I love oysters – not the insides, which my stomach decidedly can’t tolerate, but the shells. I sprinkle them here, there and everywhere in my garden because they perform a certain magic by regulating the soil’s environment.
Oyster shell can be used as an alternative to lime. Both are composed of calcium carbonate, which is an alkali, and therefore raises the pH of the soil, increasing the plant’s ability to take up other micronutrients such as zinc, iron and manganese.
Most plants, other than the obvious acid lovers – blueberries, cranberries, heathers, rhododendrons and carnivorous bog plants – like a pH of about 6-6.5. This is particularly true of vegetables. In these slightly alkaline conditions good bacteria can thrive, boosting microbial activity, which in turn boosts soil respiration. This results in happier plant roots that take up nutrients better, leading to healthier plants above ground.
If you feel that your soil, despite oodles of compost and muck, isn’t giving you the kind of growth you’d expect, it may very well be because the pH is out of balance. Traditional lime needs to be applied when the plants are dormant in winter. It’s also not fun stuff to handle – it’s a skin irritant so you shouldn’t breathe in the dust, and you certainly don’t want it near your eyes. There are alternatives: dolomitic lime is used in some organic systems, as is calcified seaweed, but both have environmental implications as neither are renewable in the short term.
Oysters, however, and for that matter whelks, can be sustainably farmed. Oyster farming, in particular, has many benefits: they are excellent filters, so purify the water; and they act as tiny carbon capturers, sequestering nitrogen and carbon dioxide as they go.
You don’t need to live by the sea to get hold of them; the oyster grit you can buy in pet shops is ideal because it’s been smashed up and washed to remove the salt. I use this in early spring before too much top growth appears, often mixing it with a little dried seaweed for good measure, scattering it across the soil before adding any homemade compost. I then leave the worms to do the rest of the work.
If you can get hold of fresh oysters then you need to rinse them, smash them up (baking them first makes this easier), and put them straight into the compost to further break down – but you’ll need to leave it for a few months. There is evidence that composted shell is better for the soil than fresh shell. The joy of this method is that it can happen any time of the year: you just need to shuck, smash and compost.