Scottish Rock Garden Club Forum
General Subjects => General Forum => Topic started by: Palustris on July 23, 2012, 09:21:14 PM
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The question is, does it really improve drainage or not?
Don't get me wrong, I have added it to compost for a long time and it seems to help water drain through quicker, but I have never been scientifically minded enough to run a proper test.
Anyone ever done it?
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I've done a few, very crude, experiments on water retention - weighing the pot. Sand does make a small difference, & marine sand seems very slightly better than river sand. Better than both is coarse (5-6mm) grit.
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I know grit is better, but if you read any of the info given out in books, Bulletins, magazines and so on about growing things in pots, the use of sharp sand is almost universally accepted as the norm. O.k Cornish silver sand is the best there is (according to my reading), but just because it is put forward as best practice does not make it so.
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I use only 5% river-sand in my potting-mixture for bulbs
but use 20-30% lava 0,5 - 1cm stones
and 10-20% perlite for the drainage
Roland
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Hi,
There are a couple of factors which need to be considered when choosing which sand to use in a compost mix - the size and shape of the particles. As you are all aware a compost mix requires good air filled porosity to allow the water to drain freely and to allow essential gaseous exchanges to take place in the root zone. Some sands may blocking these small pores due to a very small particle size, whilst a sand which has a high percentage of angular shaped grains can also affect drainage as these grains can lock together reducing the pore spaces.
That said, all sands improve drainage as they create channels or pores for the water to travel through. There are some fascinating, (and a lot of very dry ::) ) books on the subject - mainly aimed at sports turf management. We did several basic experiments at college last year as part of the soil science module. It was really interesting and the principles behind creating raised water tables by using sand / gravel was amazing. Admittedly it did take me a while to grasp this principle but once I did it explained why the drainage I installed in the old garden wasn't quite the success I had hoped for. :-[ :-X
Susan
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During 30 or so years of growing bulbs I've used all kinds of grit in the potting mix - in my experience the nature of the grit makes no difference at all. I only use sharp sand as a layer (0.5-1cm thick) underneath the bulbs; whether this makes any difference I've never bothered to check. People successfully grow bulbs in all kinds of mixes; providing it is well- drained I don't think the precise composition matters too much.
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Thank-you Susan. That was interesting.
I am not really talking about which sand or what proportion to use, just whether or not it actually does what it says on the packet. IE Does using sharp sand improve drainage? Or as someone on another site has claimed, does it have no effect or actually make the drainage worse?
That said, all sands improve drainage as they create channels or pores for the water to travel through. There are some fascinating, (and a lot of very dry ::) ) books on the subject - mainly aimed at sports turf management.
May I quote you ?
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The most crucial aspect is definitely the particle size. Fine sands make drainage worse - the small particles fall into the larger air spaces in the mix and fill them up. These spaces would have had helped the drainage, but now full of fine sand that has very small air spaces bewtween the grains, they retain water due to capillary action and suface tension. The textbooks suggest that the cut off point for whether particles drain or retain water is 1.6mm diameter. in other words, sands/grits with particles less than 1.6mm diameter will hinder drainage whilst those with particles larger than 1.6mm will help drainage.
Paul
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Thank-you, that is exactly what I needed. Most grateful.
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So,how do I start to measure the grain size of my sand ;D
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So,how do I start to measure the grain size of my sand ;D
!. Buy the sand.
2. Buy an extremely accurate digital micrometer.
3. Buy a pair of fine nosed tweezers.
4. Use tweezers to place grain of sand in jaws of micrometer (remember to hold jaws of micrometer in a vertical postion or the grain of sand will fall off).
5. Adjust micrometer until light pressure is applied (don't overdo it or you might end up with 2 grains of sand).
6. Record measurement and place measured grain of sand in appropriate bucket. I'd suggest buckets for each 0.1mm grain size to ensure you get the results you want and hence good drainage control.
7. Repeat 4 to 6 until you have the required quantiy of sand of your chosen grade.
Please report back on your results as it will be of great assistance to your fellow forum members.
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Martin, you have provided detailed and very pertinent information. I feel however, that you may have missed out a most important step needed before your suggested procedure : get new spectacles :-X
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Some time ago Ian did some experiments in which he replace the loam in his bulb mix with sharp sand. The initial results were not encouraging but I wonder whether he persisted &, if so, what he thinks now?
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So,how do I start to measure the grain size of my sand ;D
At work (back when we used to do this sort of thing) we would use a stack of calibrated sieves on a shaker and measure the proportion of the sand retained by each sieve in the stack. If it were all retained by sieves bigger than Paul's 1.6mm then it is OK to use.
