Soil Organisms

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1. Why must plant materials continuously be added to the soil to maintain an equilibrium level of soil organic matter?

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• Because it’s constantly breaking it down and being released as CO2 and this will slowly deplete OM if we don’t add it back.
Plant materials must continuously be added to replace the continuously depleting OM taken up by Microorganisms. OM in the soil improves soil structure, aeration, water intake, and soil water holding capacity (these are physical characteristics of the soil that OM help).

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2. Explain why the organic matter content of soils under grass vegetation tends to decrease gradually with increasing depth whereas, under forest vegetation, most of the soil organic matter is concentrated in the surface inch or two?

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• The roots under the grass break down every year while the tree roots don’t die and contribute back to the soil every year so all the nutrients get locked up at the surface under forest vegetation.

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3. What are the three major organic components of all plant tissue and what are their relative rates of decay compared to each other in the soil?

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• Three Groups
• Carbohydrates: fast decomposition
• Lignins: complex compound slowest group to be broken down
• Proteins: not very resistant to decay hold nitrogen, Sulfur and Phosphorus which are not found in carbohydrates. So the microorganism go after them

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4. What do the terms heterotrophic, aerobic and autotrophic refer to in describing the more common decay organisms?

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Heterotrophic: organisms that are not self sustaining they must oxidize the carbon from an organic source. Obtaining nourishment by digesting plant or animal matter, as animals do, as opposed to photosynthesizing food, as plants do
Aerobic: Are organisms that can only live in the presence of free oxygen. They require free oxygen (these are the primary decomposers)
Anaerobic: get oxygen from compounds
Autotrophic: Organisms that are self sustaining and get carbon from an inorganic source (things composed of minerals rather than living matter). Many of these are involved in the oxidation of inorganic ions.
Saprophytic: heterotrophic organism that eat dead things
Parasitic: get food from living things
Symbiotic: two organisms living together mutual benefit.
Bacteria example and fungi example in book.
• Rhizobium bacteria: lives off the roots of legume bean clovers. Lives inside roots and creates a nodule or cyst. This organism fixes N2 from air into plant useable as NH4+ (ammonium). And the plant gives the organism sugar from photosynthesis.
• Mycorrhizal fungi: why you can’t transplant Woody perennials is like an extended root system it helps the roots bring in water/nutrient absorption.
• Live on and inside roots of woody perennials- they absorb water and nutrients from soil and transfer them into roots of plant.

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5. How does cultivation of the land effect the organic matter content of the soil initially and why does this happen?

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The OM decreases because increase of aeration, loss of top soil from erosion and higher temperature

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6. Explain why assimilation of the elements derived from decaying organic matter into the new microbial tissue is a means of conserving or maintaining organic matter in the soil?

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When elements are immobilized in microbial tissue the element is removed from the readily available sources of that element in the soil. Until, the microbial tissue dies breaks down and goes through a slow mineralization process the plants are unable to use the elements that were assimilated into the tissue of the microbe conserving organic matter in the soil.

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7. What are the simple end products of organic matter decay, and why would excess moisture in the soil slow the decay process?

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• Co2, H20, ATP and inorganic ions (mineralized Ion example: Sulfate)
• Because decay is aerobic process
Excessively moist soil limits O₂ in the soil which prevents aerobic organisms from contributing to the decay process.

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8. What is the general relationship between the C/N ratio and the relative rate of decay of fresh plant tissue?

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• Inverse relationship.
If we have a low carbon to Nitrogen Ratio 20:1 == Rapid rate of Decay.
If we have a High Carbon to Nitrogen Ratio 150:1 == Slow rate of Decay.
What would be the C/N ratio of a plant that contained .4% N and 52% Carbon? 130:1

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9. What is humus and how does it compare with fresh plant tissue with respect to stability toward decay and principal chemical components?

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Humus: a dark-brown organic semi stable portion of soil that is derived from decomposed plant and animal remains and animal excrement. Humus improves the water-retaining properties of soil, adds nutrients, makes it more workable and has a high cation exchange capacity. It’s a work in process lignins and proteins

How does it compare: humus is more stable and resistant to break down then fresh plant tissue.
Fresh plant tissues have a high concentration of carbohydrates. What happens to the cellulous and Lignins when they die and form Humus.
Principal components Plant residue Humus
Carbs 45% High Low
Lignins 20% low High
Proteins 8% low High: because a bunch of dead and living microorganisms contain a lot of protein and because the % carbs drop off so much.

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10. Why is it necessary to add additional nitrogen fertilizer to the soil if a heavier than normal crop residue is added to the soil? What might be the consequences if this practice was not followed?

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• More residue extended nitrate depression
It is necessary to add additional nitrogen fertilizer to the soil because the microbes in the soil will grow exponentially under these conditions and compete with plant life for required nutrients. If this practice is not followed the majority of the essential elements will be immobilized in microbial tissue and the crop will struggle to survive until Mineralization occurs.
Nitrogen speeds up the rate of decay in the soil

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Heterotrophic:

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organisms that are not self sustaining they must oxidize the carbon from an organic source. Obtaining nourishment by digesting plant or animal matter, as animals do, as opposed to photosynthesizing food, as plants do

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Aerobic

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Are organisms that can only live in the presence of free oxygen. They require free oxygen (these are the primary decomposers)

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Anaerobic:

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get oxygen from compounds

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Autotrophic:

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Organisms that are self sustaining and get carbon from an inorganic source (things composed of minerals rather than living matter). Many of these are involved in the oxidation of inorganic ions.

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Saprophytic:

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heterotrophic organism that eat dead things

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Parasitic:

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get food from living things

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Symbiotic:

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two organisms living together mutual benefit.

