7 Oct. Water Use & Management: Moisture Availability, ET
9 Oct. Water Use & Management: Moisture Availability, ET
24. How Soils Hold Water
- Adhesion--water adheres to soil minerals, clays and humus. Adhesive forces bind water to soil surfaces, wetting them.
- Cohesion--water bonds to itself. Cohesive forces add layer upon layer of water, causing the water film to thicken and pulls water into soil pores. Coarse mineral grains hold water in the spaces between the grains by cohesive forces.
- The work required to remove water from a soil is called the water potential. The water drains from large pores easily, the water held in small pores is more difficult to remove, and the water adhering to soil colloids (humus and clay) is the most difficult to remove.
25. Soil Water Contents
- When all soil pores are filled with water, the soil is saturated.
- When gravity has drained water from larger pores, the soil is at field capacity. As the soil texture becomes finer and soil pores smaller, increasing amounts of water remain in small pores and as films coating soil particles at field capacity.
- When plants wilt because they can remove no more water, the soil is at the wilting point.
- Plant-available water is the field-capacity water content minus the water content at wilting point.
26. Soil Water Flow
- Water flows in response to a driving force and the rate it moves depends on the force and the water (hydraulic) conductivity of the soil.
- Gravity drives saturated flow, the flow conditions between saturation and field capacity. Saturated flow rates depends on the mean diameter of soil pores.
- Unsaturated flow pulls water from moist soil into dry soil when the water content is below field capaity. Unsaturated flow rates depends on how dry the soil is because flow becomes slower as water films become thinner.
- Saturated flow is far more rapid than unsaturated flow.
27. Water Uptake and Use By Plants
- Plants absorb water through their roots and lose water through their leaves.
- Transpiration, the loss of from plant leaves, is highest when the relative humidity is low and wind transport water vapor from the land surface by turbulent mixing of the lower atmosphere.
- The relative humidity varies with temperature--if the water content of the air remains the same, then raising the temperature lowers the relative humidity.
- Water loss by evapotranspiration increases as root volume (water absorbing capacity) and leaf canopy (water losing capacity) expand during the growing season.
12 Oct. Climate Change: Energy Cycle, Carbon Cycle
14 Oct. Climate Change: Energy Cycle, Carbon Cycle
28. Heat Transfer Between Soil and Atmosphere
- All of the solar energy reaching the earth is either reflected or radiated out into space, otherwise the earth would quickly become hotter.
- Earth atmosphere contains carbon dioxide that absorbs radiant energy emitted by the earth, radiant energy that would otherwise pass directly into space if the carbon dioxide were absent--this how carbon dixode absorbs energy and heats the earth atmosphere.
- The atmosphere is heated three ways: carbon dioxide absorbs radiant energy emitted by the earth surface, the lower atmosphere becomes heated when it comes in contact with warm earth surface, moisture in the soil and open bodies of water absorb energy when they evaporate and release it into the upper atmosphere when the water condenses into clouds. The heat trapped in evaporated moisture and released when it condenses is called latent heat.
29. Heat Flow in Soils & Soil Temperatures
- Heat absorbed by the soil surface raises the temperature of the soil surface. The temperature of the soil depths increases more slowly as heat diffuses into the soil profile.
- Moist soil must absorb much more heat before it temperature rises than dry soil because water can absorb much more heat than soil minerals for each degree of temperature rise.
- The temperature of the soil surface changes almost as much and almost as rapidly at the air temperature in contact with it, but the slow rate of heat diffusion downward into the soil and upward from the soil depths causes temperatures below the soil surface to change less and lag behind temperature changes at the soil surface.
- Beyond a certain depth, the soil temperature does not change at all because heat does not have sufficient time to diffuse that far into the soil during the warm season before heat begins to diffuse back upward during the cool season.
30. Carbon Storage in the Soil & The Global Carbon Cycle
- A small, but very important, fraction of solar energy is trapped as biomass and, subsequently, humus through the process of photosynthesis by plants.
- Photosynthesis by plants has altered the composition of earth atmosphere, reducing the carbon dioxide content to much below the primordial level and adding virtually all of the oxygen now in the atmosphere.
- Because soil temperatures determine the rate of biological activity, the rate plant residue decomposes to humus and eventually carbon dioxide, water and ash slows as the soil become cooler. For that reason, soils in cool regions tend to store much of its carbon in the form of humus while soils in warm regions tend to store much of its carbon in active biomass.
16 Oct. Climate Change: Energy Cycle, Carbon Cycle
19 Oct. No Class
21 Oct. No Class
23 Oct. Population Growth: Fertility & Plant Nutrition
26 Oct. Population Growth: Fertility & Plant Nutrition
31. Nutrient Uptake by Plants
- All nutrients absorbed by plant roots are ions--charged atoms--dissolved in water.
- The structure of the plant root system and the plant root itself is designed to bring the plant in to close contact with as much of the soil as possible--in essence, the plant roots grow toward the nutrients rather than pulling the nutrients to the root.