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I don't remember Ian saying the results were not encouraging. :-\
For quite some time now all our potted subjects have been in a general mix of 2:2:1 - that is sharp sand, grit and leaf mould. There seems to be no difficulty in growing good plants in this mix.
The very best sand we ever had was a large load of packs of lovely silica sand that we obtained from an oil company, when it was rejected by them because the packets were punctured and they needed the contents to be bone dry. It was clean, relatively even in size (around 1.8 to 2mm).
Nowadays we have to use the much more run of the mill sharp sand ;D
P.S. our loam used to come from rotted turf but we ran out of that long ago and that is why we went over to this extended use of sand.
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Thanks Maggi.
I don't remember Ian saying the results were not encouraging. :-\
Bulb log 34/05
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Well remembered, Gerry! That log is here: http://www.srgc.org.uk/bulblog/log2005/240805/log.html (http://www.srgc.org.uk/bulblog/log2005/240805/log.html)
Yes, to start with he found that the bulbs needed extra feeding and he compensates for that now with more attention to feeding through the growing season.
Bit more about sand and gravel here : http://www.srgc.org.uk/bulblog/log2007/210607/log.html (http://www.srgc.org.uk/bulblog/log2007/210607/log.html)
To me it seems like the "structure" of the potting mix remains more stable with the loam-less mix. If repotting is missed for a year ( or two! :-[ ) then it doesn't deteriorate in the way that the "old" mix used to, when it would look very tired and dusty.
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Darren's description is very accurate. The sand must be dry of course otherwise it will stick in the sieves.
It is usually possible to buy sieved sand from the people who supply grit and sand for industrial use. They have it available in batches with minimum and maximum grain size. Unfortunately they want to sell it by the truck load. However, with a small donation to the "coffee funds" you might get a bucket or two informally. That is the way it works here anyway. the "sand" is quite often crushed rock so it is very sharp indeed.
Göte
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To me it seems like the "structure" of the potting mix remains more stable with the loam-less mix. If repotting is missed for a year ( or two! :-[ ) then it doesn't deteriorate in the way that the "old" mix used to, when it would look very tired and dusty.
Maggi - "tired & dusty" is a good description of my 2-year old JI based bulb mix. It could also be said of me in the middle of crocus repotting. A fairly dispiriting experience this year; two very cold winters have not been kind to crocus.
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Maggi - "tired & dusty" is a good description of my 2-year old JI based bulb mix. It could also be said of me in the middle of crocus repotting. A fairly dispiriting experience this year; two very cold winters have not been kind to crocus.
Same for me Gerry. I usually find when I'm re-potting that I get very chesty as result of the dust from running the old compost through a sieve to find the last of my very small Crocus corms. This year I bought a supply of face masks to use. It turned out to be a waste of money though, I put the face mask on and my specs steamed up and I couldn't see a thing.
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!. Buy the sand.
2. Buy an extremely accurate digital micrometer.
3. Buy a pair of fine nosed tweezers.
4. Use tweezers to place grain of sand in jaws of micrometer (remember to hold jaws of micrometer in a vertical postion or the grain of sand will fall off).
5. Adjust micrometer until light pressure is applied (don't overdo it or you might end up with 2 grains of sand).
6. Record measurement and place measured grain of sand in appropriate bucket. I'd suggest buckets for each 0.1mm grain size to ensure you get the results you want and hence good drainage control.
7. Repeat 4 to 6 until you have the required quantiy of sand of your chosen grade.
Please report back on your results as it will be of great assistance to your fellow forum members.
Memo to self: Never listen to advice from retired engineers :P
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I use builders sand for my plunge beds and the best sized bulbs such as crocus and corydalis and the quickest to reach flowering size are those that self seed into it. It is heavy and wet but I have not plucked up courage to use it in my mixture.
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So,how do I start to measure the grain size of my sand ;D
I currently have two small bags of horticultural sharp sand which look very similar. One - J. Arthur Bowers - simply claims to be "graded", the other - Silvapearl - claims to be "3mm down". Visual inspection suggests that the bulk of the Silvapearl is "down" which seems to be confirmed by the fact that most of it passes through a very fine sieve - a teastrainer.
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Maybe you can use Stainless-steel-window netting
I use 40x60 VMH crates with different holes
Roland
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I use sand not so much for drainage purposes as my compost drains well anyway, but because roots just love it. Any plant will make new and healthy roots as it hits the sand, or so it seems to me. I use only sand for pots/trays of cuttings for instance. It seems to encourage the new, very fine hair-like roots.