additions
crop residues
manures
composts
soil organic
matter
Figure 3.1. Additions and losses of organic matter from soils.
losses
CO2
(respiration of soil
organisms)
erosion
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.
percent carbon
3.2
alfalfa
3.0
2.8
2.6
corn
2.4
0
1
2
3
4
5
years
Figure 3.3. Organic carbon changes when growing corn silage or
alfalfa. Redrawn from Angers (1992).
a) corn silage
b) corn grain
Figure 3.4. Soil surface after harvest of corn silage or corn grain. Photos by Win Way.
organic matter (%)
1
2 3 4
1
2 3 4
3
6
9 12
3
6 9 12
depth
1 ft
2 ft
3 ft
a) forest soil (litter layer on
top of mineral soil may be
30% or more organic)
b) agricultural soil
(originally forest)
c) prairie soil
d) agricultural soil
(originally prairie)
Figure 3.5. Examples of soil organic matter content with depth. Modified from Brady and Weil,
(2008).
100
Free particulate
organic matter
90
80
70
Particulate
organic
matter inside
aggregates
portion of
60 organic matter (%)
50
40
30
20
Mineral-associated organic matter
10
0
Increasing soil organic matter
Figure 3.6. Organic matter changes in soil as practices favoring buildup are implemented. Redrawn and
modified from Angers (1992).
1.8
1.6
700
beginning at
0.5% SOM
500
1.2
1.0
400
0.8
300
beginning at
1% SOM
200
100
0
1.4
% SOM
lbs per acre
600
0.6
0.4
0.2
0
0 20 40 60 80 100
60 80 100
years
years
b) increase in percent organic
a) net organic matter added per
matter
acre
Figure 3.7. Net organic matter additions and changes in % organic matter for
soils.*
0
20
40
*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.