Abstract
Layered double hydroxides (LDH) are anionic clays that have potential as slow-release fertilizers; however, their formulation as powders makes them difficult to apply, and their slow-release properties are impaired due to instability under acidic conditions. In the work reported, Zn-Al LDH containing interlayered 15NO3− was synthesized for use as powder (LDH-N) or for encapsulation in alginate beads (LDH-AN), and then authenticated by X-ray diffraction, attenuated total reflectance-Fourier transform infrared spectroscopy, and elemental analyses. The two LDHs were compared to K15NO3 for evaluating their slow-release properties through (i) a kinetic study of NO3− release in water under dynamic conditions, and (ii) a growth chamber experiment designed to estimate fertilizer N uptake efficiency (FNUE) by growing pearl millet (Pennisetum glaucum L.) on an acidic Oxisol in the absence of N losses. Both LDH materials exhibited slow-release properties in the kinetic studies, and NO3− release was reduced for LDH-AN as compared to LDH-N. Because of these properties, FNUE measurements in the growth chamber experiment should have been lower with the LDHs than with K15NO3, but this was not the case for LDH-N, which was attributed to the structural instability of powdered LDH in the presence of soil acidity and to the exchange of NO3− by more competitive anions such as CO32−. A significant decrease in FNUE was observed for LDH-AN, demonstrating retention of slow-release behavior that most likely resulted from the presence of a physicochemical barrier having high cation-exchange and buffering capacities while limiting exposure to soil acidity and anion exchange. Alginate encapsulation expands the practical potential of LDH for slow-release NO3− fertilization.
Highlights
The growing demand for food and feed by a burgeoning world population leaves no alternative to modern cereal production systems with intensive use of synthetic N fertilizers
X-ray diffractograms are presented in Figure 1 for KNO3, alginate, Layered double hydroxides (LDH)-N, and LDH-AN
The residual presence of KNO3 is indicated by the four peaks labeled as (*), which coincide with the corresponding peaks obtained for KNO3 (Figure 1a) but are absent from the diffractogram obtained for LDH-AN (Figure 1d)
Summary
The growing demand for food and feed by a burgeoning world population leaves no alternative to modern cereal production systems with intensive use of synthetic N fertilizers. Most of these fertilizers supply NH4 + , reflecting universal dependence on the Haber-Bosch process for NH3 synthesis [1]. Except for the use of NH4 NO3 in UAN solutions and of CaNH4 (NO3 ) (CAN) as a major N source in some parts of the world, NO3 − fertilizers currently have a very limited presence in the commercial marketplace, due in part to their low N content that makes them more expensive and less practical than ammoniacal fertilizers. Owing to the inherent risk of N loss by leaching or denitrification, NO3 − applications must be synchronized with crop N demand, creating a logistical complication that becomes especially problematic for large farming operations
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