Hydroxyapatite/urea hybrid materials: what is the basis for the enhanced nutrient efficiency?

The growing demand for food production worldwide has led to the increased use of fertilizers contributing to a range of environmental problems. To reduce these problems, the development of urea–hydroxyapatite (HAP) materials as nutrient-efficient fertilizer carriers has gained considerable attention as a more nutrient-efficient alternative to conventional nitrogen (N) and phosphorus (P) fertilizers. Conventional N fertilizers, such as urea, possess high solubility and rapidly release nitrogen leading to significant nutrient losses through leaching and volatilization. Conventional P fertilizers suffer from quite the opposite problem: they are quickly immobilized in soil and P release becomes very slow. HAP is a naturally occurring mineral and has been postulated, at the nanoscale, to release P at a controlled rate although risks associated with HAP nanoparticle occupational and environmental toxicity remain. HAP/urea hybrid materials present a unique opportunity for N–P–(Ca) fertilizer material design where innate properties of the parent materials, urea and HAP, are altered due to the purported chemical interactions, thus resulting in a novel and improved nutrient management paradigm. This review summarizes the developments in their synthesis, nutrient release and plant uptake while scrutinizing the reported underlying chemical interactions between both parent compounds, critical to the enhanced efficiency in soil.