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Developing efficient crop fertilization practices has become more and more important due to the ever-increasing global demand for food production. One approach to improving the efficiency of phosphate and urea fertilization is to improve their interaction through nanocomposites that are able to control the release of urea and P in the soil. Nanocomposites were produced from urea (Ur) or extruded thermoplastic starch/urea (TPSUr) blends as a matrix in which hydroxyapatite particles (Hap) were dispersed at ratios 50% and 20% Hap. Release tests and two incubation experiments were conducted in order to evaluate the role played by nanocomposites in controlling the availability of nitrogen and phosphate in the soil. Tests revealed an interaction between the fertilizer components and the morphological changes in the nanocomposites. TPSUr nanocomposites provided a controlled release of urea and increased the release of phosphorus from Hap in citric acid solution. The TPSUr nanocomposites also had lower NH3 volatilization compared to a control. The interaction resulting from dispersion of Hap within a urea matrix reduced the phosphorus adsorption and provided higher sustained P availability after 4 weeks of incubation in the soil.

A route is proposed to produce a hydrotalcite-like layered double hydroxide structure ([Mg-Al]-LDH) for phosphate fertilization. The mechanism of controlled phosphate release from the structure was investigated. The preparation strategy resulted in a phosphorus content of around 40 mg·g–1 LDH, which was higher than previously reported for related fertilizers. The release of phosphate into water from [Mg-Al-PO4]-LDH continued over a 10-fold longer period, compared to release from KH2PO4. Analysis using 31P NMR elucidated the nature of the interactions of phosphate with the LDH matrix. In soil experiments, the main interaction of P was with Fe3+, while the Al3+ content of LDH had no effect on immobilization of the nutrient. Assays of wheat (Triticum aestivum) growth showed that [Mg-Al-PO4]-LDH was able to provide the same level of phosphate nutrition as other typical sources during short periods, while maintaining higher availability of phosphate over longer periods. These characteristics confirmed the potential of this preparation route for producing controlled release fertilizers, and also revealed fundamental aspects concerning the interactions of phosphate within these structures.

The coating of fertilizers with polymers is an acknowledged strategy for controlling the release of nutrients and their availability in soil. However, its effectiveness in the case of soluble phosphate fertilizers is still uncertain, and information is lacking concerning the chemical properties and structures of such coatings. Here, an oil-based hydrophobic polymer system (polyurethane) is proposed for the control of the release of phosphorus from diammonium phosphate (DAP) granules. This material was systematically characterized, with evaluation of the delivery mechanism and the availability of phosphate in an acid soil. The results indicated that thicker coatings can change the maximum nutrient availability toward longer periods, such as 4.5-7.5 wt % DAP coated, that presented the highest concentrations at 336 h, as compared to 168 h for uncoated DAP. In contrast, DAP treated with 9.0 wt % began to increase the concentration after 168 h until it results in maximum release at 672 h. These effects could be attributed to the homogeneity of the polymer and the porosity. The strategy successfully provided long-term availability of a phosphate source.

The development of smart and eco-friendly fertilizers is pivotal to guarantee food security sustainably. Phosphate rock and struvite are promising alternatives for P fertilization; nevertheless, the solubility of these sources is a challenge for consistent use efficiency. Here, we propose using a polysulfide obtained via inverse vulcanization as a novel controlled-release fertilizer matrix in a system containing either Bayóvar rock (Bay) or struvite (Str). The polysulfide provides S for plants after being biologically oxidized to sulfate in soil, generating local acidity for P solubilization. After 15 days of soil incubation, the composites with 75 wt % Str and 75 wt % Bay achieved, respectively, 3 and 2 times the S oxidation from the elemental sulfur reference. Results indicated that P content stimulates the soil microorganismsʼ activity for S oxidation. The matrix had a physical role in improving Bay dissolution and regulating the rapid release from Str. Moreover, the available P in soil was 25–30 mg/dm3 for Bay composites, while for pure Bay, it was 9 mg/dm3.

The rapid hydrolysis of urea applied to the soil surface causes high rates of NH3 volatilization, leading to adverse environmental impacts and decreased uptake of N by crops. One approach that can be used to improve the efficiency of urea use involves strategies to control its release, such as the coating of granules with polymers. However, the effectiveness of this method can be limited by poor interaction between the coating and the granule surface. We, therefore, propose a novel class of nanocomposite fertilizers, based on clay exfoliation in urea matrices, with or without polymerization using formaldehyde as a strategy to increase the interaction between urea and the additives. A comparative study was performed using various slow-release fertilizers, determining the amounts of volatilized ammonia, dry matter production, and efficiency of urea-N uptake by ryegrass, in a trial carried out in a greenhouse. Interaction, such as solubility, thickness, and chemical composition of the composites revealed aspects of the interaction that affected the slow-release behavior of urea in soil and the availability of N for plants. It could be concluded that the controlled release of urea from the nanocomposites decreased NH3 volatilization, resulting in a more constant N availability in the soil and better synchronization with the nutritional demands of the plants. The new fertilizers offer a practical option for increasing urea-N efficiency, reducing environmental impacts caused by NH3 loss and improving the quality of forage grown on low fertility soils, such as oxisols.

Charcoal‐based amendments have a potential use in controlling NH3 volatilization from urea fertilization, owing to a high cation‐exchange capacity (CEC) that enhances the retention of NH $ _4^+ $. An incubation study was conducted to evaluate the potential of oxidized charcoal (OCh) for controlling soil transformations of urea‐N, in comparison to urease inhibition by N‐(n‐butyl) thiophosphoric triamide (NBPT). Four soils, ranging widely in texture and CEC, were incubated aerobically for 0, 1, 3, 7, and 14 d after application of 15N‐labeled urea with or without OCh (150 g kg−1 fertilizer) or NBPT (0.5 g kg−1 fertilizer), and analyses were performed to determine residual urea and 15N recovery as volatilized NH3, mineral N (as exchangeable NH $ _4^+ $, NO $ _3^ - $, and NO $ _2^ - $), and immobilized organic N. The OCh amendment reduced NH3 volatilization by up to 12% but had no effect on urea hydrolysis, NH $ _4^+ $ and NO $ _3^ - $ concentrations, NO $ _2^ - $ accumulation, or immobilization. In contrast, the use of NBPT to inhibit urea hydrolysis was markedly effective for moderating the accumulation of NH $ _4^+ $, which reduced immobilization and also controlled NH3 toxicity to nitrifying microorganisms that otherwise caused the accumulation of NO $ _2^ - $ instead of NO $ _3^ - $. Oxidized charcoal is not a viable alternative to NBPT for increasing the efficiency of urea fertilization.

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