Growth promoting substances residue Growth promoting substances Active (particulate, light fraction) CEC passive (humus, organo-mineral complexes) During the decomposition process growth promoting substances are produced (A), soil structure and water holding capacity are increased (B) nutrients are mineralized (during C, D, and E), CEC is produced (C, D, and E). Direct effects on nutrient availability 1) Relatively high amounts of mineralization of available nutrients produced by a combination of rapid decomposition plus previously accumulated POM or a high amount of added residues. — Rapid decomposition is stimulated by intensive tillage, good soil drainage, coarse texture, and alternating wet and dry conditions. 2) CEC produced during decomposition process. Indirect Effects on nutrient availability 1) Growth promoting substances Better root growth and healthier roots leads to more soil volume explored and more surface area for nutrient interception and for mass flow of nutrients to roots 2) Better soil structure and improved water holding capacity Better soil structure may enhance root development and exploration (see above). Good soil structure and plentiful humus content contributes to higher amounts of plant-available water following rains or irrigation. This results in better plant growth and health and more nutrient movement to roots. Figure 18.1. Influence of soil organic matter and its management on nutrient availability. D A E B Better soil structure and improved water holding capacity C