It is not so much a too many slugs problem; it is really a too few ducks problem...
I used to put all my slugs and snails on the compost heap. The trouble was that they enjoyed it so much there that they bread quite rapidly and my allotment was over run with the blighters. Now I take off all the slugs and snails and take them home to put into the council's green bin. You have to put a weight on top of the lid to keep them from escaping but it is an easy way to get rid of them without killing them.
I used Nemaslug nematode (Phasmarhabditis hermaphrodita) worms last year, spraying them only in areas where I know snails and slugs congregate. I am also using them this year. While I would not say that this has been 100% effective, it does seem to have reduced damage so far this year. (I watered them on in March and again in May 2011) I am hoping that the nematodes have reduced the breeding adults so that they will not lay eggs.
While I can understand the agonizing that many of us undergo when attempting to produce food that is grown with as few human made chemicals as possible, we must be reasonable. The slug and snail pellet ferric phosphate FePO4 is indeed an inorganic chemical. All this means, in chemical terms, is that it does not contain carbon. The confusion comes when we apply the term organic to horticulture. Organic in biology means related to life or organisms. If we replace the metal iron with the metal calcium in this compound then we get a major component of bones – calcium phosphate which, although making bones, is an inorganic chemical. Does this mean that the strict advocate of organic gardening should not use blood fish and bone as a fertilizer? I don't think so.
I would rather not use ferric phosphate as a slug and snail killer because I would rather remove as many slugs as I can by hand – gloved if possible. There is little evidence about the effect that ferric phosphate has on other soil organisms and is probably best avoided if you are trying to be organic – as in the biological meaning of the word.
A tidy allotment or garden is also a good deterrent. If there is no habitat for the molluscs to live in then there will be many fewer of them.
If you are going for the hand collection method, it is easier if you use traps like upturned flower pots, upturned orange or grapefruit skins or planks of wood. I have seen people use newspaper soaked in sugar solution and covered with a plank to attract slugs and snails. Other people do not use these methods of trapping because they think that it attracts slugs and snails. I use them regardless.
I really don't think that the rough, sharp surfaces idea works. This involves putting crushed egg shells, course sand, soot or nut shells around sensitive plants. It would seem, although I have not seen any evidence for this, that slugs and snails do not like crawling over surfaces laced with sharp or rough surfaces. However, it is always worth a try especially if you are overwhelmed with a surfeit of broken egg shells.
There are several natural predators of slugs and snails. The ground beetle is one of them and I go around looking under logs and stones to try to catch as many of these as I can to put onto the allotment. They will need a home to live in such as under planks of wood. I have put loads of toads on the allotment but I can never get them to stay. I have actually seen a toad eat a slug. Toads have remarkably long tongues. I would love to have both hedgehogs and slow worms on the allotment but sadly I have never had that luxury. Thrushes used to be the very best snail predator. They have used paving slabs on the allotment as their anvil to break open snail shells. Anything that will attract natural predators will help in the control of slugs and snails.
If you have the opportunity to have ducks on your allotment then they are ideal slug eaters.
Beer contains organic chemicals. You could use this as a trap because slugs and snails seem to be attracted to it. I have heard that you can make a diluted honey and yeast mix to trap slugs and snails as well. There are other recipes such as sugar, flour and yeast, or just sugar and yeast. I am going to try dark brown sugar and yeast. Also, it seems, milk has a similar effect on slugs and snails. I am going to try this one.
I have never used copper rings to prevent slugs from reaching plants. There is some evidence from heliculture that this is a way of preventing slug escape leading to the suggestion that copper rings might keep slugs and snails away from choice plants. The most that we can say is that copper bands will delay the movement of slugs and snails because after just one hour 25% of slugs had crossed the barrior in research.
I have experimented this year with beer traps and they have been very effective. The poor animals drown in the beer. When they seem to have gone over, I add the contents of the beer traps to the charcoal bins. Next year I will cover the traps to prevent rain from overflowing them. (I have done this with bricks June 2011)
There is a lot of evidence that coffee grounds are well worth while putting onto your compost heap.
There is little evidence that coffee grounds will deter slugs.
There is some evidence that caffeine does have an effect on slugs, however I would expect that you have already dissolved most caffeine out of your coffee beans before it gets anywhere near your garden slugs. Together with rain and general dampness of the ground, I would suggest that this is a very ineffective way of combating slugs and snails. Have a look at this:
Research in USA suggested that spraying a solution of 2% pure caffeine would prevent slug attack.
So don’t drink your coffee, spray it on your plants and even then you cannot guarantee it will be strong enough to have an effect.
Using the well tried and tested ways of gloved hand picking, nematodes and beer traps seem to be the only effective ways for the organic gardener to rid the allotment of these pesky animals.