Electrospun Fibers as Fertilizer Application Agent

Seedlings germination

Availability of low cost fertilizer has enabled farmers to significantly increase their crop yield. However, this has also led to excessive applications resulting in wastage and pollution when the fertilizers get washed off to the rivers and streams. In times of rising oil prices, cost of petroleum derived fertilizers would also increase correspondingly.

Electrospinning has been used to produce fibers in the submicron scale for the delivery of active ingredients. Given the large surface area of the fibers, it allows quick release of the encapsulated agents or tailored for more controlled release. In agriculture, a potential application is for the delivery of fertilizers. For this application, another advantage of electrospun fibers is that it forms a highly porous network which is not easily washed away by rain or blown away by wind. This reduces unintended leaching of the fertilizers and gives a more targeted application.

Controlling the release of fertilizer to the plant is partly affected by the method which the fertilizer is loaded into the electrospun fibers and the degradation rate of the fibers. Materials selected for this application should be degradable in soil environment so as not to build-up residues. Castro-Enriquez et al (2012) used elecrospun wheat gluten fiber membrane for immersion into urea solution for adsorption of the urea. Since the urea was coated on the surface of the fiber membrane, about 98% of urea was released within the first 5 h. To enable a sustained release of fertilizer, Kampeerapappun and Phanomkate (2013), used core-shell electrospinning with the fertilizer loaded in the core component. Biodegradable polyhydroxybutyrate was used as the shell while biodegradable polylactic acid was used as the core material containing the fertilizer, NPK 21-21-21. After optimizing the material composition (eg. flow rate, loading amount), fertilizer release up to a month has been recorded. Complete degradation of the fiber was found to be 3 months. Javazmi et al (2020) constructed compared single and double layered hollow nanofibrous yarns using electrospinning for the release of urea. The hollow nanofibrous yarns were constructed by electrospinning poly L-lactic acid (PLLA) nanofibre onto a polyvinyl alcohol (PVA) multifilament yarn. For double-layered hollow nanofibre yarns, polyhydroxybutyrate (PHB) nanofibres were electrospun on top of the poly L-lactic acid (PLLA) nanofibre layer. PVA multifilament yarn was subsequently removed by dipping the layered yarns in water to obtain the hollow nanofibre yarn. Urea in both single and double-layered hollow yarns were loaded into PLLA nanofibres. The urea loaded yarns were soaked in water and their nitrogen release rate was determined. In the first 12 h, the release rate of nitrogen for single and double-layered hollow yarns were 79% and 24% respectively. Single layer hollow yarns showed a burst release in the first 48 h but the release rate of double-layered yarn was more gradual. The much slower release rate of double-layered hollow yarns was due to the PHB nanofibres layer that acts as a physical barrier around the PLLA layer which delays the release of urea. Cesare et al (2019) proposed the use of electrospun nanofibrous polycaprolactone/polyhydroxybutyrate thin membranes loaded with catechol (CL-NMs) for the release of catechol as an iron-chelating natural agent to mobilize insoluble Fe for transfer to duckweed. Under controlled hydroponic conditions, Fe-deficient duckweeds supplied with CL-NMs showed recovery in physiological and growth performances. There are also no signs of short term toxicity to duckweeds. Since both polycaprolactone and polyhydroxybutyrate are biodegradable, their use in the loading and provision of catechol as plant fertilizer will not result in plastic waste on the land. In agriculture, it is preferable that the release of fertilizer is slow and gradual. This is to match the gradual nutrient uptake of plants so that there will not be excessive nutrients being washed into the river systems and causing pollution. Javazmi et al (2021) showed that a sandwich assembly of electrospun membranes is very effective in reducing the release rate of urea fertiliser. In their assembly, an electrospun single-layer poly L-lactic acid (PLLA) nanofibres membrane loaded with urea was sandwiched between two electrospun layers of polyhydroxybutyrate (PHB) nanofibres membranes. With single layer PLLA/Urea, more than 50% of the urea was released within 3 hours. For sandwiched PLLA membranes loaded with 10% urea, less than 50% was released after 39 hours. However, with higher urea loading (20% and 40%), the sandwiched PLLA membrane release rate has a much faster release rate comparable to that of single layer PLLA membrane. Higher urea loading resulted in a much faster release rate. In a sandwiched assembly, the PHB layers provide a barrier that slows the diffusion of urea. Hydrophobicity of electrospun PHB and PLLA at lower urea loading may reduce the rate of water penetration which in turn reduces the rate of urea release. Since urea is water soluble, higher concentration of urea in PLLA may reduce the hydrophobicity of the membrane. In a sandwich assembly, once water has penetrated the PHB layer, the PLLA/urea membrane may have facilitated wetting and hence increase its urea release.

Fertilizers loaded electrospun fibers has been shown to facilitate seed germination. For this, the seeds are wrapped in the electrospun fibrous membrane to protect them from insects while providing ready nutrients when the rain comes. This has been demonstrated by Krishnamoorthy and Rajiv (2016) who incorporated hexaaminocyclotriphosphazene (HACTP) and cobalt nanoparticles into electrospun polyvinylpyrrolidone (PVP) fibers. The seeds were encapsulated in the electrospun membrane as a package for easy application. This ensures that the fertilizer is within the vicinity of the seed during germination thereby reducing the quantity of fertilizer required and leaching into the environment. Using cowpea seeds as the test subject, the seeds showed increased imbibition rate with the fertilizer loaded electrospun PVP fibers. Similarly, cowpeas coated with electrospun polyvinyl pyrrolidone (PVP) fibers containing urea and cobalt nanoparticles have the best germination compared to cowpeas without coating, PVP fibers coating and PVP/urea fibers coating [Krishnamoorthy and Rajiv 2017].