Emerging Roles of Humus and Humic Constituents as Novel Inputs in Natural Farming: A Review

Abstract

Soil humus and its operationally defined fractions, humic acid, fulvic acid and humin, have re-entered the centre of debate on sustainable crop production as natural farming systems expand across smallholder and commercial agriculture. This review synthesises evidence on the formation, chemistry and agronomic functions of humic substances, with particular attention to their integration into natural farming practices that avoid synthetic fertilisers and pesticides. The chemical architecture of humic substances, dominated by carboxylic and phenolic functional groups arranged around aliphatic and aromatic cores, underlies their capacity to buffer soil pH, increase cation exchange capacity, chelate micronutrients and stimulate root architecture through auxin- and cytokinin-like signalling. Field and greenhouse trials across cereals, pulses, oilseeds and horticultural crops show that humic acid and vermicompost-derived humic fractions frequently increase nutrient use efficiency, microbial diversity and yield, although the magnitude and even direction of these effects depend strongly on humic source, application rate, soil type and crop. Within natural farming systems such as Zero Budget Natural Farming, farm-generated inputs including Jeevamrita and vermicompost function partly through their humic content, contributing to soil organic carbon accumulation, improved water-holding capacity and enhanced biological activity, while large-scale evaluations report comparable yields and higher farm profitability relative to conventional systems. Evidence on heavy metal immobilisation and drought and salinity tolerance further extends the relevance of humic substances beyond fertility management into environmental remediation and climate resilience. Nonetheless, mechanistic inconsistencies, a scarcity of long-term multi-site field trials and weak standardisation of humic product characterisation constrain firm agronomic recommendations. This review concludes that humic substances represent a scientifically credible, though imperfectly characterised, component of natural farming, and it outlines priority research directions for reconciling laboratory-derived mechanistic insight with field-scale reliability.