To buy or not to buy?

To buy or not to buy? – compost bags at a Garden Centre

Up to now I’ve relied on buying seed and potting compost from my local nursery. Until about 2 years ago this was the standard ‘multi purpose’ stuff and it contained peat. More recently I’ve switched to ‘Peat Free’ both to do my bit to reduce Peat extraction and because having used it (it’s the ‘Which?’ – recommended ‘New Horizon’ brand) I’ve generally liked it for it’s more flakey, open structure. I’ve also started to use my own compost as a top dressing around plants in the spring and have used specialist composts for ericaceous plants, aquatics and for long-term planting in pots.

I’ve had variable results when I’ve used the shop – bought multi- purpose compost for seed germination, as its larger fragments can be a bit much for some seeds to make contact with and get going. On the other hand, the special seed compost I’ve bought can be a bit too dense and water-retaining , so just the opposite problem. So, now I’m wondering if I should try to make up my own seed and possibly potting mixes. The main argument seems to be that by doing this you can tailor your compost to the job in hand, and more generally, for potting on different plants with their different needs. So, when a new ‘Master Composter’ Factsheet arrived from Garden Organic which contains some interesting information on making your own seed and potting composts, I thought that you might find it helpful too. So I’ll summarise the main points here – maybe by the end of the article I’ll decide what I’ll buy and what I’ll make myself!

Why Make your own compost?

  • Organic, peat free composts aren’t always available to buy
  • Even commercially produced seed and potting composts have a limited life. If you ony need a small amount buying a 50 or 60 litre bag can be wasteful
  • It seems easy to do (hmm, I’ll reserve judgment on this one)
  • The basic ingredients such as loam, compost and leaf mould can be readily available at no cost (hmm again, I’m not sure about the quality of my garden soil as the loam…)
  • You can make the amount you need as you need it
  • If you prefer using animal- free products you can be confident about the ingredients
  • You can make different mixes appropriate for different plants and purposes.

What to fill seed trays and modules with?

What to fill seed trays and modules with?

So what’s the downside?

Apparently it isn’t as simple as it first seems….

Commercial producers spend years developing their products and it’s hard to replicate the characteristics of an ideal growing medium – good structure which retains water and air well but also drains well and which provides the correct nutrients for the plant.

Although plain garden soil will grow seedlings and plants very effectively in your garden, it won’t be as effective if it’s used in a tray or pot. For starters the number of micro- organisms in garden soil is mind blowingly large, and they create just the right balance  by regulating nutrient levels, suppressing diseases etc. If this balance is disturbed by using just a small volume of soil in a tray, for example, many of the benefits of garden soil are lost and problems can occur with germination and plant development.

So, trying to come up with the right balance of nutrients in a homemade mix is a big challenge, as the levels of essential nutrients will vary in organic compost and their release is unpredictable. An over – rich mixture, for example, can be as much of a problem as one lacking in nutrients. Ideally, you don’t want a mix that breaks down, leaving trays and pots half empty, or a mix which ‘slumps’, becoming compacted and airless, let alone one which contains diseases.

What are the ingredients?

Seed and potting composts consist of two main ingredients:

  1. Bulky material to hold the plants upright. Loam, peat substitute or a mix of the two tends to be the most common material used.
  2. Nutrients to feed them

Specific ingredients are:

Loam – good quality garden soil or home-made loam from old turf stacked upside down and covered for 6 – 12 months and then sieved and pasteurised before used for seed sowing,

Peat substitute such as coir and composted fine grade wood waste. Reclaimed peat from river dredging can also be used  (but this can contain weed seeds).

Garden compost is nutrient rich and isn’t needed for seed germination, but is useful in potting mixes and for longer term growing in containers. The release of nutrients can be very variable and it can contain weed seeds.

Leaf mould – 2 year old leaf mould can be used neat for seed sowing or incorporated into mixes to improve structure. It’s rich in micro organisms but low in nutrients and might contain some weed seeds.

