Chapter 5

Nutrients to Soils

I. Classifications of nutrients

1. Macronutrients—utilized in large amounts• C, H, O, N, P, Ca, Mg…

2. Micronutrients—trace amounts utilized• Fe, Cu, Zn, Se, I, …

Nutrients come from

--atmosphere

--weathering of rocks

II. Weathering

Weathering – the process by which exposed geological substrates become soil

Rock types—igneous—from cooled volcanic flowssedimentry—deposition of mineral

particles (sediments)metamorphic—altered by heat and

pressure

The release of mineral nutrients into soil is result of weathering--depends upon:

type of rock

temperature

precipitation

wind or water erosion

III. Plant uptake

1. Acquire nutrients by uptake of dissolved ions in soil solution

2. Active transport moves solutes through the soil

3. Nutrients differ in their rate of active transport

4. Development of fine roots is response to nutrient availability

IV. Decomposers

1. Feed on dead organic materialA. High quality contains simple carbohydrates,

simple fats and proteins

B. Low quality contains complex carbohydrate cellulose and lignin

2. Lignin—decomposed by fungi

3. Mineralization—microbial breakdown of organic matter into inorganic substances

V. Nutrient cycling

Microbial action—important part of nutrient cycling

--the pathway of an element through an ecosystem

--may go through many organic forms before mineralization occurs

Nitrogen

• 2nd most important nutrient after C

• Plants live in symbiosis with microbes

--fix Nitrogen from atmosphere soil

• Soybean rotation with rice

• Important in rubisco and chlorophyll

• >50% N in leaves tied up compounds

VI. Availibility of nutrients

• pH affects availability and uptake of nutrients

• H ion competes with other cations for available spaces on soil particles

Cation exchange capacity—the ability of a soil to adsorb positive charged ions.

• Soil –foundation upon which all terrestrial life and much freshwater aquatic life depends

• Plants depend upon soil types• Pathway between organic and mineral

worlds• 3 dimensional unit—length, width and

depth--pedon

I. Soil formation

5 factors to soil formation

1. Parent material

2. Climate

3. Biotic factors

4. Topography

5. Time

1. Parent material

• Unconsolidated mass which form soils

• Derived from parent rock

• Determines overall composition of soil

• Transported by wind, water, glaciers and gravity

2. Climate

• Influences development of soil

• Temperature and rainfall govern rate

• Climate influences plant and animal life in region– In turn influence soil

3. Biotic factors

• Plants, animals, bacteria and fungi– Contribute to formation

of soil

• Input of organic matter by vegetation– Reduces erosion

• Bacteria and fungi primary decomposers– Mixes with soil mineral matter

4. Topography

• Contour of land

• Influences amount water entering the soil

• Steeper topography—more runoff– Less accumulation of water in soil– More overland water increases transport

of soil and nutrients

5. Time

• Weathering takes time– Plant material organic soil– Parent material mineral soil

• Well-developed soils – 2,000 to 20,000 years

• Younger soils not as fertile

Formation of humus –the residue of organic matter

II. Weathering

• Mechanical weathering—exposure to wind, water, temperature– Frozen water in crevices– Tree roots

• Chemical weathering—occurs along with mechanical weathering and continues after– Organisms in soil– Rain – mechanical and chemical

• Low pH due to NO3 and humus

III. Soil horizons

• Vertical cut through a pedon –soil profile

• O –organic layer

• A, E, B, and C –mineral layers

• Oi –litter layer• Fluctuates seasonally

• Oa –humus layer

• A horizon—upper layer of mineral soil• High organic content• Mixing zone

• E horizon—zone of maximum leaching (eluviation)

• Downward movement of water and suspended material alters structure

• B horizon—zone of illuviation—• accumulation of silicates, clay, iron,

aluminum and humus

• C horizon—weathered material• R horizon—unweathered parent

material

IV. Soil characteristics

• Soil texture affects pore space

• Big role in movement of air and water

• Primary soil particles– Gravel >2 mm– Sand 0.05 – 2 mm– Silt 0.002 – 0.05 mm– Clay <0.002 mm

• Clay—characteristics– Plasticity– Cation exchange capacity– High water retention

• Soil particles held together in shapes called aggregates or peds

• Aggregate arrangement—soil structure

• Dissolved cations in soil are bound on organic matter and clay particles– Plate-like clay particles = micelles– Have a negative charge –attract

cations– Number of sites = cation exchange

capacity

• Cations bind differently– H+ > Al2+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+

– H+ and Al2+ strongest attraction

V. Soil orders

• Large array of soil orders

• Produced by climate, vegetation, time and degree of weathering

• Distinct set of features

• Range from Aridisols to Histosols

VI. Life in the soil

• Diverse ecosystem• Same requirements as other organisms• Important in development of soil• Vast range in size and habitat• Many live within pore

spaces• Soil disturbances result

in loss of diversity

VII. Erosion

• Stripping land of vegetation results in erosion

• Caused by wind or water• High clay content soil—slow absorption of

water subject to erosion• Sheet erosion—water movement across top of

soil eroding top layer• Rill erosion—water is concentrated in small

channels• Gully erosion—larger channels