1. Marianne Sarrantonio University of Maine
Organic Matter Budget
Marianne Sarrantonio
University of Maine

2. Now, here, you see, it takes all the running you can do to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that.
-Lewis Carroll
In Through the Looking Glass

3. Benefits of Soil Organic Matter
Increases soil aggregation
Increases soil water holding capacity (WHC)
Improves water infiltration rates
Increases cation exchange capacity (CEC)
Improves nutrient cycling
Helps solubilize some mineral nutrients
Serves as habitat / food source for soil life
Dark color helps warm soil
Smells good

4. “LONG-TERM” VEGETABLE / COVER CROP TRIAL
University of Maine

5. LONG-TERM VEGETABLE / COVER CROP TRIAL

6. LONG-TERM VEGETABLE COVER CROP TRIAL %Water Stable Aggregates 2005

7. Pools of Soil Organic Matter
Fresh organic material, Partially decomposed mat’l, Humus
The living….the dead… the very dead (Magdoff, 2000)
Active fraction Passive fraction (Brady and Weil, 2001)
Labile fraction Recalcitrant fraction
Litter, Mic biomass, Light fraction, Stable fraction, Old SOM
(Paul and Juma, 1981)

8. LITTER:
Recently killed organic residues not yet incorporated
0-20% of SOM
ACTIVE FRACTION:
Microbial Biomass
Organic residues in various stages of decomposition
Non-humic substances
2-30% of SOM (Stevenson, 1994, Janzen et al. 1992)
STABLE FRACTION:
Humic substances
70-96% of SOM

9. Plant Litter Composition
Simple sugars
Starches
Proteins and amino acids
Hemicellulose
Cellulose
Fats, waxes, oils
Lignin and polyphenolics
Ease of Decomposition

10. Chemical Structure of Plant Compounds
GLUCOSE
AMINO ACID
H-C-O
H-C-OH
HO-C-H
C-H2OH H
R - C – NH2
COOH

11. Chemical Structure of Plant Compounds
LIGNIN

12. Factors Affecting Decomposition of Organic Matter
Chemical composition of litter
Tillage intensity and frequency
Size of residue pieces
Soil-litter contact/depth of placement
Soil temperature
Soil moisture/ soil aeration
Soil pH
Soil texture
Microbial biomass size and activity

13. Soil Organic Matter at the end of a year equals the SOM at the end of last year, plus the current year’s additions of organic residues, minus the losses of each fraction through decomposition during the current year.

14. SOM1 = (SOM0 – SOM0*k1) + (NA – NA*k2)
Where SOM1 is the soil organic matter at the end of current year
SOM0 is the coil organic matter at the end of last year
NA = new additions of organic residue
k1 = the decomposition rate of SOM0
k2 = the decomposition rate of NA

15. FACTORS AFFECTING K1
Baseline for silt loam in temperate climate = approx. 2%/yr
Conventional tillage can increase by 50%
Additional tillage/ yr can increase by 30%
Drought decreases
Hot weather increases
Light soils increases/ heavy decreases
Wet soils (> 60% WFPS) decreases
Previous losses of active fraction decreases
Large additions to active fraction increases

16. FACTORS AFFECTING K2 C:N ratio
Carbon profile (i.e. % lignin, cellulose, etc.)
Age / maturity

17. Rotational Accumulation Exercise Worksheet
Start of Rotation:
Organic Matter (SOM0) __________ Balance
Year 1
Subtract SOM0 x k1 -__________ _________
Add NA- (NA x k2) +__________ _________
Add NA- (NA x k2)(2nd amendment ) +__________ _________
Organic Matter at end of Year 1 (SOM1) __________(carry)

18. EXAMPLE 1 WORKSHEET CALCULATIONS
SOIL: sandy loam with 2.5% organic matter at start of rotation
CLIMATE: Northeast (cool, humid)
TILLAGE: Conventional, once in the late spring
K1 : 3%
CROP: Grain corn
ASSUME:
1 acre of soil to 6” depth weighs approx. 2,000,000 lb kg m3)
Therefore:
Organic matter at start of rotation (SOM0) = 2,000,000lb/ac x 0.025
= 50,000 lb/ac
SOM0 x k1 = 50,000 lb/ac x .03 = 1500 lb/ac

