Home Made Potting Compost?

[Geoff]

With all the talk about removing peat from potting composts, possibly being forced by taxation or a ban, and many users being unhappy about the commercial peat free composts on offer is now the time to consider making your own potting mixtures?

First a little history. Growers in the past made their own potting mixes from a variety of materials and often in the world of competitive growing regarded their mixtures as secrets to be jealously guarded, this applied to both the basic mixture and the added fertilisers. The need for standardisation was obvious and resulted in a research project of thousands of experiments at the John Innes Institute culminating in the publication of "Seed and Potting Composts" by Lawrence and Newell in 1941.This included the two standard mixes, the base fertiliser mix and 3 rates of addition:
JI Seed Compost:
2 parts by bulk medium loam
1 part by bulk peat
1 part by bulk coarse sand
JI Potting Compost:
7 parts by bulk medium loam
3 parts by bulk peat
2 parts by bulk coarse sand (I've found this as 1 part and 2 part, must check)
JI Base Fertiliser:
2 parts by weight hoof and horn
2 parts by weight superphosphate of lime
1 part by weight sulphate of potash
JI Potting Mixes:
To make JI 1 add 4ozs JI Base and ¾oz ground limestone to a bushel of JI Potting Compost.
To make JI2 double the base, JI 3 treble the base
To make JI Seed add 1½ozs superphosphate of lime and ¾oz ground limestone to a bushel of mix.
(nowadays there is a metric equivalent of these of course).

There are some drawbacks with these formulations; what is medium loam, the loam has to be partially sterilised (define partially), the mixture should be used within 6-8 weeks. These inevitably led to variability in commercially available products, particularly as the loam element became more difficult to obtain to a high standard (not unlike the variability with new peat free formulations).

The renewed search for uniformity with commercial viability focused on the consistent and inert material peat. Work at the University of California and UK research in the 1960s produced the soilless composts such as Levington that we are familiar with today. These became very good and reliable products which is why we are so reluctant to give them up. These are all proprietary formulations that you can't make yourself so making us dependant on the commercial sector.

To reduce peat consumption to about 30% of current we could go back to JI formulations but to eliminate it we need another solution. We can continue to rely on the commercial sector and hope they develop good products, I don't see a 21st century peat free standard arising from an independent research institute, or we could go back a couple of generations and make our own. The risks with home made are variability (any worse than commercial JI or peat free?) and lack of sterility (how important is it?). One lesson we can perhaps retain from JI is nutrient consistency so perhaps all we have to do is mix something that feels right then add JI Base Fertiliser or seed mix chemicals.

The four obvious contenders to use in the mixes are soil (probably garden soil as I can't see us all stacking turf to rot down), leaf mould, the contents of the compost heap and coarse sand. There is also the Johnboy magic ingredient of composted chopped straw and Comfrey. A bit of Googling finds some "formulations":
Helen Yemm in the Telegraph:
Seed : 2 parts molehill, 1 part sand, 1 part leaf mould or compost
Potting : 7 parts soil, 3 parts compost/leaf mould, 2 parts sand, slow release fertiliser
(seems like JI with substitution of peat by leaf mould or compost to me)
Wild Yorkshire website:
half soil, half compost, FBB fertiliser
Ask Organic website:
Seed : equal parts loam, leaf mould and compost
Potting: 1 part loam, 2 parts compost, 1 part leaf mould

I have dug out my Father's old Rotasieve, that I have never used, from the depths of the garage and used it on some 2009 leaf mould, compost heap material and skimmings from a vegetable plot and produced some remarkable materials. The compost was the trickiest, I think it needed to be drier. I think I am going to try and reproduce JI Potting using:
7 parts soil
3 parts organic - half leaf mould half compost
1 to 2 parts coarse sand
I've not worked out the fertiliser yet.

