Presently agriculture all across the world is facing a wide range of challenges; the important challenges are (a) crop yield stagnation (b) decrease in arable land due to land degradation and urbanization (c) low nutrient use efficiency (d) deficiencies of more than one nutrients in soil (e) declining soil organic matter (e) water availability etc. Under these challenges, it would be difficult to produce enough food to feed the ever increasing populations, which is expected to cross 9 billion by 2050.
Nanoscience and Nanotechnology research in agriculture and horticulture are still at an elementary stage but developing rapidly. Conventional bulk fertilizer or traditional fertilizers are not only expensive for the producer, but may be harmful to humans and the environment. This has led to the search for environmentally friendly fertilizers or smart fertilizer, mainly those with high nutrient-use efficiency, and nanotechnology is rising as a promising alternative. In agriculture, nanotechnology products are being tested for various applications, such as nanoscale sensors for sensing nutrients, nanoscale pesticides, smart and target delivery of nutrients, agronomic fortifications, water purification and nutrient recovery. However, the benefits of nanofertilizers or nanomaterials are unquestionably opening new approaches towards precision and sustainable agriculture; their limitations should also be carefully considered before market implementation. In particular, the extensive release of nanomaterials into the environment and the food chain may pose a risk to environment and human health.
Like conventional fertilizers, the nanofertilizers are also nutrient fertilizers composed, in whole or part, of nanostructured formulation(s) that can be delivered to the plants, allowing for efficient uptake or slow release of active ingredients. The definition of nanofertilizer is debatable. In the literature related to nanotechnology application in agriculture, nanofertilizer is used for both materials of a physical diameter between 1 and 100 nm in atleast one dimension (e.g., ZnO nanoparticles) and those existing at the bulk scale with more than 100 nm in size but that have been modified with nanoscale materials (e.g., bulk fertilizer coated with nanoparticles). The exceptional properties of nanoparticles, such as high surface area/volume size ratio and enhanced optoelectronic and physicochemical properties, compared to their bulk counterparts, is now emerging as a promising strategy to promote plant growth and productivity. As a result of their unique properties, nanoparticles may influence metabolic activities of the plant to different degrees compared to conventional materials and have the potential to mobilize native nutrients, such as phosphorus, in the rhizosphere.
Conventional bulk fertilizers vs nanofertilizers
Farmers or growers mainly apply convention fertilizers through the soil by either surface broadcasting, subsurface placement, or as fertigation or with irrigation water. However, the fate of large portion of applied fertilizers is lost to the atmosphere or enters to water bodies, finally polluting our ecosystems. For example, the 75% N of urea after application in field lost through volatilization (as NH3 or emission as N2O or NO) or through NO3 leaching or runoff to water bodies. The current N fertilizers, therefore, face the problem of low nitrogen utilization efficiency (<20%), whereby loss of N in the environment causes eutrophication and greenhouse gas increase. It has been reported that key macronutrient elements, including N, P, and K, applied to the soil are lost by 40–70 %, 80–90 % and 50–90%, respectively, causing a considerable loss of resources. The excess phosphorus becomes “fixed” in soil, where it forms chemical bonds with other nutrients and becomes unavailable for uptake by the plants.
Important benefits of nanofertilizers over conventional chemical fertilizers rely on:
(a) Their nutrient delivery system as they regulate the availability of nutrients in crops through slow/control release mechanisms. Such a slow delivery of nutrients is associated with the covering or cementing of nutrients with nanomaterials. By taking advantage of this slow nutrient delivery, growers can increase their crop growth because of consistently long-term delivery of nutrients to plants. For example, nutrients can be released over 40–50 days in a slow release fashion rather than the 4–10 days by the conventional fertilizers.
(b) In addition, nanofertilizers required in small amount which reduce the cost of transportation and field application.
(c) An additional major advantage is over accumulation of salt in soil can be minimized as it required in small amount.
(d) Another advantage for using nanofertilizers is that they can be synthesized according to the nutrient requirements of planned crops. In this regard, biosensors can be attached to a new innovative fertilizer that controls the delivery of the nutrients according to soil nutrient status, growth period of a crop or environmental conditions.
(e) The miniature size, high specific surface area and high reactivity of nanofertilzers increase the bioavailability of nutrients.
(f) Providing balanced nutrition, nanofertilizers facilitate the crop plants to fight various biotic and abiotic stresses.
It is reported in several crops, that use of nanofertlizers and nanomaterials enhanced the growth and yield in several crops relative to plant treated with conventional fertilizers. However, the extensive use of nanofertilizers in agriculture may have some important limitations, which must also be considered and it is crucial to determine the toxicity/biocompatibility of nanofertilizers.
Environmental and health concern of nanofertilizers
The application of nanostructures or nanoparticles as agrochemicals (fertilizers or pesticides) is systematically being explored, before nanofertilizers could be used in agriculture or farming for a general farm practice. The properties of many nanoparticles are considered to be of potential risk to human health, viz., size, shape, solubility, crystal phase, type of material, and exposure and dosage concentrations. However, expert opinions indicate that food products containing nanoparticles available in the market are probably safe to eat, but this is an area that needs to be more actively investigated. To address the safety concern detail studies are required to know the impact of nanoparticles within the human body once exposed through nanofood. Researchers have to assess and develop proper assessment strategies to assess the impact of nanoparticles and nanofertilizers on biotic and abiotic components of ecosystem. Among the various issues, the accumulation of nanomaterials in environment, edible part of plants might be the important issues before use in agriculture.