Is it okay to leave peat moss out from a soil-less mix recipe?

Question

Is it OK to leave out peat moss in a soil recipe? I have this gardenbox and I'm looking for recipes for a mix to buy in bulk and ALL of them have peat moss, but from what I've read it's no good. Here's an example:

• 1/2 bushel of peat moss
• 1/2 bushel of vermiculite
• 1/2 cup of ground limestone

Answer 1

You can't just leave it out, you have to substitute it with something. Coir, for example. (I use well-composted horse manure + bedding.) If you leave it out, the soil won't hold enough water.

Mike Perry's answer to this question says:

A good "Potting Mix" is made up of 3 basic parts: * Water retaining material. * Drainage material. * Plant food (fertilizer) material.

In your example recipe, the peat moss is for water retention, the vermiculite is for drainage, and you don't have any plant food. (You might leave out fertilizer from a seed-starting mix if you are going to quickly prick out the germinated seedlings into another potting container that has some food.)

You could substitute coir for the peat moss in that recipe, or if you wanted to add plant food to the mix and you had a compost with good water retention you could try using it for double duty (food & water).

See my answer to the same question for the peat-less recipe that I use (which does have soil, so may not suit your application).

Answer 2

You can of course leave any organic medium out of your soil mix!

This is how lots of vegetables etc. are grown professionally, even many bonsais.

However, you will have to modify your fertilizing and watering scheme accordingly:

1. Water thouroughly, typically daily. Do not just wet the substrate, but have the water flow right through the pot. This will also flush out any surplus fertilizer and prevent the substrate becoming salt-laden.
2. Fertilize regularly and heavily, typically two to four times the amount prescribed. Liquid fertilizers are easiest to use for that purpose.

This will work with all kinds of inorganic substrate, from stone-wool to lava, pumice, clinoptilolite, crushed bricks, … and lead to fine, healthy roots.

Also make sure, that your mineral substrate does not contain any dusty fractions, e.g. by sieving or intensive flushing before planting. No particle should be less than 2-3 mm in diameter. A good size for small pots is approx. 4 mm, for large pots 8 mm or even some more.

If you insist on using organic material, you might also try a compound, which does not decompose too fast, e.g. crushed pine bark. Coir does decompose rather fast, especially, if it is fine. This will clutter up your substrate in time, which is suboptimal for the root system due to the fine roots not getting enough air.