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Soils 5


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Permenant Wilting Point - ? bars
-15 bars or -1.5 mPa
What enhances water availability?
Horizon types
Soil depth
Percipitation and other water supply
Components of the hydrologic cycle
Infiltration rate
Rate at which water enters the soil
Volume per unit of time
I=Q(quantity - a volume)/A(area)*time
Factors effecting infilitration rate
Compaction - decreasee
Crust - decrease
Macropores (texture- aggregates) - increase
Mulch - both
Saturation - decreases
Surface and subsurface run-off
Surface run off happens to the fraction that doesn't infiltrate
Subsurface run off is water that infiltrates but drains by gravity (such as down a hill), through capiliarity or because it hit a pan layer to a stream or body of water.
Change in soil storage
Equals inputs minus outputs. Usually done on a watershed basis.
Factors - precipitation, leaching to creek or aquifer, overland flow, transpiration, evaportation
Outputs from decomposition
CO2,Humic compounds, more organisms, nutrients, non-humic compounds
Physical composition of plant litter
75-90% water, 25-5% dry mater
Chemical composition of plant litter
44% C, 8% H, 40% O, 8% other
Types of compounds found in plant litter
Sugars, starches, protiens, hemicellulose, cellulose - all 40-90%; fats and waxes - 1-10%, lignin 5-40%
Litter quality
Refers to litters susceptability to decomposition
Litter quantity
Varies by eco-system type (temp and water). Tropical grass -high, tundra - low, trpical forest - most, temperate forests between.
Detrital pool
undecomposed litter, partially decomposed, and build of organic matter
Turnover rate
How fast all litter mater decomposes. Slow in waterlogged soil - build up of detrital pool. Fast in warm humid climate so less accumulation.
An oxidation reaction (oxegen taking electrons) by heterotrophic organisms (get energy and carbon from eating others).
Decomposition reaction
OM + O2 (microbes and enzymes) -> CO2 + H2O + Heat Energy + Humus + New microbes + Nutrients + Non-humic substances.
Describe the basic trophic levels in the soil food web.
The primary producers are the plants and soil organic matter. There are several levels of consumers beginning with bacteria, mycorrhiza, mites, and other fungi as primary consumers. Highest levels of consumers include earthworms and small mammals that feed from the soil. (Ref Soil Org PPT slide 2 and pg 319 of Soil Elements)
Components of organic matter in soil by percentage.
33-50% humus
33-50% active decomposition
10% fresh residue
<5% organisms
Where do organisms live in the soil?
On and around soil aggregates
A good soil is a ...
LIVE soil.
Microbial biomass increases or decreases with depth? Why?
Decreases. As decrease in depth, decrease in organic matter to feed microbes and other soil organisms and decrease in oxygen for respiration. (Ref: Soil Org PPT slide 5)
Describe the cycles of soil organism activity.
Activity levels change daily, probably in relation to temperature, and seasonally in relation to temperature, nutrient availability, etc. Most active in early summer, may decrease in late summer and pick up again in fall. Lowest level after frost. (Ref Soil Org PPT slide 6)
How does the biomass of soil organisms differ from ecosystem to ecosystem?
Total biomass may vary, different types may be present in different amounts. Influenced by climate, soil type, etc. (Ref Soil Org PPT slide 8 & 9) Bacteria dominate in grasslands, fungi in forests. (Ref Soil Org PPT slide 23)
Discuss how to measure the activity/diversity of a food web.
Count larger individuals and plate counts of cultured bacteria. Measure respiration (CO2 production), nitrification, or decomposition. Measure biomass of C, N, and P, DNA and RNA, or enzymes. (Ref Soil Org PPT slide 10)
Discuss complexity in soil food web.
Soil food webs contain both generalists that feed on many organisms and across trophic levels and specialist that live only on a limited number of organisms. Complexity varies by ecosystem type with the rainforest as being the most complex and caves the least. Complexity is valuable for resilience. Complex relationships can increase productivity, eg: bacteria and bacteria feeding nematods on blue grama grass. (Ref Soil Org PPT slides 11-13)
Very smal, single celled, prokaryotic organisms. Very diverse and abundant in soils. Important in all transactions in soil food web.