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• Rhizobium bacteria:

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lives off the roots of legume bean clovers. Lives inside roots and creates a nodule or cyst. This organism fixes N2 from air into plant useable as NH4+ (ammonium). And the plant gives the organism sugar from photosynthesis.

Front 19

• Mycorrhizal fungi:

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why you can’t transplant Woody perennials is like an extended root system it helps the roots bring in water/nutrient absorption.
• Live on and inside roots of woody perennials- they absorb water and nutrients from soil and transfer them into roots of plant.

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how to categorizes soil organisms

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i. Based on size
ii. Based on life form

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· How do we further subcategorize these organisms

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Based on their food source
And where they get O2 from

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Based on size

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Macro: you can see with your eye
Meso:
Micro: can not see with your eye

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Based on life form

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Flora: plant life (flora uses the most organic matter

Fauna: animal life

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Most important/Active primary decomposers are the

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micro flora.
1) Examples: Bacteria and Fungi (these are considered plant life)
2) Micro fauna is a protozoa and it eats Bacteria

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Autotrophic:

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self sustaining they get carbon from an inorganic source (are things composed of minerals rather than living matter). Many of these are involved in the oxidation of inorganic ions.
® Example: to add nitrogen to soil on the cheap you buy ammonium and it gets converted to nitrite by bacteria and then another bacteria comes around and converts it to nitrate.

If you add ammonium these bacteria will go nuts in population because it has more food and creates

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Heterotrophic:

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not self sustaining they must oxidize the carbon from an organic source.

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Saprophytic:

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Use dead or decaying plant or animal life to oxidize the carbon.

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Parasitic:

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use living plant and animals to oxidize the carbon.
► Parasites: cause damage to plants, bunch rot in grapes powdery mildew.
► How do they survive when there are no plants they convert into a saprophytic until plants come back. When the plants come back they convert back to parasitic organisms.

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Aerobic:

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require free oxygen they breath (these are the primary decomposers)

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Anaerobic:

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functions in the absence of free oxygen. They get it from Ions and compounds.
◊ How do they get oxygen then?
► Water logged land causes an anaerobic environment in the soil. And the anaerobic bacteria will attack the nitrate and steal the oxygen from the element releasing Nitrogen gas into the air. Seeping all your nitrogen from the soil. (this specific bacteria that does this is call a denitrifying bacteria.)

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Carbon makes up what % of Organic Matter

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58%

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If you looked at primary decomposers bacteria and fungi how would you categorize:

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them Micro Flora heterotrophic saprophytic aerobic they release CO₂

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Microflora

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are primary decomposers they are breaking down dead and or decaying plant or animal residues.

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Microfauna:

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Live on these microflora (they eat them)

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Misofauna:

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live on microflora and fauna but at some point in time become food for lower organisms.

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The most limiting nutrient is

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Nitrogen

Front 37

The rate of decomposition of organic material is based on its

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Carbon/Nitrogen Ratio

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If we have a low carbon to Nitrogen Ratio 20:1_______rate of Decomposition.

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fast

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If we have a High Carbon To Nitrogen Ratio 150:1 ________ rate of decomposition.

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slow

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mineralization:

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Organic compounds are broken down by soil microbes and release as carbon dioxide, water and simple inorganic ions. N is released as ammonium S as sulfate.
► This release products for the plants to take up.

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Immobilization:

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where components from the breakdown of Organic Matter are picked up and reutilized to make new microbial tissue.
The problem is these guys are super competitive for these nutrients and compete with the plants.

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3 beneficial effects of soil Organisms

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1)Important in recycling nutrients through decomposition of organic material. Must be decomposed before plants can use.
2)Microbes immobilize nutrients from breakdown of OM- helps to insure continued productivity of soil.
3)Microbe decomposition of OM helps create aggregates in the soil this leads to improved structure and aeration of soil: Earthworms for example. This increases water intake improves soil structure in the macro and micro level.
○ Why do some soils have more earth worms? Soils with more OM have more food for the organisms.

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3 negative effects of soil Organisms.

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1)Bacteria and or Fungi which are causal agents of plant diseases. These are the two primary groups causing diseases.
2)The Initial immobilization of N or S by microbes could lead to crop deficiency.
3)Potential Release of nutrients from plant available to non available form.
Example: the denitrofication process that’s when the nitrate gets broken down and released as N2 gas and it depletes the nitrogen in the soil. If the soil is water logged this happens 72 hours in this condition.

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□ Analyzed a soil sample of 100 grams and found 1.2grams of C.

What is % OM in this Soil?

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1.2 x 1.72=2.064 grams of OM
® (2.064/100)x100=2.064%OM

Front 45

OM in soil improves what soil physical characteristics of the soil:

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structure soil, aeration soil, water intake, and soil water holding capacity (these are physical characteristics of the soil that OM help).

Front 46

If I find 4.25% OM in 200 grams of soil what would be my total weight of C in grams in this Sample?

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4.25%x200=8.5
8.5X.58=4.93
4.93 grams of carbon

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Plants are made up of

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75% water and 25% Dry Matter

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Dry matter:

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>90% of plant dry weight is C, O H
Elemental Composition of plant Materials:
► Carbon =42%
► Hydrogen= 8%
► Ash= 8%
► Oxygen= 42%
► Total= 92%

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Organic Components

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Proteins
Carbs
Lignins

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Carbohydrate like substances: only contain

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Carbon
Hydrogen
Oxygen

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Average living bacteria has a C/N of

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6:1

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Why do we care about Humus

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because it has High CEC (Cation Exchange Capacity)