Worm compost (from wormeries) is ideal in mixes needing plenty of nutrients and has good water – holding capacity, so is useful in hanging baskets. It can also be spread on the top of these and other containers and watered in where an additional feed is required during the growing season.

Manures – well rotted strawy farmyard manure provides both bulk and nutrients and is best used in rich mixes for long term use, such as tomatoes and peppers growing in pots.

Horticultural grit is needed to ensure good drainage if the loam is too heavy.

Organic fertilisers  – bone meal, hoof and horn etc. can be added to a mix to provide the necessary plant nutrients. These ‘slow release’ materials are used in small quantities. Sea weed is another option for those who prefer animal – free fertilisers.

Worm compost- a good top dressing for containers and hanging baskets

Worm compost- a good top dressing for containers and hanging baskets

What sort of compost?

Different composts suit different needs (all parts are by volume, not weight):

  • Seed composts do not need to be very nutritious as seeds come with their own initial power store. Once a pair of true leaves has formed the seedlings need to be potted up into a richer mix.  For seed compost use mature leaf mould or a mix of equal parts loam and leaf mould.

  • Temporary potting mixes need to be rich enough to sustain a plant during its early life, aiming for sturdy plantlets. This is where making a mix suitable for different types of plant and different stages in their growth process comes in. It might be tempting to make up a rich mix but it’s probably better to be a bit mean on the nutrients to encourage plants grow up ‘hard’. Too much nutrient can lead to lots of soft, lush foliage which is prone to weather damage, and attack by pests and diseases. There are different recipes to try out: either equal parts loam, garden compost and leaf mould; or, 2 parts peat substitute, 1 part grit, 3 parts loam and half part garden compost; or, 2 buckets loam,2 buckets coir/ leaf mould, plus 225g seaweed meal,110g bonemeal and 85g hoof and horn; or 3 parts leaf mould, 1 part worm compost

  • Long term potting mixes– a good balance of slow release nutrients is needed to sustain plants in containers for a long time. Try either 4 parts loam (no need to pasteurise) with 2 parts leaf mould or 3 parts loam; or, 3 parts loam, 1 part manure, 1 part leaf mould – this is especially good for hungry, fast growing plants like tomatoes, aubergines and peppers in pots.

Tips for success

  • Pasteurise loam when used in a seed sowing mix
  • Don’t sow seeds or transplant seedlings into neat garden compost – it may well be too rich
  • Add horticultural grit to ensure good drainage wherever necessary
  • Make different mixes for different things
  • Loam-based mixes tend to be more successful than peat or peat substitutes
  • Mix ingredients thoroughly
  • Don’t store home – made mixes
  • Try out small quantities and monitor plant reactions
  • Don’t feed to a rota, but respond to plant growth and needs
  • Keep mixes in use only slightly moist – wet soil rots seeds, so if in doubt DON’T WATER! (I think this is where I’ve been going wrong with my shop – bought seed compost)
My usual choice of 'mulit-purpose' compost- other brands are available...

My usual choice of ‘multi-purpose’ compost- other brands are available…

So will I take the plunge and start making my own seed and potting compost?

I can see the advantages of mixing different things for different plants and stages in development and as seed compost is quite expensive to buy it is probably well worth me making this up for myself. As to potting mixes, I think I don’t produce enough leaf mould and compost and have a good enough loam to make it a realistic option on any scale. It might be best overall if I use most of my own leaf mould as a soil improver in the autumn and garden compost as a mulch in the spring as now, but perhaps use some of these ingredients (and grit) to amend the multi – purpose (peat free) compost I normally buy to make up different potting mixes according to the plants and their stages of development.

What’s your experiences of shop- bought and home-made compost? I’d love to hear from you.

Source: ‘Potting composts: Making your Own’ – Master Composter Factsheet 5 – Garden Organic

Old School Gardener

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