Volcanic rock

TheAfricanGarden@aol.com TheAfricanGarden@aol.com
Fri, 26 Jul 2002 03:25:03 PDT
Hi Mary Sue and all, 
On the topic of media I'd just like to say that I use peat based composts for everything I grow here, however I will say that most composts sold here to amateurs in the UK have very low air filled porosities. 

Some are blends of Irish and dark Somerset sedge peats, and the dark Somerset peats have a tendancy to compact. 

The composts I favor come from Irish Moss peats, my favorite is from a manufacturer called 'Bulrush' (just in case it is exported). I actually buy their growbags which are sold for tomatoe use, it also works out to just 3p per litre, whats that, 2 cents per litre, if you open them they contain many different grades of peat, and to anyone looking at it, it looks really rough stuff, but gets better once the twigs etc have been taken out. However it has a fantastic air filled porosity, and probably over 13%. The mix also contains a lot of potash for the tomatoes, which is ideal for a lot of the bulbs here. 

All the SA bulbs I grow here produce a fantasticly large and strong root system, however there are two other factors along with the compost that I take into consideration. 

All the pots have gravel placed on top, and this to prevent the sun directly drying out the surface of the compost. The sun is also good on terracotta here as being maritime our summers are often cooler, so when the sun decides to come out, the pots get a lot of radiant heat and will be a lot warmer than the air temperature, and this heat can be stored by the pot for longer. This giving a lot of the South African bulbs the heat that they would get and require for growth and flower production. 

And I use terracotta pots to grow all the bulbs, irrigation can be tricky and time consuming in the summer but in the winter, the compost is able to breathe through the porous pot and hence the bulbs rarely get waterlogged and hence frosted during their dormancy. 

Very rarely do I put sand or pumice in any mix and primarily because I find that some can clog a mix and make it heavy. Remember sand can be good draining or bad draining dependant on the size of the grains in it. Fortunately this is easy to test. 

The only problem with the compost here, worms. I never considered them a pest until a few years ago but if a plant starts failing there'll probably be a worm or worms in the pot, and this has become more troublesome than vine weevil or sciarid fly, the compost gnat. 

The worms actually compact the compost, roots then rot, and the worms then start eating the decaying rots. This has actually been a significant pest on growing Tulbaghia. It can be remedied by soaking the pots with a benomyl fungicide, as this fungicide is know to kill worms as well as fungi. 

Any other problems are easily sorted by repotting as the compost is very cheap, and it washes off roots easily if the compost needs to be replaced because of a pest etc. 

From what I've seen of pumice, expansion and breakdown usually takes place after a time because it soaks up too much water and isn't as strong a material as lecca (lightweight expanded clay aggregates). Small grade lecca pellets used in hydroponics may be a better alternative, however they can be very expensive. 

In theory you can grow anything in literally anything, just look at what is used in the hydroponic industry, eg. rockwool, lecca, straw bails. However, standard growing and composts require a portion of the ingredient to hold onto nutrients for sustainable culture, in peat, it is 'humic acid', which is produced by the breakdown of the peat, and in soil based composts it is 'clay'. I don't see coir as an alternative as it lacks the ability to hold onto nutrients unless mixed with either a loam, or other organic compost. 

Best Wishes, 
Dave (Plymouth, UK) 

N.B. Noting that terracotta pots would probably dry out too quickly in warmer climates, such as the Pacific Coast in full sun. 

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