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Soil

Terms in this set (27)

soil
layer of chemically and biologically altered material that overlies bedrock or other unaltered material at Earth's surface
parent material
the layer of bedrock that underlies soil and plays a major role in determining the type of soil that will form above it
soil composed of:
minerals from parent material, organic material from plants, air, water, living plant roots, microorganism, and decomposing organisms
horizons
-distinct layers of soil forming a soil profile, commonly referred to by letters (O, A, B, C, D, etc.)
O horizon
-layer composed of dead organic material
-1 m2 = 10,000 springtails, 100,000 mites, 1,000,000 nematodes
A horizon
-eluvial
-layer rich in humus, consisting of partially decomposed organic material mixed with mineral oil
-leaching out takes place
-roots
leaching
process in which groundwater removes some substances by dissolving them and moving them down to lower soil layers
Why is soil formation affected by rain?
Rainfall increases plant production and breaks down parent material; areas with low precipitation have slow soil formation.
B horizon
-illuvial
-too deep for plant roots
-accumulation takes place
C horizon
composed of large rocks and broken material, caused by freeze thaw
D horizon
unaltered bedrock
What will determined what horizons look like?
-time, climate, parent material, vegetation, and local topography
Why are "young soils" typically more fertile?
less time for leaching
Soil Skeleton:
>2mm =
0.05-2mm =
0.002-0.05mm =
< 0.002 mm =
>2mm = gravel
0.05-2mm = sand
0.002-0.05mm = silt
< 0.002 mm = clay
What determines how much water can be held?
surface area
Smaller particles (clay) have larger SA relative to volume.
Hold more water, but that is held very tightly.
loam soils
best for growing plants, contains 40% sand, 40% silt, and 20% clay
Soil composition affects field:
capacity and wilting point
Polarity of Clay
1. silica (Si4+) at core, 4 O2 form tetrahedron around (negative charges face outwards)
2. group together into micelles
3. grouping alters cation (+ ions such as K+, Na+, Ca++, H+, Mg++, NH4+ exchange
4. high cation exchange capacity, more potential soil fertility
5. soil structure is important! Overal soil fertility is a balance of soil particle sizes.
laterization
-the breakdown of clay particles, which results in the leaching of silicon from the soil, leaving oxides of iron and aluminum to predominate throughout the soil profile
-occurs in tropical and subtropical regions where soils weather to great depths
-clay breakdown lowers soil cation exchange capacity, and minerals readily leached
-new minerals from parent material too deep for plants
-primary productivity depends on the rapid cycling of nutrients close to the surface
good soil
well-aggregated, has structure and "tilth," structure allows for air and water flow. Soil tilth = property suitable for plant growth, particularly at root zone. Refer to degree of aggregation.
What contributes to tilth?
-dependent on glomalin protein produced by mycorrhizae
-"Biotic glue."
-forms stable, lattice-like aggregates.
-value of undisturbed soil. cultivation? C loss? increases carbon in the atmosphere and global climate change
-more deforestation, more carbon in the atmosphere
gravitational water
drains through large pore spaces quickly
capillary water
-water in small pore spaces retain by capillary action against pull of gravity
-main source for plants (use root hairs)
hygroscopic water
-adsorbed on surface of soil
-clay high surface area, holds tightly to water
-virtually unavailable to plants
Field capacity =
-water held in soil 1-3 days after a soaking
-max. amount of water held by soil against force of gravity
-affected by soil texture
-well-textured: 1/2 volume = space
-macropores = largest pores
wilting point
-megapascals becomes more negative as soil dries
-clay has a more negative value
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