Filamentous organism, similar to fungi, though much smaller and prokaryotic so now considered bacteria. Most numerous next to bacteria. Contribute to decomposition, nitrogen fixing, and protection against other microrganisms through production of antibiotics.
Decompose carbon compounds
Improve OM accumulation
Retain nutrients in the soil
Bind soil particles
Food for the rest of the food web
Mycorrhizal fungi
Compete with plant pathogens
(Ref Soil Org PPT slide 18)
Mobile, single celled creatures that capture and engulf thier food. Include amoebas, ciliates, and flagellates. Move through soil water. Prey on bacteria.
Unsegmented roundworms, usually to small to see with the naked eye. Predatory both on many soil organisms and on plants. Live in soil water and more course soils with room to swim.
Most important macroanimal in soils. Eat detritus/O.M. Important in macropore formation (aeration, water infiltration, plant root growth), increasing nutriet availability by soil mixing and consolidation of OM and bacteria in casts.
Describe some ways that land management can effect soil organisms
Tillage reduces OM, earthworms, mychoriza, etc.
Compaction reduces organisms because reduced aeration,etc.
Biological crusts sensitive to disturbance.
Pesticides reduce soil organisms.
Spodisols and OM
OM goes down in elluvial E horizon but back up in Bhs horizon. Unusual. Most soils OM decreases at some rate with depth.
Soil productivity is dependant on
Support, water, aeration, nutrients and symbionts.
What is a nutrient
Element that is necessary for growth or completion of life cycle.
Number and name of nutrients needed by plants.
17-18 nutrients. C H O P K I N S Ca Fe Mg B Mn Cu Zn Cl Mo Co Ni (cobalt, just legumes)
Plant nutrients that are positively charged cations
Ca +2, MG +2, K +1, NH +4, Cu +2, Zn + 2
Soil nutrients that are typically available as negative anions.
PO4 -3, S04 -2, N03 -
Forms of nitrogen found and soil.
NH4+ - ammonium, N03 - nitrate, and N02 - nitrate (rare)
>500 ppm
C H O P K N S Ca Mg
Fe, Mn, Zn, Cu, Mo, Cl, B, Co, Ni
The law of the minimum
Plant growth is constrained by the essential elment that is most limiting. Barrel stave example.
Discuss nutrient deficiencies and visual symptoms on plants
Use visual symptoms because hard to test for. Nutrient deficiencies in immobile nutrients will be at the tips. In mobile nutrients, in the older foliage. Color change is most common symptom.
Does a visual symptom of deficiency in a plant mean the nutrient is not in the soil?
No, just not available to the plant. pH could be limiting availability. Could be in wrong form.
Discuss nutrient uptake and energy needs
Plants usually taking up nutrients against the concentration gradient so need energy from respiration. Respiration requires O2.
Discuss selective uptake
Plants generally take up nutrients selectively by need but concentration in plants can be influenced by a high concentration (large gradient) outside the plant.
Discuss electrical balance of nutrient uptake.
Plants electrically balance nutrient uptake by dumping H+, OH-, or HC03 -. Can change pH around plant roots.
Factors that effect nutrient uptake
Plant metabolism factors: 1) oxygen supply, 2) temperature, 3) availability of products of respiration and photosynthesis. Soil factors: 4)water content, 5) root distribution, 6) nutrient availability, 7) symbionts
Changed area just outside of plant roots. Where exudates accumultation, microorganisms feed off of exudates, nutrients are extracted.
Sources of nutrients in soil
Parent material, atmospheric deposition, secondary minerals, soil organic matter, and exchange sites.
How are nutrients made available to plants from various sources (i.e. put into soil solution)
weathering of primary minerals, exchange/desorption from CEC sites, decomposition (mineralization) of organic mater. Availability depends on the source and the speed of the reaction that makes nutrients available.
