I'm not an expert on house plants (I much prefer to be outside with my plants!) but the other day I received a question about what compost to use when repotting house plants that were starting to look a bit sad, so here's a quick "Compost 101" introduction to the subject.
When we say "compost" in this context, we mean "potting compost" rather than "made-it-ourselves garden compost".
Why? Home-made compost - and this includes that old standby, "well-rotted manure", now known as "organic matter" - is often unsuitable for potting and for houseplants, for several reasons.
Firstly because it may be "too rich" in nutrients, causing the plants to grow too much foliage at the expense of flowers. Incidentally, I've put that in quotes, because in one sense, soil can never be too rich, as plants can only take up as much as they can use. However, with many plants, too much of one particular nutrient - nitrogen - will create tons of leafy growth at the expense of flowers and fruit, so it's usually better to use something which is balanced, ie which contains a desirable balance of the three main nutrients, N (Nitrogen) P (Phosphorous) and K (Potassium). Incidentally, why couldn't they call Phosphorous PH? Then Potassium could be P, and it would be a lot easier to remember. Just sayin'.
Secondly it tends to have a very dense texture, which means it soaks up a lot of water: ideal for the garden, not so good for indoors, as it makes pots very heavy.
Thirdly, this denseness (density?) can cause it to become water-logged, which is very bad for plants in pots.
Fourthly, once the waterlogging dries out, it often becomes rock hard - again, not good for potted plants.
Fifthly (if there is such a word) it may well contain weed seeds, and it certainly will contain an awful lot of creepy-crawlies, including bugs and worms. Now, to have really successful potted plants, you need what is termed "soil life" which means microbes, bacteria, and fungi: these are the engines which break down the nutrients you add in the form of liquid feed and ordinary water, and which make those nutrients available to the plant. Sterile, dead soil will not support healthy plants. However, bugs, and worms in particular, are bad for potted plants: worms create tunnels within the root ball, and air on the roots is very bad. Likewise, there are many bugs that you really don't want on your indoor plants.
For all these reasons, it is better to buy in clean new compost for house plants, and now we are now able to buy "perfect" compost, both for our potting, and for our houseplants, and the name is Multipurpose Compost.
That nice Mr Titchmarsh once said "House-plant compost is multipurpose compost in small bags, especially for people who only need a little at a time".
Yes, it's the same material, sold in small quantities. Does that seem unfair? Not at all: once you open a bag of compost, the nutrients within it start to degrade, so there is no point having a whopping big bag of the stuff cluttering up the shed, if you only need enough to top-dress a couple of house plants.
But if you already buy it for the garden, there is nothing to stop you using it for houseplants as well. I buy cheap multipurpose compost from what are called the Sheds, ie places like B&Q/Homebase, and my absolutely unbreakable rule for cheap compost is to sieve it before using it.
I ALWAYS sieve my compost. Two reasons: firstly, it gives you a chance to pull out all the contaminants, ie large bits of twig, broken glass, bits of plastic, hard shards, stones, etc. No, I'm not kidding, here's a photo of what came out of just one large bag:
Rubbish aside, the second reason is that by sieving it, you end up with three types of material:
1) Large bits of material, usually woody lumps, which are too big to be incorporated in a pot. They are picked out of the sieve (along with the rubbish) and go in the bin.
2) What falls through the sieve is fine, soft, clean, nice to handle, easy to use, and is the basic compost.
3) The third stuff, which is left in the sieve, is what I call the "nuts" of compost: lumps which are too big to go through the sieve, but which are not inorganic or big bits of wood. I use these as a top dressing on pots - perfect for preventing soil compaction, reducing weed growth, and (for those times when you didn't catch the weeds before the got a hold) easy to shake off the top layer, weeds and all, and replace with fresh nuts for an instant top-dressed look.
John Innes compost is "the best stuff" because it is soil-based (rather than made from coir, bark, or any other agricultural by-product) (which is not meant as a criticism, by the way!) but very expensive, which is why you find so many recommendations for using it in combination with cheaper multipurpose compost. Being soil-based, it contains soil life - those helpful microbes, bacteria and fungi - so using at least some of it will help to get your pots started.
But all compost, John Innes included, only contains nutrients for about 6 weeks (honest - it says so on the pack!) so whatever you use, the houseplants will need some help along the way: that's why we give them liquid feed, foliar feed, slow release fertiliser, an annual top-dressing of fresh compost etc: all to make up for the fact that they have exhausted all the nutrients from whatever compost they started out with.
So yes, you can use multipurpose compost for houseplants, as well as for potting up, and to summarise the Handy Tips:
Only buy a small bag - even though it is more expensive - if you only need a little.
Don't open the bag until you are ready to start using it: and if you don't use it all, close the bag tightly to keep air, water and light out of it until you are ready to finish it.
Sieve it before using it, and if it feels dry to the touch, dampen it down before using it.
There you go!