Phosphorus is one of the primary nutrients essential for plant growth and crop production (Mengel and Kirkby, 2004). At present, most commercial phosphorus (P) fertilizers on the market such as single superphosphate are derived from phosphate rock (Barker and Pilbeam, 2006). However, world reserves of currently exploitable phosphate rock are rather limited and increasingly dissipated (Abelson, 1999). For these reasons, alternative technologies to recover P from waste materials have received a great deal of interest in recent years (de-Bashan and Bashan, 2004; Shu et al., 2006).
Struvite (MgNH4PO4·6H2O) is a potentially marketable product for the P fertilizer industry, which can be recovered at municipal wastewater treatment works from anaerobically digested sludge liquors (de-Bashan and Bashan, 2004; Gaterell et al., 2000). However, the production of struvite at full scale remains limited, partly because of the costs associated with chemical addition. In particular, anaerobically digested sludge liquors tend to be deficient in magnesium (Mg) (de-Bashan and Bashan, 2004; Nelson et al., 2003). Recently, scientists at the Spanish Research Council (CSIC) have developed a new, economically advantageous method for recovering struvite using a low-grade MgO-containing byproduct of the magnesite industry as a Mg source (Quintana et al., 2004, 2006, 2008). This technology is now being implemented at full scale in a municipal wastewater treatment plant in Madrid, Spain.
Several studies on struvite recovered from municipal wastewater have been focused on its production (e.g., de-Bashan and Bashan, 2004, and references therein), but its potential for use as a crop fertilizer requires additional research. In particular, positive agronomic results in terms of yields and P uptake have been reported for chickpea (Cicer arietinum L.) (Ghosh et al., 1996), lupin (Lupinus albus L.) (González-Ponce and García-Lòpez-de-Sá, 2008), and ryegrass (Lolium perenne L.) (González-Ponce and García-López-de-Sá, 2007; Johnston and Richards, 2003; Plaza et al., 2007). To date, however, no data are available in the literature for lettuce (Lactuca sativa L.), which is a leading vegetable crop in many parts of the world with a high demand for P (Kelling et al., 1998; Mou, 2008).
The objective of this work was to evaluate the potential of struvite recovered by the CSIC pilot process (STR) as a source of P for the cultivation of lettuce under greenhouse conditions. Single superphosphate (SUP), a common P fertilizer derived from phosphate rock, which is considered to be fully plant-available (Johnston and Richards, 2003), was used as a reference for comparison.
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