Smart fertilizers: What should we mean and where should we go?

The current agricultural system faces several challenges, the most important being the ability to feed the increasing world population and mitigate climate change. In this context, the improvement of fertilizers’ agronomic efficiency while reducing their cost and environmental impact is one of the biggest tasks. Available literature shows that many efforts have been made to develop innovative fertilizers defined as ‘smart fertilizers’, for which, different interpretations and definitions have been used. This paper aims to define, classify, and describe the new frontier of the so-called smart fertilizers with a particular focus on field-scale studies on herbaceous species. Most of the analysed papers associate the ‘smart’ concept to the controlled and/or slow release of nutrients, using both terms as synonymous. Some others broadened the concept, including the controlled release of nutrients to reduce the environmental impact. Based on our critical analysis of the available literature, we conclude that a fertilizer can be considered ‘smart’ when applied to the soil, it allows control over the rate, timing, and duration of nutrients release. Our new definition is: ‘Smart fertilizer is any single or composed (sub)nanomaterial, multi-component, and/or bioformulation containing one or more nutrients that, through physical, chemical, and/or biological processes, can adapt the timing of nutrient release to the plant nutrient demand, enhancing the agronomic yields and reducing the environmental impact at sustainable costs when compared to conventional fertilizers’