Describe the sources of nutrients to soils and speed of availability
Readily available in soil solution. Moderately available on exchange sites depends on number of sites and exchangable ions available. Slow availability from organic matter as decomposes - depends on size of matter.
If you were to look at three things to assess soil nutrient availability what would they be?
Soil texture - influences exchange sites. Amount of organic matter. pH to make sure nutrients will be available.
Methods of nutrient movement through the soil.
Mass flow and diffusion
Describe mass flow of nutrients
Nutrients move proportionally to water movement and concentration in water. Fast way to move nutrients. Driven by transpiration.
Describe nutrient movement by diffusion.
Nutrients move through soil solution in response to concentration gradient. Plant uptake reduces concentration around roots, but diffusion is slow so causes a zone of depletion by roots. Rate influenced by that nutrients diffusion coefficient.
Role of nycorrhizae
Symbiotic relationship with plant roots. Sends out fungal hyphae and picks up nutrients provided to the plant in exchange for food. Dramatically increases surface area for nutrient uptake.
Two types of mycorrhizae
Ecto - cover the outside of the root, common in forests.
Endo - vesicular arbuscular, penetrate the root.
Describe nitrogen fixers
Bacteria and actinomycetes that take atmospheric N2 which is strongly bonded together and convert it ammonium for plant uptake in exchange fore organic carbon from the plant. Symbiotic. Rhizobium is very common nitrogen fixer in legumes. Does not provide all the N a plant needs. Also lichens fix nitrogen.
Mobile nutrients
No3-, K+, Ca +2, Mg+2. Move readily through the soil.
Immobile nutrients
Nutrients that are strongly adsorbed or precipitate out of solution (therefore low concentration in solution). PO4 -3, and Fe +3.
3 stages of decomposition
1. Physical Fragmentation: soil Fauna- insects, termites, earthworms

2. Leaching : soluble compounds

3. Chemical Breakdown and Synthesis:
- micro organisms
- enzymes, break things apart and form new compounds.
- formation- release of oxidation, by products and nutrients
- form compounds resistant to further decay (humus and organic acids)
9 factors affecting soil decomposition (in the soil and environmentally)
Aeration/water, precipitation, temperature,abundance of soil flora/fauna, pH, litter quality, nutrients, toxic compounds, humans
How does precipitation effect decomposition?
Increases with precip until waterlogged
How does temperature effect decomposition?
Increases up to about 40 C. (Organisms die). In fresh litter increase more dramatic than humus which decomposes more slowly.
How does pH effect decomposition?
Do not like it too acidic or too akaline - extremes inhibit activity. More akaline soils host more bacteria and worms, acidic soils, more fungi.
How do nutrients effect decomposition?
More nutrients decomposes faster, less, slower. Depends on essential nutrient availability and C:N ratio. Woody components take much longer. More N per C.
How do toxic compounds and humans influence decomposition.
Negatively. Some plants (walnut, ecalyptus) release phytotoxins to prohibit other plants growth. Humans reduce decomposition through tillage, deforestation, etc.
Humus and Humins
Outputs of decomposition. Humus is complex mixture of brownish-blackish amorphous organic substances resulting from microbial decomposition and synthesis. Does not have a chemical formula. Humin: is very resistant. It is bound to mineral material.
Benefits of Humus
1. high surface area = high CEC, which is pH dependent
2. high water holding capacity
3. micro-pores
4. adhesives that form aggregates in the soil.
5. nutrient source - N, P, S ...
6. Carbon source for organisms (microbe food)
Detriments of humus
You can have too much of a good thing.
1. physical properties such as anchorage is poor if you have all OM 2. can have phytoxins 3. diseases- can have an increase in diseases if you have too many microorganisms such as root rot.
Where is SOM?
Forest (oak) 202 above ground, 184 below ground = 386 tons Prairie = 7 tons above ground + 345 below ground + 353 tons of SOM prairie has many fine roots, high turn over, creates alot of SOM.

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