in the MPC, because generally, for container plants, I don't want explosive growth.

    When I have finished with a container, I tip the contents onto the flower beds, it makes a great mulch.

    Can I ask a question on a different topic please. Where have the wasps and acorns gone to?

    For the last four years, my mature Oaks which used to produce huge crops of acorns, have not produced a single one??? And so far this year I have only seen a single wasp. Normally this time of the year with the fallen fruit, the wasps are out in force - this year - NONE???

    1. That seems very little nutrition to me Mr.S. Perhaps you liquid feed instead.

      As to your question about oaks I don't really know about the national situation but I do know that yields of acorns are very variable in acorn production. Maybe next year will be a 'mast' year and there will be a massive production.
      Don't mention lack of wasps to me. We have just arrived in Sorrento and after Brenda was stung at our Manchester hotel on Sunday night her swollen arm is not yet back to normal..... And what a who ha we had at my recent wedding when ten year old Bibi was stung just as dinner was served!

    2. Hi Roger, my land is very fertile reclaimed Wash marshes, plus it is full of mulched vegetation, charcoal, and regular additions of ground limestone and sulphur. This year, one patch of nettles reached 8', so the soil does not need additional feed.

      The 50:50 mix with peat based MPC (I use PH brand) not only has the natural fertility of the soil, plus the food added to the MPC, but the 'wildlife' in my fertile soil are being fed with a load of organic (Peat), with plenty of limestone to prevent the innevitable acidification that comes from the digestion of Peat.

      If anything, the mix starts off a little bit too 'hot' and early growth can be a bit too lush if I use the mix too quickly after making it, so I tend to keep a heap of it at the ready, and add a bag of MPC and a bag of soil to keep it topped up.

      But, if I am potting up Ragwort, then I make the mix fresh, because it gives a boost to transplanted Ragwort crowns.

    3. Yea !! saw my second wasp of the year this afternoon.

  2. We use a lot of the vermiculite and find it produces very good results for us. We sow seeds on the surface of compost and cover with vermiculite with no problem. You do have to be careful when you start watering though whilst the vermiculite is still very light. I also makes vermiculite in with compost when taking cuttings which seem to root very well in it.

    1. This is the beauty of gardening Sue, we all have our different approaches.
      I want in my posts to encourage independant thinking and not just slavishly following whatever Mr what's it says on the tele!

  3. Interesting and informative post as usual Roger, as you already know I am certainly not one to follow Mr TV Gardener although I do have a soft spot for The Beechgrove Garden to me the only gardening programme worth watching. My potting mixture is made up from cheap multi-purpose compost with some peat and, if necessary, grit, for seed compost I sieve the compost and although I have used vermiculite in the past I find a good sharp neutral grit combined with peat to be more satisfactory and dare I say natural. I return all the old compost to the garden which has been responsible for the improving by clay/sand soil to something manageable.
    I do not have a heated glasshouse so have no need whatsoever for clean pots and sterile compost, if plants could think I doubt they would prefer sterile surroundings. Using acid mediums I get plenty of liverwort and pearlwort but as they sit on a layer of grit they are easily removed, after all we never saw bitter-cress until the advent of containerised plants.
    Loved the comment about superstores, I know someone who works for B&Q who told me that they buy in plants so cheaply that it's not worth looking after them properly.
    Yourself and Peter are the experts but I have to question the use of lime and peat together as I was led to believe that this would cause some sort of chemical reaction.

    1. There might be but any reaction is much more gentle with chalk or limestone rather than garden lime Rick
      After all lime is used in making John Innes composts and that is a quarter peat and multipurpose peat composts are near neutral so must have had lime in their formulation

    2. Thanks for that Roger, I had forgotten about John Innes composts, which is surprising considering the amount I have made in the past!

  4. Your background makes your content unreadable on a phone

    1. Thanks Unknown, I will go to my technical advisors. I am at my son's at the moment and he agrees with you

  5. The website is designed to read on a phone, you need to press the "reader mode" symbol, this also allows you to increase the font size, and is far the best place to print from, we are using the responsive blog application provided by Google, sorry if it upset your reading of this informative blog.
    The Roger Brook No Dig Gardener Technical Team

    1. Paul, my son says androids do not have a reader mode. What can I tell him?

    2. thanks! but increasing the font size doesnt help, the text is unfortunately unreadable with that dicentra behind it. it's also a disaster in the right hand navigation on all browsers but if roger prefers bells and whistles over content and functionality that's his decision.
      'and is far the best place to print from' this defeats the object of a website. also mobiles arent generally connected to printers. to be environmentally friendly, would I bury the paper afterwards with or without digging?


Note: only a member of this blog may post a comment.

Related Posts Plugin for WordPress, Blogger...