Rather a long post but what do you think? Is home made worth a go? Do any of you make it already? What sort of mixtures do you make? Have you ever seen any evidence that lack of sterility is a problem? I haven't been brave enough yet to contemplate a seed sowing mixture.

[Johnboy]

Hi Geoff,
I think that we should all be very grateful for your very timely posting.
The time that you have spent to find a solution has resulted in real food for thought for us all.
I do make my own seeding and potting mixtures but not without the use of fossil fuels because the machine that pulverizes my straw uses quite a lot of electricity. Without pulverizing straw my compost would not work because pulverizing make the straw super absorbent and when the Comfrey is added the resulting liquid produced is taken into the straw and without that there would be just a soggy mess and masses of flies about.
My pulverizer was built to produce insulation from newspaper for housing.
I originally bought it at auction for the engine. I do not know who actually produced it because apart from the words Made in Sweden there is nothing to tell me where it came from. No makers name. When this finally gives up the ghost I do not know what will happen.
The trouble is that not very many gardeners could ever have this equipment so it puts them at a great disadvantage.
Even the price of straw at present would deter many gardeners from using it.
I am convinced that Comfrey is capable of supplying sufficient nutrition and there are several methods of capturing this nutrient and the best I know is Comfrey Concentrate which is the way to extract the goodness without the awful smells involved with other methods.
Once again I thank you for your posting because it seems the first step on a long haul, with I hope, plenty of debate without any bickering.
Sincerely,
JB.

[Tony Hague]

There was once (long ago) an article in KG giving some homemade compost recipies, but don't ask me to find it !

For what it is worth, I grew my tomatoes last year half in New Horizons, and half in :

3 buckets sieved garden compost
1 bucket medium vermiculite
2 handfulls seaweed meal

Mixed in the wheelbarrow. The tomatoes were the same mix of varieties, mixed around in the positions in the bed so no difference in conditions. I could discern no difference in the results, aside from the need to weed the ones in the homemade mix !

I have alos started making leaf mould with the intention of using it for seed compost - the only thing I currently use that is peat based. I does seem to have quite a peat-like character. Don't know how well it works yet, but if it was good enough for Geoff Hamilton, it must be worth a try.

[Geoff]

Another thought on fertiliser. When I last looked for JI Base I couldn't find it and I haven't any Hoof and Horn in stock so I can't make it to the formula. Looking on the web I have found a supplier that says JI Base is NPK 5:7.7:10. I use Fish, Blood and Bone which is 5:5:6.5 and I have both Superphosphate and Potash so with a bit of maths I think I can get there.
Askorganic.co.uk includes a bit about leaf mould:
"To make leafmould with added nutrient, mix comfrey leaves in a bucket with 2 year old leafmould, alternating ingredients every 10cm. After 2 or 3 months, the nutrient from the comfrey will have mixed in with the leafmould. Alternatively, if you have a leafmould collecting bay, simply mix leaves and comfrey as you fill it up".
So the first method might be an alternative to your chopped straw method, JB. Not convinced by the second point as I make my leaf mould in an open container so I think the comfrey contribution would wash out.
Tony, the only Geoff Hamilton book I have is the "Gardeners World Vegetable Handbook" and there is no planting mixture / leaf mould discussion in there so are you referring to another book? Askorganic list various seed sowing mixes based on leaf mould:
"Leafmould alone or Coir alone - seedlings need to be transplanted quickly
Leafmould + loam 1:1 - needs careful watering, not suitable for very small seed
Loam + leafmould + garden compost 1:1:1 - a good, general mix
Leafmould + wormcast 3:1 - a rich mix"
I don't think I would sow many seeds a year if I relied on collecting wormcasts!
I've made some of the mix I suggested in my first post and pricked some Antirrhinums out into it so I'll see what happens.

