Abstract
Nitrate (NO3
-) and Phosphate (P) are the most important limiting nutrients in crop production
systems. The current practices of using soluble forms of NO3
- and P have limited agronomic
effectiveness because of the significant fractions lost irreversibly by fixation and precipitation
in soils. It is also a threat to the quality of surface water as the NO3
- and P runoff and leaching
from fields lead to eutrophication. Hence, there is increased attention from the agriculture
industry for technology to increase the sustainability of current NO3
- and P use through the
development of slow-release NO3
- and P fertilizers to increase crop production rate while
conserving our mother nature and protecting the health of the consumers.
This thesis assesses layered double hydroxides (LDHs) as a substitute and NO3
- and P fertilizer
sources. LDHs are a class of anionic lamellar materials with a two-dimensional layered
structure and an ion-exchanging power. Although LDH is derived from the structure of
positively charged mineral brucite, Mg(OH)2, unlike the brucite structure, some of the divalent
metal ions are replaced with trivalent metal ions in the LDH structure, and the anion-like NO3
-
and P are loaded into the interlayer. The LDHs have the potential to release NO3
- and P anions
slowly, through exchange with other anions like carbonate (CO3
2-) present in the soil.
The NO3
- loaded LDHs were synthesised from magnesium nitrate hexahydrate, zinc nitrate
hexahydrate and aluminium nitrate hexahydrate utilising the co-precipitation method. To
confirm the presence of LDHs, prepared materials were characterised using powder x-ray
diffraction (XRD), Attenuated total reflectance-Fourier transform infrared spectroscopy (ATRFTIR)
measurements and thermogravimetry analysis (TGA). Powder XRD revealed a
characteristic LDH structure for all LDH samples. TGA showed decomposition usually
occurred through a multi-step process as expected for LDHs. Subsequently, the NO3
- loaded
LDHs were used as starting materials to synthesize P loaded LDHs.
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Encapsulation of the best performing LDH in terms of slow release and buffering capacity in a
biopolymer matrix (LB) was done to stop the direct contact of powdered LDH with soil, retain
water, and enhance the water-holding capacity of the soil. The slow-release properties of the
materials were evaluated using a kinetic study of anion release in aqueous medium and under
farming conditions using two different plant growth experiments. The release ability of LDHs
and encapsulated LDH was compared with a soluble nitrate source and commercial slowrelease
fertilizer.
In the first part of the study, a co-precipitation method was utilised to prepare the LDHs (Zn/Al
LDH, Mg/Al LDH, Mg-Zn/Al LDH) loaded with NO3
− and made alginate beads (LB) using
powder Mg/Al LDH. Physicochemical characterization revealed that the LDH was able to
maintain the layered structure. The NO3
− content in LDHs was 54.7, 55.2, and 54.4 mg/g for
Zn/Al LDH, Mg/Al LDH and Mg-Zn/Al LDH, respectively. The release rate of NO3
− from
LDHs and LB in tap water and soil solution was much more gradual than soluble nitrate source
(KNO3), and Mg/Al LDH was the best material in terms of buffering capacity. The materials
were used as a fertilizer for coriander (Coriandrum sativum) growth. LDHs showed similar
effectiveness for plant growth and produced dry matter that contained nitrate similar to that of
soluble fertilizer. LDHs also increased the soil pH value, which enhanced the accessibility of
nitrate to the plant by diminishing the uptake of NO3
− by the mineral phase of the soil. LB was
able to supply water to plants for extended periods but showed lower productivity than LDHs
and KNO3 in coriander growth since the release rate was much slower than LDHs. The
findings demonstrated the potential of LDHs as slow-release fertilizers, as reliable matrices to
retain nitrate levels, and soil pH and to avoid nitrate leaching. Encapsulation of LDH showed
further reduction in the release rate which supports its usage as a long-term crop fertilizer.
In the second part of the study, an ion-exchange method was utilized to prepare the LDHs
(Zn/Al LDH, Mg/Al LDH, Mg-Zn/Al LDH) with subsequent encapsulation of powder Mg/Al
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LDH in alginate matrix to produces composite beads. The slow-release characteristics of Pexchanged
LDHs were confirmed by kinetic study. The release rate of P in soil solution was
almost instantaneous for the Soluble P source (KH2PO4), while the LDHs show P release at an
average of 40% at the end of 168 h.
The materials were used as a fertilizer for tomato growth in two systems: hydroponic and soil.
We compared the dry matter production of powdered LDHs and LB with a soluble P source
(KH2PO4) and commercial water-soluble hydroponics nutrient fertilizer (Nutrifeed) and
commercial slow-release fertilizer-Wonder plant starter (PS) for a comparative assessment.
The ability of LDHs to produce dry matter in the soil system was much higher than that of the
hydroponic medium. The P release from LDHs was the slowest in LB which retained more
than 70% of P within the LDH matrix, thus producing less dry matter. The pH-increasing
(liming) effect was higher in Mg/Al LDH than in other LDHs. Differences in P uptake among
treatments with LDHs and other sources of P fertilizer were minor.