Nanosized Calcium Phosphates as Novel Macronutrient Nano-Fertilizers

Abstract

The need for qualitatively and quantitatively enhanced food production, necessary for feeding a progressively increasing World population, requires the adoption of new and sustainable agricultural protocols. Among them, limiting the waste of fertilizers in the environment has become a global target. Nanotechnology can offer the possibility of designing and preparing novel materials alternative to conventional fertilizers, which are more readily absorbed by plant roots and, therefore, enhance nutrient use efficiency. In this context, during the last decade, great attention has been paid to calcium phosphate nanoparticles (CaP), particularly nanocrystalline apatite and amorphous calcium phosphate, as potential macronutrient nano-fertilizers with superior nutrient-use efficiency to their conventional counterparts. Their inherent content in macronutrients, like phosphorus, and gradual solubility in water have been exploited for their use as slow P-nano-fertilizers. Likewise, their large (specific) surfaces, due to their nanometric size, have been functionalized with additional macronutrient-containing species, like urea or nitrate, to generate N-nano-fertilizers with more advantageous nitrogen-releasing profiles. In this regard, several studies report encouraging results on the superior nutrient use efficiency showed by CaP nano-fertilizers in several crops than their conventional counterparts. Based on this, the advances of this topic are reviewed here and critically discussed, with special emphasis on the preparation and characterization approaches employed to synthesize/functionalize the engineered nanoparticles, as well as on their fertilization properties in different crops and in different (soil, foliar, fertigation and hydroponic) conditions. In addition, the remaining challenges in progress toward the real application of CaP as nano-fertilizers, involving several fields (i.e., agronomic or material science sectors), are identified and discussed.

Keywords: ACP; calcium phosphates; chemical doping; hydroxyapatite; nano-fertilizers.

Figure 1

Schematic representation of the joint SAXS and WAXTS analysis used to elucidate the ACP-nAp core-crown arrangement of the nanocomposites platelets present in both biogenic and biomimetic apatite “Reprinted with permission from Ref. [17]. 2021, Elsevier”.

Figure 2

A schematic comparison of soluble P, nano-sized solid P and solid P fertilizers and their environmental properties. “Adapted with permission from Ref. [38]. 2014, Springer Nature”.

Figure 3

Cumulative release of nitrate (a) and calcium (b) from nitrate-doped nano-apatite (blue curve) and ACP (red curve) and non-doped nano-apatite (green curve); (c) Structural evolution of nitrate-doped nano-apatite, apatite and ACP nanoparticles after their suspension in water during days 1 and 3. “Adapted with permission from Ref. [26]. 2020, Springer Nature”.

Figure 4

(a) TEM image of nanoU-NPK nanoparticles. (b) Cumulative urea release from granular urea (blue curve) and nanoU-NPK (magenta curve) embedded in a simulated solid medium, mimicking an inert soil (a low-diffusive medium) during water irrigation. The inset shows a graphical representation of the column used in the leaching experiments “Reprinted with permission from Ref. [31]. 2020, ACS”.

Figure 5

(a) FTIR and PXRD of Nano-UACP, (b) TEM-images of NanoU-ACP nanoparticles showing spheroidal morphology, (c,d) FTIR spectra of Urea (black curve), Nano-U-ACP (red curve), and non-functionalized ACP (Nano-ACP, grey curve) in the regions of wavenumber from 1850–1550 cm⁻¹ and 1600–1350 cm⁻¹, respectively. The shift observed in the vibration mode assigned to the urea for the material Nano-U-ACP and pristine urea indicating the main Ca-NH2 interactions are highlighted. “Reprinted with permission from Ref. [56]. 2021, Springer Nature”.