Powdered bat guano fertiliser

Depending on the batch, its analysis ranges from 3 to 4% nitrogen, of organic origin, and from 12 to 14.5% phosphoric anhydride (P2O5); the label on your bucket gives the exact value. Phosphorus supports root formation, then that of flowers and fruit; organic nitrogen, for its part, is released only as micro-organisms in the soil break it down. The guano therefore acts gradually, over a period of about eight weeks, whereas a soluble fertiliser acts within a few days and leaches out faster. A dose of 10 g per litre of grow medium thus supplies from 1.2 to 1.45 g of phosphoric anhydride to each litre of your potting compost.

This guano is mixed into soil and potting compost, never into the water of an aquaponic system: an organic powder added to a circuit where fish live would cloud the water, break down while using up oxygen and feed algae. It does, however, have its place around your aquaponic setup: in your pots and soil-filled tubs watered by hand, in the vegetable beds that take the tomatoes and squashes too big for the grow beds, or in the compost of the plants that will not join the system. Plants intended for the grow beds, for their part, are started in a neutral grow medium such as rockwool, with no soil or fertiliser, so that nothing is added to the fish water.

When a deficiency appears in an aquaponic system, the correction relies on products that are safe for fish. Iron, the element that is most often lacking, is supplied as DTPA chelated iron at a rate of 2 mg per litre, repeated every three weeks or as needed, as recommended by New Mexico State University. Potassium and calcium arrive with the pH correctors: by alternating potassium hydroxide and calcium hydroxide, or by using potassium bicarbonate, you bring the pH back up towards 7 while feeding the plants. These additions are explained step by step in the guide which fertiliser to use in aquaponics.