Pietra pomice per orticoltura in granuli da 3 a 6 mm

These pores make pumice a biofilter, whose volume is sized according to the fish feed ration. Allow roughly 1 L of volcanic gravel per gram of feed given each day, compared with 0.5 L for plastic media designed for filtration. A 33 L bag therefore provides the surface area needed for around 33 g of feed per day, which is the ration for about 0.7 m² of lettuces, since a bed of leafy vegetables receives 40 to 50 g of feed per square metre per day. In a NFT or deep water system, where the baskets hold only a few handfuls of pebbles, this role falls to a separate biofilter, which the pumice can also fill.

The pH governs the work of these bacteria. Nitrification works best above pH 7.5 and stops below 6, whereas plants prefer more acidic water, so aquaponic systems are run at around 6.8 to 7.0, with an alkalinity of at least 100 mg/L of CaCO3. As nitrification gradually acidifies the water, the pH tends to fall over the weeks. Pumice, which does not release carbonates, keeps this drift easy to read, and you correct it by alternating small doses of hydrated lime and potassium hydroxide, which also supply calcium and potassium to the plants. The article on regulating pH in aquaponics covers these corrections in detail.

On its own, pumice works as a grow medium for cuttings and seedlings: it stays airy, does not compact and does not encourage collar rot. Mixed with potting compost or coco fibre, it lightens the grow medium, improves pot drainage and limits settling over repeated watering. One part pumice to two or three parts compost already gives a noticeably freer-draining mix for potted plants, herbs and succulents; you can then adjust the proportion to suit the plant and how often you water. For a compost that also needs to hold a reserve of water and cations, zeolite can be mixed in the same way, at 10% of the volume.

Being mineral, pumice does not degrade over time and does not lose its structure. After a crop, remove the roots, rinse the grains thoroughly, then leave them to dry before storing them. The 33 L bag, which weighs around 25 kg straight from the quarry, becomes noticeably lighter once the grains are dry, since the bulk density of dry pumice falls to 430 to 530 kg per cubic metre. In hydroponics, where pumice supplies no nutrients, a longer rinse between two crops flushes out the salts of the previous solution left in the pores, before you start afresh with a new solution prepared with a hydroponic fertiliser.