BIOL 4120 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782 |
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The
creature above has 8 walking legs (note that it has lost one)
but is not a spider or a mite. What is it? |
Lecture 5 The Terrestrial Environment
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Loss of surrounding aqueous environment introduces a new stresses:
Light hits vegetation before it hits the surface of the Earth and is usually measured as PAR by ecologists, not total radiant energy of the Sun
Light reaching the vegetation is
Soil is like pornography (as defined by the US Supreme Court): you may not be able to define it exactly but you know it when you see it. It is the interface between plants and the rock that constitutes the continents. It is a mixture of small rock particles, air, water, organic debris, and organisms (bacteria, fungi, plant roots, burrowing animals, and protists). It is fantastically complex when closely examined.
Weathering produces the small rock particles and can be either accomplished through mechanical or chemical destruction of larger rock
- Mechanical weathering - uneven expansion as temperature changes crack rock, expansion of water as it freezes in cracks splits rock, and wind and waves move small particles and make them even smaller.
- Chemical weathering - chemical alteration of rock by process of oxidation once exposed to air, by acids produced by plant roots and by decomposition of plant litter.
Parent Material - the underlying rock that is the basic source for minerals in soils (although wind can blow in minerals from other places)
Leaching - loss of soluble minerals from soil as precipitation percolates through the soil to the groundwater and then flows into river, lakes and aquifers
Erosion - loss of soils as water carries them downslope, where they can be deposited on bottomland or, if they enter a stream, carried downstream where the particles either become part of the stream or river bottom or reach the ocean and settle onto the ocean floor
Humus - organic matter that is no longer recognizable as being from a particular source. This is an important soil component and is mostly plant material. Lignified wood persists longest in the soil.
Soils have characteristic colors, textures, structures, moisture and depth
Color - Black colors indicates high humic content, iron oxide (rust) imparts a red color (common in foothills of the Appalachian Mountains) when oxidation is complete but partial oxidation of iron can give a yellow-brown color to soils
Texture - depends on profile of fine particle sizes present in soil
- From largest to smallest - gravel, sand, silt, and clay
- smallest clay particles are so small that it takes hours for them to settle out of water (the accepted way to measure size profile of soil)
- gravel is not part of fine particles so texture depends on relative proportion of sand, silt and clay
- equal portion of all three is a Loam
- Coastal plain from Texas to New Jersey is dominated by sand
- Recall that air and moisture are important components of soil
- amount of either depends on the pore space in the soil which is an outcome of texture
- gravelly, sandy soil has lots of pore space which is good for aeration but will not hold water (less surface area to which the water can adhere)
- clay soils have less pore space but retain water. Large surface area means the exchange of ions between minerals and soil solution is fastest here.
soil depth varies greatly but is usually less on ridge tops and greatest in bottomlands
New minerals often enter the soil from the bottom (from the parent material) and organic material from the top, as leaves die and become debris (litter), so these things mix into the soil from opposite sides and the soil develops Horizons, vertical layers.
- O Horizon - Leaf litter, unconsolidated (loose) where plant litter on upper side becomes humus on lower side as animals, fungi, and bacteria process it
- A Horizon - topsoil - upper layer of mineral soil, usually black due to presence of humus from O horizon, often nutrient rich and full of fine, nutrient absorbing roots
- E Horizon - Eluviation (washout) layer - area where leaching removes small particles and leaves behind insoluble minerals, under the A and on top of the B layer. These particles have been leached of soluble nutrients and so are nutrient poor. Found most often in soils under wet forests but rarely in grasslands because rainfall is to little to form the layer
- B Horizon - subsoil - Illuviation (wash in) layer- region where the particles that come from the E horizon accumulate. Usually denser in texture (due to small particle accumulation) and harder for roots to penetrate.
- C Horizon - broken parent material that is beginning to weather (but more slowly than minerals in B or A horizon) and so is chemically nearest the parent material
- Bedrock - unbroken parent material
Plants get both moisture and soluble nutrients from soil and different soils have different capacities as reservoirs of each.
Soils are saturated when additional water drains away rather than be absorbed. All of the pore space is filled with water and no air is left. However, if saturated soil is placed over a mesh, some water will drain from it due to gravity. The water lost is drainage and will be lost soon after the rain stops. The moisture that remains is what will sustain the plant through the periods between rains.
