The iron sulphate solution was released through a hose trailing in the ship’s propeller wash while she spiralled around the drifting buoy in widening concentric circles one km apart.
Since the iron is rapidly taken up by the biota or converted into insoluble colloidal rust, the inert gas sulphur hexafluoride (SF6) was continuously added in trace amounts to the iron solution in order to mark the fertilised patch as SF6 can be measured at very low concentrations. A total of 480 g of this biologically inert gas is sufficient to mark the entire patch. A tank was emptied in about 2.5 hours and was filled by teams of scientists while the contents of the other was being released. Iron sulphate tablets are used to treat patients suffering from anaemia and we used the same quality grade sold in gardening shops and department stores for treating lawns. Nevertheless, the substance is converted into rust which stains clothing and large amounts of the dust can irritate eyes and nose so we took maximum
precautions to reduce exposure to the minimum by having those doing the job wear protective clothing and masks. An area of 300 km2 was fertilised with a total of 10 tonnes of iron sulphate which took 30 hours to complete. We administered only half the quantity originally planned because the mixed layer was only half as deep as expected.
As expected, diatoms were the first phytoplankton group to respond to iron fertilization but their further growth was limited by silicon deficiency.