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.

Add organic matter Increased biological activity (& diversity) Decomposition Nutrients released Aggregation increased Pore structure improved Humus and other growth-promoting substances Reduced soilborne diseases, parasitic nematodes Improved tilth and water storage HEALTHY PLANTS 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] Harmful substances detoxified

Figure 2.4. The cycle of plant nutrients.

- - a) cations held on humus b) cations held on clay particle c) cations held by organic chelate Ca ++ Mg ++ K+K Ca ++ Mg ++ K+K+ Zn Figure 2.5. Cations held on negatively charged organic matter and clay.

Figure 2.6. Changes in soil surface and water-flow pattern when crusts develop. a) aggregated soilb) soil crusts after aggregates break down runoff infiltration

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.

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. crop harvest carbon dioxide (CO 2 ) (0.04% in the atmosphere) root respiration and soil organic matter decomposition erosion carbon in soil organic matter crop and animal residues photosynthesis respiration in stems and leaves

crop harvest crop and animal residues nitrogen in soil organic matter ammonium NH 4 + leaching to groundwater nitrate NO 3 - erosion NH OH - NH 3 + H 2 O immobilization legumes free-living bacteria nitrogen fixation NH NO 3 - nitrogen gas (N 2 ) (78% of atmosphere) volatilization plant uptake denitrification NO 3 - N 2 + N 2 O atmospheric fixation or fertilizer production 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.