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Laboratory Manual

Experiment 6. Decomposition of Plant and Animal Residues in the Soil

A close relationship exists between the nitrogen and organic matter contents of soils. Generally, the higher the organic matter, the higher the nitrogen content of a soil. A rather constant relationship exists in the proportions of carbon and nitrogen in the soil. The ratio of carbon to nitrogen (ranging by weight from 8/1 to 15/1 with the median being 10/1 or 12/1.) For a given soil in a given climate the ratio is fairly stable. Adding plant material or animal manures to the soil tends to temporarily widen the C/N ratio of soils. The C/N ratio of legumes and farm manures ranges from about 20/1 to 30/1 whereas materials such as woody and strawy residues may exceed 100/1. Microbes in the soil have C/N ratios of about 4/1 to 9/1.

On the whole, material with a C/N ratio wider than 35/1 will cause microbial immobilization of available nitrogen (nitrate and ammonium) in the soil. Residues with C/N ratios between 25/1 to 35/1 tend to have little short-term effect on the level of nitrate and ammonia in the soil although in the initial stages of decomposition they may cause a tie-up of the mineral nitrogen, which will be released later. If the C/N ratio is less than 25/1 some of the organic nitrogen will be released as ammonia during the rotting process. Any time the C/N ratio exceeds 35/1, additional fertilizer N should be applied to compensate for the N immobilization by soil organisms. About one lb. of N per 100 lb. of residue will satisfy the requirements with most materials which cause immobilization of N.

During decomposition of soil organic matter, a change in the microbial population occurs. Cellulose-decomposing and lignin-decomposing fungi increase as well as actinomycetes and bacteria. Bacteria cannot decompose lignin but can decompose cellulose. Fungi can breakdown cellulose and lignin. Actinomycetes appear to act on cellulose. Certain soil fauna may digest cellulose and hemicellulose. Lignin, cellulose, and hemicellulose are constituents of plant cell walls.

As decay occurs the C/N ratio of the residue decreases since C is lost as CO2 and the nitrogen is conserved. As humification of the organic matter nears completion, the decay organisms subside due to the lack of easily oxidizable carbon. At this time the C/N ratio is near to that of the soil.

Exercise

1. You are provided with an infertile soil.

Determine its textural class.______________________________________________

2. Obtain four flower pots. Fill each pot loosely, without compacting, to within 1 inch of the top of the pot. Dump the soil in separate piles on the bench and mix 100 g of the organic material into each pile, then add a nitrogen fertilizer to provide 0, 0.5, 1, or 1.5 g actual N.

NOTE THE NITROGEN FERTILIZER THAT WAS USED _____________________________

LABEL EACH POT FOR ORGANIC RESIDUE AND N APPLIED

3. Add 10 g of limestone, 2g of bonemeal, and 10 g of wood ashes to each pot of soil-organic matter mixture.

4. Transplant one tomato plant into each pot. Place pot on bench in greenhouse and water with 200 ml of tap water.

The estimated C/N ratios of the residues are given below.

------------------------Nitrogen Added, g/100g of organic material-------------------------------------

Organic material

-----------0-----------

----------0.5----------

-----------1-----------

-----------1.5----------

None

n/a

n/a

n/a

n/a

Paper (0.01%N)

5000

98

50

33

Sawdust (0.1%N)

500

83

45

31

Dead leaves (0.3%N)

170

62

38

28

Peat moss (0.5%N)

100

50

33

25

Compost (1% N)

50

33

25

20

Cow manure (1% N)

50

33

25

20

Alfalfa (2.5% N)

20

17

14

12

*In calculations of C/N ratio the fertilizer N is considered part of the rough organic matter. It is estimated that the plant material is 50% carbon.

5. As the plants grow in the greenhouse, note their appearances. At harvest record the appearance in the following table. Record plant heights and weights in the tables on the next page.