19. Rotational Accumulation Exercise Worksheet
Example 1
Start of Rotation:
Organic Matter (SOM0) _50,000____ Balance
Year 1
Subtract SOM0 x k1 -__1,500____ __48,500_
Add NA- (NA x k2) +__________ _________
Add NA- (NA x k2)(2nd amendment ) +__________ _________
Organic Matter at end of Year 1 (SOM1) __________(carry)

20. ASSUME: 5000 lb/ac corn residue remains in the fall
EXAMPLE 1 WORKSHEET CALCULATIONS
ASSUME: lb/ac corn residue remains in the fall
K2 corn grain residue: 70% (0.70)
THEREFORE:
NA – (NA x k2) = 5,000lb/ac - (5000 lb/ac x 0.70)
= 5,000 lb/ac – 3,500 lb/ac
= 1,500 lb/ac

21. Rotational Accumulation Exercise Worksheet
Example 1
Start of Rotation:
Organic Matter (SOM0) lb/ac __50,000___ Balance
Year 1
Subtract SOM0 x k1 -__1,500___ _48,500__
Add NA- (NA x k2) +__1,500___ _50,000__
Add NA- (NA x k2)(2nd amendment ) +____0____ _50,000__
Organic Matter at end of Year 1 (SOM1) __50,000___(carry)

22. EXAMPLE 2 WORKSHEET CALCULATIONS
SOIL: sandy loam with 2.5% organic matter at start of rotation
CLIMATE: Northeast (cool, humid)
TILLAGE: Conventional, once in the late spring
K1 : 3%
CROP: Silage corn (k2 = 0.75)
Organic matter at start of rotation (SOM0) = 2,000,000lb/ac x 0.025
= 50,000 lb/ac
SOM0 x k1 = 50,000 lb/ac x .03 = 1500 lb/ac

23. EXAMPLE 2 WORKSHEET CALCULATIONS
ASSUME: lb/ac corn residue remains in the fall
K2 silage residue: 75% (0.75)
THEREFORE:
NA – (NA x k2) = 1,000lb/ac - (1,000 lb/ac x 0.75)
= 1,000 lb/ac – 750 lb/ac
= lb/ac

24. Rotational Accumulation Exercise Worksheet
Example 2
Start of Rotation:
Organic Matter (SOM0) ___50,000__ Balance
Year 1
Subtract SOM0 x k1 -__1,500___ _48,500__
Add NA- (NA x k2) silage +__250____ _48, 750__
Add NA- (NA x k2)(2nd amendment ) +____0____ _ 48,750__
Organic Matter at end of Year 1 (SOM1) __48,750__(carry)
LOSS: 1,250 lb/ac or 2.5% of SOM0

25. EXAMPLE 2A WORKSHEET CALCULATIONS
SOIL: sandy loam with 2.5% organic matter at start of rotation
CLIMATE: Northeast (cool, humid)
TILLAGE: Conventional, once in the late spring
K1 : 3%
CROP: Silage 1000 lb/ac residue; k2= 75% (0.75)
COVER CROP (planted previous fall): Hairy 3000 lb/ac
k2 = 80% (0.80)

26. Rotational Accumulation Exercise Worksheet
Example 2A
Start of Rotation:
Organic Matter (SOM0) _ 50,000___ Balance
Year 1
Subtract SOM0 x k1 -__1,500___ _48,500__
Add NA- (NA x k2) +__250____ _48, 750__
Add NA- (NA x k2)(2nd amendment ) +__600____ _ 49,350__
Organic Matter at end of Year 1 (SOM1) _49,350__(carry)
LOSS: 650 lb/ac or 1.3% of SOM0

27. EXAMPLE 2B WORKSHEET CALCULATIONS
For FOUR YEAR ROTATION
YEAR 1
TILLAGE: Conventional, once in the late spring, once in fall
K1 : 3.9 % (increase by 30% for additional tillage)
CROP: Silage 1000 lb/ac residue; k2 = 0.75
COVER CROP (planted previous fall): Hairy 3000 lb/ac; k2 = 80% (0.80)
COVER CROP (Fall seeding) Cereal rye