- Field Capacity - measurement of maximal amount of water soil can hold due to adhesion (capillary attraction) between soil and water, usually given as a % of total soil weight when saturated calculated as the difference between the weight of the drained saturated soil and the same soil when totally dried in an oven. Sandy soils have large pores but small surface areas and so they do not hold much water (most is lost as drainage). Clay soils have smallest particles and largest surface area and hold most water (and least drainage).
- Capillary water - water in soils held there by adhesion between water and soil particles.
- Wilting Point - moisture level at which plants can no longer draw water from soil. This also depends on adhesion between soil and water, which increases as soil texture becomes finer. So clay has greater field capacity but greater wilting point also.
- Available Water Capacity (AWC) - difference between Field Capacity and Wilting Point percentages
Soluble nutrients in soil are charged particles, Ions, and, because the soil particles have charge, they interact with the soil. Plants must compete with the soil particles for these ions.
Cations are positively charged, (e. g. Ca++, Mg++, NH4+) and Anions are negatively charged (e. g. NO3-, SO4-, Cl-)
- Ion Exchange Capacity - total number of charged sites on soil particles in a standard volume of soil - a measurement of the holding power of the soil for soluble nutrients
- usually temperate zone soils have an excess of negative charged sites in the soil (many due to humus) and so have a larger Cation Exchange Capacity than Anion Exchange Capacity
because ions can become attached to the soil particles at these charged sites, they are not in solution (even though they are soluble) and do not leach out of the soil, a good thing for plants
the paucity (scarcity) of positive sited in temperate soils means that there are fewer places for anions to attach and these ions (PO4--- and NO3-) are leached from soils and are often added to the soil by farmers in fertilizer
the ions on the soil sites and in soil solution are in a dynamic equilibrium and where most are (bound or free) depends on their relative attraction for the soil sites.
Al+++ > H+ > Ca++ > Mg++ > K+ = NH4+ > Na+
This means that, as pH of soil decreases, the ions will go into solution in the order from right to left, with aluminum being solubilized last (remember that acid rain can solubilize it and it is poisonous to many organisms)
plants secrete H+ ions when they wish to free mineral ions to absorb as nutrients. They give up H+ and get mineral nutrients, which is the reason exchange is in the expression "ion exchange capacity" and its not something like "ion holding capacity"
Pedology is the study of soils and Pedogenesis is the formation of soils. There are many types and variant of soils and we will not memorize any specific names here. However, climate often influences the soils and we can understand this through an examination of the major soil-forming processes.
For you information, many of the soils in our area are Utilisols. They are old soils that have been strongly leached and so laterization is the most influential process around here. Our utilisols are often red due to iron oxides (the famous "red clay" soils of the American South)
Important soil formation processes
Laterization - when precipitation greatly exceeds evaporation and transpiration in warm climates, water rapidly percolates through the soil and into the groundwater, where it flows into rivers and streams. Soluble soil nutrients are constantly leached out of the soils, leaving behind the less soluble ions (Al+ and Fe+++), which precipitate and give the soils their color (whitish for Al and red for Fe) and H+ ions, which make the soil acidic. Loss of nutrients to leaching results in nutrient-poor soils.
Calcification - when loss of soil moisture due to evaporation and transpiration exceeds precipitation water leaves the soil through the surface rather than through flow of groundwater. The minerals dissolved move upward from the groundwater with the water and the least soluble (typically CaCO3) are deposited where the precipitate, usually in the B horizon, where, if enough precipitate, they can form a hard layer called a Caliche
Salinization - happens most often in very dry climates and is basically the same process as calcification except that the precipitate is salt (NaCl) and it occurs in the A layer because there is little rainfall to move down through the soil and offset the upward movement due to evaporation. Can result in a Salt Crust on the surface of the soil. The salt concentration is such that plant growth is inhibited.
Irrigation of dry lands can result in salinization where none existed before and can poison the land. Irrigation water dissolves salt as it soaks into soil and brings the salt to the surface as it evaporates. Huge problem in the US southwest, in Australia, and in Northern Africa, major areas of dryland irrigation.
Podzolization - Cool, moist regions often have conifer forests, where the needles fall and are not quickly decomposed due to low temperature. Accumulation of humus and humic acids causes soils to become acidified. The acidification can become so intense even Al and Fe ions are solubilized and the lower portion of the A horizon is left with only sand made of silica (glass - SiO2). Soluble nutrients are leached and soil is nutrient poor.
Gelization - constantly wet soils have little air and so O is scarce for decomposition, which is very slow. Accumulating organic materials are acidic and the low pH means mineral nutrients are lost to leaching. Soils often grey-black due to incompletely oxidized state of iron.
Last updated January 21, 2007