Slow‐release fertilizers using lignin: Challenges and future prospects

Abstract

Lignin is an integral part of the plant cell wall and is the largest single source of aromatic carbon in the world, which makes it suitable for the production of feed, chemicals, fuels, and biomaterials. Total lignin production is greater than 50 million tons per annum; however, currently, less than 5% of the lignin is being valorized, and 95% is burnt to create steam and power. Burning aromatic carbons produces heat and power, making them low‐value materials. However, the macromolecules of lignin can be modified to use for several industrial applications, such as surfactants, paints, hydrogel, absorbents, and fertilizer. In this review, we provide a systematic and comprehensive summary of the last 10 years of the development of controlled release fertilizer (CRF) from lignin, which is a renewable material. Lignin‐based CRFs are green, environment friendly, and are considered efficient for agricultural use in place of urea and diammonium phosphate. Coating the fertilizer with lignin could increase fertilizer's 20–30 times slower release vis‐à‐vis uncoated fertilizer. However, the structure of lignin varies from one process to another process; it is therefore highly dependent upon the fractionation process and pulping methodologies adopted. A high degree of heterogeneity, complex structure, and low reactivity of the lignin results in an uneven coating, large surface porosity, and cracks in the coatings. The similarity of the lignin can be improved by chemical modifications in its molecule; however, any chemical processing increases the cost of slow‐release fertilizer.