1. additions crop residues manures composts losses CO 2 (respiration of soil organisms) erosion soil organic matter Figure 3.1. Additions and losses of organic matter from soils.

2. Figure 3.2. Root systems of annual wheat (on at left in each panel) and wheatgrass, a perennial at four times of the year. Approximately 25 to –40% of the wheatgrass root system dies back each year, adding considerable amounts of organic matter, and then grows back again. Compared to annual wheat, it has a longer growing season and has much more growth both above ground and below ground. Wheatgrass was 12 and 21 months old when the first and last photos were taken. Photo by the Land Institute.

3. Figure 3.3. Organic carbon changes when growing corn silage or alfalfa. Redrawn from Angers (1992). alfalfa corn years percent carbon 3.2 3.0 2.8 2.6 2.4 0 1 2 3 4 5

4. Figure 3.4. Soil surface after harvest of corn silage or corn grain. Photos by Win Way. a) corn silageb) corn grain

5. Figure 3.5. Examples of soil organic matter content with depth. Modified from Brady and Weil, (2008). a) forest soil (litter layer on top of mineral soil may be 30% or more organic) c) prairie soil b) agricultural soil (originally forest) d) agricultural soil (originally prairie) depth 1 ft 2 ft 3 ft organic matter (%) 1 2 34 1 2 34 3 6 912 3 6 9

6. Increasing soil organic matter Mineral-associated organic matter Free particulate organic matter Particulate organic matter inside aggregates 100 90 80 70 60 50 40 30 20 10 0 portion of organic matter (%) Figure 3.6. Organic matter changes in soil as practices favoring buildup are implemented. Redrawn and modified from Angers (1992).

7. 020406080100 0 200 300 400 500 600 700 years beginning at 0.5% SOM beginning at 1% SOM lbs per acre a) net organic matter added per acre 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 020406080100 % SOM years b) increase in percent organic matter Figure 3.7. Net organic matter additions and changes in % organic matter for soils.* *Estimated for soil starting at 0.5% or 1% organic matter, receiving a total of 5,000 lbs of residue per acre per year; 20% remains after one year, and soil organic matter decomposes at the rate of 3% a year.