[Geoff]

You can tell it's raining today!
Thanks for the lead Tony, found "Making Your Own Potting Compost" by Joyce Russell November 2004 p.20.
In summary she generally does not advocate fertiliser relying on compost from the heap or sometimes well rotted manure then liquid feeds (not defined) after six to eight weeks but mentions fish, blood and bone as an option. Also she tends to use less loam than a JI type formulation.
There are two seed mixtures - 1 compost, 2 leaf mould, 1 sharp sand or equal parts loam, leaf mould and sand.
There are various potting mixes, for example - equal parts leaf mould, manure, compost, perlite - 3 loam, 2 leaf mould, 1 manure, 1 sand.
She also mentions the leaf mould / comfrey mix but gives it 2 to 5 months.
Microwave, flame weeder and fire pit are all mentioned as optional sterilising methods - I'm thinking of trying overnight in the bottom of the Aga when the wife isn't looking.
I'm experimenting in the flower garden first as I can stand any failures better.

[Johnboy]

Hi Geoff,
I have not sterilized any soil for some time now but the last time I did I made a contraption over the top of a garden incinerator with a sheet of corrugated iron over the top and passed the soil over the very hot CI. I don't know how sterilised the soil was but I certainly didn't get any weed seeds in the mix that I made. Ultimately it was made up of around 20% Peat 20% Horticultural fine grade grit and 60% sterilized soil all topped off with vermiculite fine grade. It worked very well but what I would use instead of Peat now I am not quite sure.
JB.

[Nature's Babe]

like you Geoff I was going to suggest comfrey mixed with leafmould as part of the mix, I believe the comfrey helps to speed up decomposition.
Also I have three big wooden compost bays and often take soil from under where the compost stood its nice and dark and rich to add to the mix, and free from weeds after six months or more with compost on top.

[Tony Hague]

The book I have is "The Orgainc Garden Book" by Geoff Hamilton. Whilst I could not find any leaf mould recipies directly, he does say of leafmould that it is too good to use as a mulch or soil conditioner, but should be saved for making seeding or potting compost. He gives recipies for homemade peat and loam based potting composts (on p252 of my copy).

Before growbags were 99p from Asda, my dad always made his own tomato compost. It involved loam from stacked turves (from a turf growing friend, presmably rejects), setrilised by setting it in a steel tray on some brick piles over a raging fire, and splashing in a bit of water from the can to make steam. This was mixed with peat or leaf mould from the wood of a nearby friendly farmer.

[Geoff]

One of the trials I have decided to run is "how long do I have to put home made potting compost in the bottom oven of the Aga to kill weed seeds?".
It hasn't been run long enough yet but the intermediate results are interesting. I mixed equal parts garden soil, compost, leafmould and coarse sand - not because I thought it was a valid formulation but to make sure I tested all four components. I filled a quarter seed tray then put the rest in the oven, after 12 hours I filled another tray then filled the final tray after 24 hours. I then watered and sowed half of each tray with Catalogna Lettuce and covered the seeds with Vermiculite. This was April 1st and they have been in the greenhouse until today.

Compost Trial 7th.jpg (111.27 KiB) Viewed 6427 times

The labels don't show too well but on the left is "As Mixed", then "12 Hours" and "24 Hours". A few things to note:
1. limited weeds in the as mixed
2. different germination speeds across the three trays
3. better looking seedlings in first tray - might just be because they were quicker
I offer no explanation except when you read about John Innes formulations it says "partial sterilisation is beneficial", maybe this is what is happening here. I'll review in another week.

[snooky]

Used homemade compost to line a trench and I sowed carrots in it and something is digging it up.I don't think it is the resident allotment fox but birds.There is something there they like.I hope that the carrots like it as much

[Mike Vogel]

I have to admit that I just open up my compost bins and sieve the extract. Then i use the sieved compost for sowing. During the sieving, as I put the stuff into bags, I add organic seaweed meal and some leafmould.

I also have courser stuff which I use for potting on. I haven't had any cause to complain or bewail "bad luck" either with the sowings or wityh the potted on plants..