Humic substances (HSs) are typically found in nature in soil, plant, and natural water, and they are fundamentally created from organic matter decomposition. The main active components of HSs contain humic acid, humus, ulmic acid, fulvic acid, humin, and certain microelements. HSs are currently not only natural, but also harvested and engineered, and have been widely exploited in soil, plant, and animal productivity for the past 20 years. In the soil, HSs have the potential to improve a wide range of soil and play an essential role in the microorganism’s activity in the environment. In addition, HSs are a part of soil humus and they represent the largest pool of recalcitrant organic carbon in the terrestrial environment. Also, these valuable compounds have bio-stimulative effects on plants, where they could boost the photosynthetic activity, and improve root system and plant growth, as well as reduce stress damages. The HSs also could affect specific metabolic pathways in plants leading to improved tolerance of abiotic and biotic stresses. Today, our soils are low on these HSs, and as a result, animals and chickens are not receiving adequate amounts in their regular diet. Many researches have indicated that when soil humus percentages fall below 2%, the soil cannot provide sufficient amounts of HSs materials into the crops grown for the quantities needed by livestock animals. Recently, HSs have been used as one of the alternative feed additives in animal husbandry to improve the economics and ecology of animal production by increasing the growth rate, improving feed efficiency and immunity, reducing the risk of disease, and increasing animal products quality. This book chapter discusses the functions that HSs play in soil, plant, and animals to keep agricultural sustainability. We assemble and describe the applications and role of such HSs in agricultural and environmental ecology.