- The plant root alters the chemistry and the biological activity in the soil immediately surrounding it so as to increase the availability of plant nutrients, this zone is called the rhizosphere.
- Plant nutrients reside in three soil reserves: unweathered minerals, humus, and adsorbed to the surface of soil colloids--clays and humus. Mineral weathering releases plant nutrients from minerals while microbial decomposition of humus--mineralization--releases plant nutrients from humus.
- Macronutrients (N, P, K, Ca, Mg, S) account for a few to several percent (parts per hundred) of the dry weight of a plant. Micronutrients (Cu, Fe, Zn, B, Mo) account for a very small portion of the dry weight of a plant, usually a few parts per million.
32. The Nitrogen Cycle
- Most nitrogen in the earth system is atmospheric nitrogen gas (N2). Certain microorganisms transform atmospheric nitrogen (N2) into living biomass. This transformation is called: biological nitrogen fixation.
- Most biological nitrogen fixation results from the symbiotic (mutually beneficial) association between bacteria and legume plant roots. The bacteria create nodules on legume roots, providing the ammonium (NH4+) nitrogen needed by plants in exchange for sugar needed by the bacterium.
- The ammonium nitrogen released from decomposing plant residue and humus is easily adsorbed to clay surfaces, but some microbes transform ammonium (NH4+) nitrogen into nitrate (NO3-) nitrogen which clays do not adsorb and which is easily leached from the soil by percolating water.
- The nitrogen cycle is completed by yet another community of soil microorganisms--denitrifing bacteria--that transform nitrate (NO3-) nitrogen into atmospheric nitrogen (N2).
28 Oct. Population Growth: Fertility & Plant Nutrition
33. Phosphorus & Iron Nutrition
- The phosphorus used plants is released by mineral weathering and mineralization of humus. Most phosphorus cycles between living biomass and humus, with small inputs form mineral weathering
- Soil clays bind phosphorus so strongly to their surfaces that adsorbed phosphorus is unavailable for plant uptake.
- The release of phosphorus from humus and plant residue depends on the level of biological activity. The release of phosphorus from minerals or adsorbed to soil clays depends on soil acidity--phosphorus is most soluble and, therefore, most available to plants when the pH is mildly acidic to neutral (pH 5 to 7).
- Though the iron content in all soils is more than sufficient to meet plant nutrient requirements, the solubility of iron is very sensitive to soil acidity.
34. Plant Nutrient Deficiencies
- Nitrogen is the most common deficient nutrient.
- Plant nutrient deficiencies interfere with normal growth, causing leaf damage or abnormal growth. Long before visible symptoms appear, plant growth is slowed.
- A plant is deficient in a particular nutrient if the plant shows a positive growth response (i.e., it produces more tissue) when a soluble form of the nutrient is added to the soil. Eventually, as the amount of the deficient nutrient is increased the growth response diminishes until the plant has a sufficient supply to meet its growth needs.
30 Oct. Population Growth: Fertility & Plant Nutrition
2 Nov. Erosion: Water Movement, D&T, Erosion Control
4 Nov. Erosion: Water Movement, D&T, Erosion Control
35. Common Themes in Soil Erosion
- Silty soils are the most erodible. Clays resist erosion because they are the adhesive that binds soil aggregates together. Wind and water cannot transport soil aggregates and sand because they are too large.
- Erosion requires the detachment of fine particles and their transport in suspension. Detachment can occur by raindrop or saltation. Both wind and water, if moving with sufficient velocity, can hold fine particles in suspension long enough to transport them large distances.
- Cover, whether from vegetation or residue, protect the soil surface from detachment and reduces both wind or runoff velocity sufficiently to reduce transport.
- Planting windbreaks or building terraces accomplish the same thingÑthey reduce the velocity of the transporting medium be it wind or water.
- Climate contributes to erosion risk, influencing rainstorm intensity, prevailing wind velocities and amount of vegetative cover.
36. Impact of Soil Erosion
- Because humus and clay are so easily transported, they are preferentially lost during erosion. The loss of humus and clay will accelerate the process of erosion because soil aggregate stability diminishes and detachment becomes easier.
- Highly weathered soils often contain B horizons with a high clay content, as is common in southeastern U.S. Because water percolates slowly through clayey soils and because there is less plant-available water in clayey soils, removal of the surface A horizon exposing the clayey B horizon will increase runoff and reduce the plant-available water. Erosion of these types of soils results in land degradation that cannot be reversed.
- Eroded sediments degrade the environment both in transportÑlowering visibility during dust storms or harming aquatic life in streams carrying high sediment loadsÑand upon depositionÑburying the land surface or filling lakes and reservoirs.
- The nutrients bound to eroded clay and humus can upset nutrient cycles, stimulating algal blooms and killing aquatic organisms by depleting the water of dissolved oxygen.
6 Nov. Erosion: Water Movement, D&T, Erosion Control