Figure 2.1. A nematode feeds on a fungus, part of a living system of checks and balances. Photo by Harold Jensen.
Figure 2.2. Partially decomposed fresh residues removed from soil. Fragments of stems, roots, and fungal hyphae are all readily used by soil organisms.
HEALTHY PLANTS Increased biological activity (& diversity) Add organic matter Reduced soilborne diseases, parasitic nematodes Decomposition Aggregation increased Pore structure improved Nutrients released Humus and other growth-promoting substances Harmful substances detoxified Improved tilth and water storage Figure 2.3. Adding organic matter results in many changes. Modified from Oshins and Drinkwater (1999). [Designer: Note odd font of item lower right; should be like the others]
- - Zn++ Ca++ Ca++ Ca++ Mg++ - - - - - - Ca++ - - - - - - K+ - Mg++ - Ca++ - K+ - a) cations held on humus b) cations held on clay particle c) cations held by organic chelate Figure 2.5. Cations held on negatively charged organic matter and clay.
infiltration runoff a) aggregated soil b) soil crusts after aggregates break down Figure 2.6. Changes in soil surface and water-flow pattern when crusts develop.
Figure 2.7. Corn grown in nutrient solution with (right) and without (left) humic acids. Photo by R. Bartlett. In this experiment by Rich Bartlett adding humic acids to a nutrient solution increased the growth of tomatoes and corn as well as the amount and branching of roots.
carbon dioxide (CO2) (0.04% in the atmosphere) photosynthesis crop harvest crop and animal residues carbon in soil organic matter erosion root respiration and soil organic matter decomposition respiration in stems and leaves Figure 2.8. The role of soil organic matter in the carbon cycle. Losses of carbon from the field are indicated by a dark border around the words describing the process.
atmospheric fixation or fertilizer production nitrogen fixation volatilization crop and animal residues NH4++ OH- NH3 + H2O plant uptake erosion immobilization denitrification leaching to groundwater nitrogen gas (N2) (78% of atmosphere) NH4++ NO3- crop harvest legumes free-living bacteria nitrogen in soil organic matter ammonium NH4+ NO3- N2 + N2O nitrate NO3- Figure 2.9. The role of organic matter in the nitrogen cycle. Losses of nitrogen from the field are indicated by the dark border around the words describing the process.