Organic fertiliser made with bat guano

In an aquaponic system, the fish feed provides most of the nutrients the plants need, phosphorus included: their waste, converted by the bacteria in the filter, feeds the grow bed. The deficiencies most often encountered there concern iron, potassium and calcium, not phosphorus. Bat Guano, whose phosphorus is insoluble and which breaks down into organic matter, therefore has no reason and no place in the pond water, where it would cloud the water and use up the oxygen the fish depend on. Instead, correct iron with a DTPA chelated iron, at around 2 mg/L roughly every three weeks, and potassium with a compatible salt such as potassium bicarbonate.

Guano does, however, suit the soil-grown crops that often surround a system: raised beds, planters and pots in the greenhouse. Some hydroponic growers use it as an infusion, after soaking for at least 12 hours and filtering through muslin, but this cloudy solution is reserved for a separate crop and never goes into the pond, or into any water that flows back to it. For plants destined for the grow bed, germinate the seeds in rockwool cubes without organic fertiliser, then rinse the root ball before transplanting. The article which fertiliser to use in aquaponics sets out what can enter the system.

The best time remains when preparing the grow medium, at repotting or planting, because the guano then has time to break down before flowering. For a tomato planted in May in a 20-litre pot, mix 200 g into the compost, then top up on the surface during the summer if the foliage turns pale or the soil looks exhausted. In the vegetable garden, a spring application on the fruiting vegetable bed is often enough, raked into the top few centimetres and followed by generous watering, and a second light application can support recovery after a first harvest.

Guano does not replace a liquid fertiliser when a plant shows an immediate need, since its action takes several weeks. The two approaches combine well: guano forms the base reserve, and an organic liquid fertiliser applied at watering meets the peaks in demand of vegetables in full production. In a tub of strawberries, for example, you mix the guano in at planting, then a few enriched waterings at fruiting time cover the whole season without overloading the grow medium.

The 1 litre tub is enough to prepare around a hundred litres of potting compost, enough to fill a balcony's window boxes or a few large planters. The 10 litre bucket is intended for a family vegetable garden or a greenhouse grown in soil, where applications are counted in hundreds of grams per bed. In both cases, keep the container closed between uses, in a dry place, because damp makes the powder clump together and harder to dose.

The powder provides far more than its phosphorus. According to the June 2024 technical data sheet, its nitrogen, 3.3%, is entirely bound to organic matter, so it is only released at the pace of life in the soil. Phosphorus reaches 14.9% expressed as oxide, and potassium 4.5%. Guano also supplies a lot of calcium, 27.7% expressed as oxide, with a little magnesium, and its reaction is alkaline, with a pH of 8 to 9 measured in suspension in water. Being a natural product, guano varies from one batch to another, and the analysis printed on your tub may differ slightly from these values. During flowering, a liquid organic fertiliser such as Alga Bloom tops up the base reserve without replacing it, and Royalmix, an organic potting mix with worm castings, forms with the guano a fully organic base for fruiting vegetables in pots.