Class on Soil composition & dynamics:
Services Supplied by Soil
Soil represents an important component of a nation’s assets, one that takes hundreds to hundreds of thousands of years to build up and yet very few years to be lost. The loss of productivity through mismanagement is thought to have ushered many once flourishing societies to their ruin (Adams 1981). Today, soil degradation induced by human activities afflicts nearly 20 percent of the Earth’s vegetated land surface (Oldeman et al. 1990).
In addition to moderating the water cycle, soil provides five other interrelated services. First, soil shelters seeds and provides physical support as they sprout and mature into adult plants. The cost of packaging and storing seeds and of anchoring plant roots would be enormous without soil. Human-engineered hydroponic systems can grow plants in the absence of soil, and their cost provides a lower bound to help assess the value of this service. The costs of physical support trays and stands used in such operations total about US$55,000 per hectare (for the Nutrient Film Technique Systems; FAO 1990).
Second, soil retains and delivers nutrients to plants. Tiny soil particles (less than 2 microns in diameter), which are primarily bits of humus and clays, carry a surface electrical charge that is generally negative. This property holds positively charged nutrients—cations such as calcium and magnesium—near the surface, in proximity to plant roots, allowing them to be taken up gradually. Otherwise, these nutrients would quickly be leached away. Soil also acts as a buffer in the application of fertilizers, holding onto the fertilizer ions until they are required by plants. Hydroponic systems supply water and nutrients to plants without need of soil, but the margin for error is much smaller—even small excesses of nutrients applied hydroponically can be lethal to plants.
Third, soil plays a central role in the decomposition of dead organic matter and wastes, and this decomposition process also renders harmless many potential human pathogens. People generate a tremendous amount of waste, including household garbage, industrial waste, crop and forestry residues, and sewage from their own populations and their billions of domesticated animals. A rough approximation of the amount of dead organic matter and waste (mostly agricultural residues) processed each year is 130 billion metric tons, about 30 percent of which is associated with human activities.A wide array of decomposing organisms—ranging from vultures to tiny bacteria—that extract energy from the large, complex organic molecules found in many types of waste.
The simple inorganic chemicals that result from natural decomposition are eventually returned to plants as nutrients. Thus, the decomposition of wastes and the recycling of nutrients—the fourth service soils provide— are two aspects of the same process. The fertility of soils—that is, their ability to supply nutrients to plants—is largely the result of the activities of diverse species of bacteria, fungi, algae, crustacea, mites, termites, springtails, millipedes, and worms, all of which, as groups, play important roles.Some bacteria are responsible for "fixing" nitrogen, a key element in proteins, by drawing it out of the atmosphere and converting it to forms usable by plants and, ultimately, human beings and other animals. Certain types of fungi play extremely important roles in supplying nutrients to many kinds of trees. Earthworms and ants act as "mechanical blenders," breaking up and mixing plant and microbial material and other matter (Jenny 1980). For example, as much as 10 metric tonnes of material may pass through the bodies of earthworms on a hectare of land each year, resulting in nutrient rich "casts" that enhance soil stability, aeration, and drainage (Lee 1985).
Finally, soils are a key factor in regulating the Earth’s major element cycles—those of carbon, nitrogen, and sulfur. The amount of carbon and nitrogen stored in soils dwarfs that in vegetation, for example. Carbon in soils is nearly double (1.8 times) that in plant matter, and nitrogen in soils is about 18 times greater (Schlesinger 1991). Alterations in the carbon and nitrogen cycles may be costly over the long term, and in many cases, irreversible on a time scale of interest to society. Increased fluxes of carbon to the atmosphere, such as occur when land is converted to agriculture or when wetlands are drained, contribute to the buildup of key greenhouse gases, namely carbon dioxide and methane, in the atmosphere (Schlesinger 1991).
Soil-water dynamics:
Processes which alter soil structure
1. Leaching- is the removal of dissolved materials by water percolating down. What moves down is often a function of the pH of the water itself...see otes on cation-exchange..
2. Illuviation- the deposit of leached materials in lower layers- especially Fe, Al, clay, humus in middle layers; other nutrients settle even further down.