28. Rotational Accumulation Exercise Worksheet
Example 2B
Organic Matter (SOM0) ___50,000__ Balance
Year 1
Subtract SOM0 x k1 -__1,950___ _48,050__
Add NA- (NA x k2) silage +__250____ _48, 300__
Add NA- (NA x k2)(2nd amendment ) +__600____ _ 49,900__
Hairy vetch
Organic Matter at end of Year 1 (SOM1) _48,900__(carry)
LOSS: lb/ac or 2.2% of SOM0

29. EXAMPLE 2B WORKSHEET CALCULATIONS
For FOUR YEAR ROTATION
YEAR 2
TILLAGE: Conventional, once in the late spring
K1 : 3.0 %
CROP: 2000 lb/ac residue; k2 = 80% (0.80)
COVER CROP (planted previous fall): Cereal 6000 lb/ac
k2 = 75% (0.75)
COVER CROP (Fall seeding) none

30. Rotational Accumulation Exercise Worksheet
Example 2B
Organic Matter (SOM1) __48,900___ Balance
Year 2
Subtract SOM1 x k1 -__1,907 ___ __46,993_
Add NA- (NA x k2) soy +__ 400____ __47,393_
Add NA- (NA x k2)(2nd amendment ) +__1,500___ __48,893_
Cereal rye
Organic Matter at end of Year 2 (SOM2) __48,896__(carry)
CUMULATIVE LOSS: 1,104 lb/ac or 2.2% of SOM0

31. EXAMPLE 2B WORKSHEET CALCULATIONS
For FOUR YEAR ROTATION
YEAR 3
TILLAGE: Conventional, once in the late spring, once in fall
K1 : 3.9 % (increase by 30% for additional tillage)
CROP: Grain 5000 lb/ac residue; k2 = 70% (0.70)
COVER CROP (planted previous fall): none
COVER CROP (Fall seeding) Cereal rye

32. Rotational Accumulation Exercise Worksheet
Example 2B
Organic Matter (SOM2) ___48,893__ Balance
Year 3
Subtract SOM2 x k1 -_1,906____ __46,987_
Add NA- (NA x k2) grain corn +_1,500___ __48,487_
Add NA- (NA x k2)(2nd amendment ) +____0____ __48,487_
Organic Matter at end of Year 3 (SOM3) _48,487___(carry)
CUMULATIVE LOSS: 1,513 lb/ac or 3% of SOM0

33. EXAMPLE 2B WORKSHEET CALCULATIONS
For FOUR YEAR ROTATION
YEAR 4
TILLAGE: Conventional, once in the late spring
K1 : 3.0 %
CROP: 2000 lb/ac residue, k2 = 0.80
COVER CROP (planted previous fall): Cereal rye, k2 = 0.75
COVER CROP (Fall overseeding) Hairy vetch

34. Rotational Accumulation Exercise Worksheet
Example 2B
Organic Matter (SOM3) __48,847___ Balance
Year 4
Subtract SOM3 x k1 -__1,455____ _47,032__
Add NA- (NA x k2) soy +___400____ _47,432__
Add NA- (NA x k2)(2nd amendment ) +__1,500____ _48,932__
Cereal rye
Organic Matter at end of Year 4 (SOM4) __48, 932__(carry)
TOTAL LOSS AFTER 4 YEARS: 1,068 lb/ac or 2.14% of SOM0
TOTAL LOSS AFTER 8 YEARS: 2,115 lb/ac or 4.23% of SOM0

35. WAYS TO INCREASE OM
REDUCE K2
REDUCE K1
INCREASE NA

36. Best Ways to Reduce K1 Reduce intensity of tillage
Reduce frequency of tillage
Reduce depth of tillage
Reduce area of tillage (e.g. zone till)
Reduce mechanical cultivation (uh oh!)

37. BEST WAYS TO LOWER K2 Species / amendment choice
Allow covers to grow longer
All other factors are already rolled into k1

38. BEST WAYS TO INCREASE NA
Plant cover crops in timely way
Allow cover crops to grow longer
Fill short niches with covers
Full-year fallow with cover crops
Species choices
Supplement with manure and compost
Use more small grains

39. TO DO LIST
Include manure (k2 = 0.20 – 0.50) and compost (k2 = 0.10 – 0.30)
Develop as Excel worksheet
Account for OM protection by clays and aggregates
Account for “small-grain effect”
Include more vegetable residues
Account for erosion

40. ….and in all things, it is most important to keep your sense of humus.
- Anon