**Chapter 8 139**

Chemical Characteristics of Humic Substances in Nature *by Claudio Fernando Mahler, Nicoly Dal Santo Svierzoski and Cassiano Augusto Rolim Bernardino*

### **Chapter 9 153**

Sewage Sludge to Fertilise Durum Wheat: Effects on Crop and Soil *by Pilar Mañas and Jorge de las Heras*

Preface

Present in soil, water, and geological organic deposits, humic substances are the most widespread organic compounds, naturally occurring from the physical, chemical, and microbiological transformation of biomolecules. Humic substances represent about 25% of the total organic carbon on the Earth and comprise up to 75% of the dissolved organic carbon in water, making them important for multiple

Based on their solubility, humic substances can be fractionated into humic acid, fulvic acid, humin, and some microelements. Humic acid includes long-chain aggregates soluble in alkali, while fulvic acid is a mixture of short-chain compounds soluble in both acidic and alkaline solutions. Humin, in contrast, constitutes the non-soluble fraction of the humate. This classification is based only on superficial criteria and does not indicate chemical behavior or provide insights into chemical

Humification, or the set of processes leading to humic substances, is a set of reactions occurring either in sequences or simultaneously and affecting at varying speeds an extreme diversity of substrates. These reactions are essentially biochemical in nature, but purely chemical processes can also be involved. Physical factors, such as temperature or the humidity of the surrounding environment, also determine the formation of humic substances at various stages. Humification, therefore, appears to be a complex process in which biological activity plays a major role in the short term, but the slow evolution of humic compounds due to external conditions may gradually

Despite many decades of extensive studies, the formation mechanisms of humic substances, as well as their structure, are still a subject of discussion and controversy. The content of this book revolves around the structure and dynamics of humic substances, their physicochemical and biological properties, and the technics of their extraction and characterization. The book also sheds light on recent advances and applications of humic substances in agriculture, environment, industry, and

**Abdelhadi Makan**

Tétouan, Morocco

Abdelmalek Essaadi University,

environmental processes in both soil and aquatic systems.

structures.

medicine.

modify the properties of the humus.