_______________________________________________________________________________

Plant Appearance at Harvest. Results . Night

------------------------Nitrogen Added, g/100g of organic material-------------------------------------

Organic material

-----------0-----------

----------0.5----------

-----------1-----------

------------1.5---------

None

__________________

__________________

__________________

__________________

Paper (0.01%N)

__________________

__________________

__________________

__________________

Sawdust (0.1%N)

__________________

__________________

__________________

__________________

Dead leaves (0.3%N)

__________________

__________________

__________________

__________________

Peat moss (0.5%N)

__________________

__________________

__________________

__________________

Compost (1% N)

__________________

__________________

__________________

__________________

Cow manure (1% N)

__________________

__________________

__________________

__________________

Alfalfa (2.5% N)

__________________

__________________

__________________

__________________

_______________________________________________________________________________

_______________________________________________________________________________

Plant Heights at Harvest, cm. Results . Night

------------------------Nitrogen Added, g/100g of organic material-------------------------------------

Organic material

-----------0-----------

----------0.5----------

-----------1-----------

-----------1.5----------

None

__________________

__________________

__________________

__________________

Paper (0.01%N)

__________________

__________________

__________________

__________________

Sawdust (0.1%)

__________________

__________________

__________________

__________________

Dead leaves (0.3%N)

__________________

__________________

__________________

__________________

Peat moss (0.5%N)

__________________

__________________

__________________

__________________

Compost (1% N)

__________________

__________________

__________________

__________________

Cow manure (1% N)

__________________

__________________

__________________

__________________

Alfalfa (2.5% N)

__________________

__________________

__________________

__________________

_______________________________________________________________________________

_______________________________________________________________________________

Plant Weights at Harvest, g/plant. Results . Night

------------------------Nitrogen Added, g/100g of organic material-------------------------------------

Organic material

-----------0-----------

----------0.5----------

-----------1-----------

------------1.5---------

None

__________________

__________________

__________________

__________________

Paper (0.01%N)

__________________

__________________

__________________

__________________

Sawdust (0.1%)

__________________

__________________

__________________

__________________

Dead leaves (0.3%N)

__________________

__________________

__________________

__________________

Peat moss (0.5%N)

__________________

__________________

__________________

__________________

Compost (1% N)

__________________

__________________

__________________

__________________

Cow manure (1% N)

__________________

__________________

__________________

__________________

Alfalfa (2.5% N)

__________________

__________________

__________________

__________________

_______________________________________________________________________________

ANSWER THESE QUESTIONS:

1. In which treatments did N deficiency appear? (Check table of plant appearances). _______________________________________________________________________________

_______________________________________________________________________________

2. What were the C/N ratios of the treatments in which N deficiencies appeared? (Check table of C/N ratios).

_______________________________________________________________________________

3. Describe any symptoms of excessive nitrogen applications.

_______________________________________________________________________________

4. What were the causes of injuries that occurred with excesses of N fertilizer or with organic matter additions?

_______________________________________________________________________________

5. Account for the cause of death or near death of any plant in the experiment. (Describe the conditions under which the plant died and specify the probable cause of death).

_______________________________________________________________________________

6. How would you avoid the plant mortality which you noted or improve the growth of plants with the treatments that severely restricted growth?

_______________________________________________________________________________

7. If you had mixed the various organic materials in a more fertile soil, would you have expected the same results as you found with this poor soil? _______

Explain why or why not:

_______________________________________________________________________________

8. Attach graphs of plant weight vs. fertilizer applied and plant weight vs. C/N ratio in organic matter.

9. Write a short paragraph summarizing your conclusions from this experiment.

_______________________________________________________________________________

References

1. Brady, N.C. 1974. The nature and properties of soils, 8th ed. Macmillan Company, New York. Chapter 6. (Any edition is an acceptable reference).

2. Russell, E.W. 1973. Soil conditions and plant growth. 10th edition. Longmans or Macmillan. Chapters 14 and 15. (Any edition is an acceptable reference).

3. Waksman, S.A. 1936. Humus. Williams and Wilkins Co. Baltimore. Chapter 7.

Produced and maintained by Allen Barker
University of Massachusetts, Amherst.

Organic material	-----------0-----------	----------0.5----------	-----------1-----------	-----------1.5----------
None	n/a	n/a	n/a	n/a
Paper (0.01%N)	5000	98	50	33
Sawdust (0.1%N)	500	83	45	31
Dead leaves (0.3%N)	170	62	38	28
Peat moss (0.5%N)	100	50	33	25
Compost (1% N)	50	33	25	20
Cow manure (1% N)	50	33	25	20
Alfalfa (2.5% N)	20	17	14	12