Lettuce growth and phosphorus leaching in media amended with phosphorus-treated magnetite

Magnetite was examined for use as a medium on which to bind phosphorus (P) for the purpose of reducing P leachate from container grown crops. Magnetite was acid washed, pH adjusted to 5.0, and then shaken in a 10,000 mgP·L ^-1 solution. The P-magnetite was chemically characterized to determine P content (176 mgP·kg^-1 magnetite). A greenhouse experiment was performed to determine the accessibility of the P from the P-magnetite to the plant and the amount of P leachate. 'Valmaine' Romaine lettuce Lactuca sativa was sown into 5.5 inch standard containers and treated with one of the following: 250 g P-magnetite (44 mg P), 150g P-magnetite (26mg P), 50g P-magnetite (8.8mg P), 150g untreated magnetite, 3g 15-9-12 Osmocote (119 mg P), 21-5-20 constant liquid feed (5.25 mg P/fert. event), and no treatment. For four weeks, weekly leachate samples were taken and analyzed for P content. At the completion of four weeks, the shoots were dried and weighed. The constant liquid feed treatment had the highest P leachate and shoot dry weights. Osmocote and 150 g P-magnetite had similar P leachate, and the shoot dry weights for the 150 g Pmagnetite treatment were significantly higher than for Osmocote. This study demonstrated that P from amended magnetite is accessible for uptake by plants and leachate was reduced using P-magnetite compared to a constant liquid feed.