Grit, perlite or vermiculite.

Hi all,

I have been watching some gardeners world stuff on the net and they keep on about free draining compost. Sometimes they use perlite some times grit and sometimes vermiculite. What is the difference in use for each?

Rusty

 

I've used Perlite and Vermiculite on their own with hydroponics systems before. both are are good at holding water within em, but Vermiculite acts more like a sponge allowing for less aeration.

Mixing grit, perlite or vermiculite with compost is done to split up the compost to improve aeration and draining. People even add sand and other things. In the past I've even used crushed(well broken up) pieces of polystyrene foam that you get in packaging from t.v.'s etc. Some people even use cat litter.

 

I used perlite once. I know a lot of people like it but I hated it and would never use it again. If you accidentally over water just once, all the perlite comes up to the top because it is so light. I also begrudge paying so much for what is essentially a tiny piece of inexpensive stone, expanded into a big bag of bubbles.

 

I didn't know either, Rusty; but this PDF document from the rhs (called "Know your compost") will tell you all you need to know

RudeBwoyUK mentioned cat litter; there was a thread on this very subject back in November

 

If your gonna fork out good money on something I'd recommend looking into Diahydro. That stuff is awesome and fizzes when it comes into contact with water lol. It repells some insects, holds quite a bit of water( i think 150% of its own weight, also has a high silica content which i think is great for roots. Some people use it as a top layer, but you can mix it into compost.

its 100% natural aswell so can use it with organic stuff if your into that sorta stuff

 

I believe you can use all or any of these - the object is to improve drainage, but still retain moisture. My view is :-

Vermiculite - in some people's view it holds a bit too much water, but is often used to sprinkle on top of seeds as a way of keeping moisture in but is light in weight and allows light to penetrate.

Perlite - This seems to be more popular than vermiculite for adding to compost. A lot of people use it, but in my experience it can get a bit wet. Its very expensive in small quantities and to my mind is only competitive when bought cheaply in 100 litre bags.

Sharp sand - You can buy this from a builders merchant and its pretty cheap. It doesn't hold moisture but improves drainage. I use a mix of 2/3 multipurpose compost to 1/3 sharp sand for almost everything. I have since read a book written by the research department of an American University, which was entirely about trials on the germination of seeds. They also used the same mix - so if its good enough for them its good enough for me. For seed sowing I sieve this to give a product which is pretty close to seed compost.

Grit - this is a larger particle size than sharp sand, but does the same. It tends to be used more in bulk to improve the clay soil in borders than in pots. Although you can always sprinkle grit on top of the surface of pots to retain moisture and discourage weeds growing.

Soil - don't use this for growing seeds as it will contain a lot of other seeds as well as being rather too heavy and wet. However there is nothing wrong with potting up larger plants with this as it is exactly what they would experience if planted out in the border. In practice I use a mix of about half garden soil and half of my compost/sand mix. This is then pretty close to a John Innes mix. Soil, especially clay, contains a lot of nutrients as opposed to compost which contains very little.

Long term potted plants such as trees should be potted up in a soil based mix. The reason is A) the soil has nutrients, B) The extra weight will give stability, and C) compost after two or three years will break down into smaller particles (the composting process) which will impede drainage, as opposed to soil that won't.

Used Compost Mix - Many people say you shouldn't reuse compost as the nutrients will be used up. But things like peat effectively contain no nutrients. The manufacturers have to add nutrient to the peat before they bag it up. So logic says why not just replace the nutrients rather than replace the whole mix. To me thats like restocking the fridge when it becomes empty, which is cheaper than buying a new fully stocked fridge. You should be feeding plants in pots on a regular basis so it shouldn't matter much if the compost has feed of its own or not. However don't reuse compost that has had diseased plants or parasites in it.

I must add that these are just my views - I am sure other people will have other views. The secret, like most gardening, is to do and use what does well for you.

 

Anybody considered "rockwool" as an alternative for seed sowing? apparently very good moisture and air retention and equally suited to soil or hydrophonic methods of growing.

 

http://www.gardenerscorner.co.uk/forum/threads/chilli-advice.49091/
The Finnish Chili grower in Pams post above uses Rockwool. Anyone tried this medium ?
I use vermiculite to sprinkle over surface sown seeds . I use perlite in the seed compost for some of the more difficult seeds I grow . I also use it in my baskets to keep the compost a little more open. As Peter said its a little expensive to use generally unless you can get a bulk supply . I get my V and P from Wilkos its £3.75 for 10 litres, my local GC charges £7.24!!!!!

 

Why is it always said of sand that it doesn't hold moisture? Sand in fact holds a great deal of moisture, a pot of wet sand will take an eternity to dry out unless it is exceptionally hot!!
Perlite is my personal favourite, this holds no moisture at all and opens up seed compost like nothing else. also adds its own warmth to the mix, I love it!!

Val

 

Val -- I was not aware that others had said that sand doesn't hold moisture. I was suggesting that it isn't porous and doesn't have a very big surface area compared to many alternatives. So it will probably hold less water on its reduced surface.

I know a lot of people like Perlite. I do use it - but I am just as happy using sharp sand. I think a lot of the secret is if you can get your watering in line with the material being used, ie use what you are comfortable with.

 

Not much to add - I tend to use a mixture of Multipurpose, JI#1 & Builders sharp sand in about 3.2.1 ratio for most things. i just tend to mix it up until it feels right for whatever job im doing. Not very scientific but seems to work for me.

 

For the first time *ever* I have bought B&Q own (Verve) "seedling and potting" compost (12ltr) plus "Vermiculite" (B&Q own; "Verve") to start my seedlings with.

Instructions: "add 50/50 vermiculite vs seed/potting compost" ... so, why not sell it at the same level??

I'll be mixing the vermiculite (10lrt) with the seedling/potting compost (12ltr) and calling it "job done"! I'm prepared for the risk (which is a huge upshoot in previous - ignoramus years).

Just irritates me that if it needs to be 50/50, why not sell "like for like"? (Hmm, "capitalism" springs to mind Counter argument: sell more if it was more explicit!)

 

Mum - I do agree it would be much easier to add them together and sell it as one item.

However, there is an arguement for selling things seperately. I do sometimes change my mix. My normal mix is 2/3 compost to 1/3 sharp sand. Some things like Brugmansia cuttings that like lots of water would probably be happy in pure compost - though I am so lazy I tend to treat most things the same. But some things like Hibiscus do like to have a very sandy compost - they need a lot of air round their roots. So I might use 1/2 or even 2/3 sharp sand for them.

But in general I find plants are very forgiving and will grow happily under a range of conditions. If they get too fussy - I tend to give them the boot anyway. I can't be dealing with spoilt plants.

 

In the past, I've been known to cover newly sown seeds in sand, as it is very easy for new shoots to poke through, but mostly because I think it reduces the risk of fungal and moss problems if you over water (which I used to do a lot).

 

You present a fair, and logical argument, PeterS

Last year (and previously) my "argument" to planting anything was: it will thrive or die!

This year (having been an avid reader of historic posts and present questions) my argument is: KISS! (Keep it Simple, Stupid!).

Having studied marketing and market research, I still believe the easier you make it for people to understand, the greater your sales will be because you are catering for the masses. However, by that same token, gardening suppliers want to profit as much as possible in an uncertain economy and so they want to appeal to the niche market (where possible). I just think (as a beginner) selling one at 12ltr and the other at 10ltr and then advising 50/50 just adds to confusion and can put people off!

Thankfully, I have you and the whole of GC to keep my feet grounded