3. Runoff & erosion can completely erode off the Organic & A layers- due to topography & agricultural practices. In the SE, most of the O/A layer is long gone, with exposure of the B layer - as typified by the red (Fe) layers you see all over the landscape ( as a result of illuviation)
4. Capillarity & evaporation can actually cause nutrients to rise-with a drought, or on hillsides, water and what is dissolved in it can flow up (hill) ! ....
Soil Biota
What are the important biological micro & macrobiota of the soil?
Observe the diagram... give me several examplea of organisms that alter soil structure and the significance of their actions....
How do theses creatures alter the soil? in terms of
- water holding capacity?
Scientists have unearthed a new clue in the search for ways to combat soil erosion, the science journal Nature reported ..Matthias Rillig from the Carnegie Institution of Washington in California found that atmospheric carbon dioxide helped bind soil particles together.A healthy soil has many small particles which are structured into clumps called aggregates. These aggregates provide a myriad of tiny caves and cracks that help hold nutrients, water, and organic matter.
"Soil aggregation is important for preventing soil loss through wind and water erosion," Rillig wrote in Nature. "Our finding that an increase in soil aggregation could be brought about by atmospheric change may have implications for studies of soil stabilization in ecosystems," he added.
Rillig and his team of researchers also found that carbon dioxide can affect soil's fertility.
The researchers examined soils from two ecosystems and exposed them to a range of different carbon dioxide concentrations for three to six years. They found that carbon dioxide appeared to stimulate changes in soil structure, leading to an increased abundance, stability, or size of aggregates.
A fungus that forms symbiotic relationships with plants seemed to be instrumental in the process, they said.
Increased carbon dioxide often leads to increased photosynthesis. Some of the extra carbohydrate produced by this is transferred to the fungi, which in turn provide the plants with nutrients.
Using some of the extra carbohydrate, the fungi produce more of a protein-sugar complex called glomalin, which appears to play a critical role in producing and stabilizing soil aggregates.
"We think the fungi may use glomalin as a kind of lubricant, allowing them to extend their hyphae &emdash; a root-like filament structure &emdash; through the soil. But it also appears to act as a kind of glue, holding particles together," said Christopher Field, one of the research team.
Physical & chemical composition of soil:
Soil profiles: their significance and production.
Carefully contrast the profile make-up of young vs. mature soils.
What causes a soil's profile to change with age?
Down South, the O and A layers no longer exist? why? how can agriculture change the face of our soils?
How long will it take to develop a new O & A layer?
Soil particle size & it's impact on soil fertility.
How do the particles which make up the soil affect soil properties? how does the soil surface to volume ratio affect its properties?
Nutrient availability - clay with the highest surface to volume ratio holds nutrients the tightest via cation-exchange availability
Clay minerals, or phyllosilicates, consist of strongly bound sheets of silica tetrahedrae and alumina octahedrae which are held together by only weak interatomic forces between the layers, often hydrogen bonding from water. In kaolinite, one of the most important clay minerals, a single sheet of corner connected Silica tetrahedrae is connected by common apex oxygen atoms to a single sheet of edge-connected alumina octahedrae (it is called a 1:1 phyllosilicate).
Water dynamics: Sand allows water to readily percolate while clay retains it. How does this affect water availability for the roots? This is based on the fact that as the diameter of the particle increases, the surface area to volume ration will decrease. It is on this surface area that water is held, thus the larger the diameter the less water held.
Given this fact, what should be the 'best' ratio of soil particle types?.... this soil known as loam is made up of all 3 size classes - clay, silt and sand.
How does soil formation occur? what forces ( wind, river, freezing etc. ) are most critical in our region?
Acidity and soil nutrient availability:
What observations can you make based on your interpretation of the graph below? what happens to nutrient availability when soils become more acidic or alkaline.
Given that plants can alter the pH of soil by addition of organic acids ( i.e.. conifer needles which are very acidic) how may they use this strategy competitively?
How does man's intervention with acidic deposition affect our forests, grasslands? how might this affect wildlife in the long run? Already animals show nutrient deficiencies which are altering bone and physiological processes in Sweden.
Soil profiles in different ecosystems:
Explain why the difference in the horizon profiles you see below. What factors including abiotic: ppt, wind, topography and bioticforces help to modify their profiles.
For example, why does the desert soil have no O or E layers? Why would you suppose that the grassland soil is the richest by far? How is it that deciduous forests have fair to poor nutrient loads of minerals given that they support so much tree mass?
