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.