Lecture 16 - Soils PDF

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Lecture 16 - Soils...

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LECTURE 16 -- SOILS SOILS Like the Biosphere, Soils are at the Interface between the Atmosphere, Hydrosphere, and Lithosphere, and are part of the Biosphere Soil is like the living skin of Mother Earth -It covers most of the continental surfaces, It is made of weathered rock material -- mineral particles broken down from rock Plus air, water, and many living organisms Soil is a VERY SLOWLY RENEWABLE Resource Our Biosphere cannot live without it So we need to Conserve it Because it takes many thousands of years for soil to develop from bare rock And even hundreds of years for soil to begin forming in sediments, such as river flood deposits or sand dunes The mineral part of the soil includes oxygen, silica, iron, aluminum, potassium, nitrogen, calcium, magnesium, sodium, phosphorous, sulphur, and many others --provides nutrients for plants to grow and for maintenance of their bodies, These are minerals we need for growth, also, which we get by eating plants, or animals that have eaten plants Water is a major component of soil -Even dry soils have some water -thin films of water on mineral and organic particles -- see Fig. 23-3, p. 251 Water is necessary for the growth of plants and soil organisms for development of soil humus -- broken down organic material and is important in the development of a mature soil Water carries materials from the land surface down into the soil

Soil has Atmosphere -Air between mineral particles -enriched in Carbon dioxide, with less nitrogen and oxygen than upper air FACTORS IN SOIL DEVELOPMENT CL-O-R-P-T CLimate Organic component, Organisms Relief -- topography Parent material Time Soils are very different, develop differently, due to these five factors So they vary from place to place Climate -- particularly Temperature and Precipitation influences how soils develop If HOT and MOIST -- rapid weathering of mineral component Much Leaching of minerals -Water dissolves and oxidizes minerals in rock and carries them down deep in the soil -- especially iron and aluminum -- red, brick soils -- LATERITE These soils are not nutrient rich -- become depleted rapidly if the massive forest vegetation is removed If HOT and DRY -- Desert soils -- very slow to develop Much Calcium Carbonate accumulates in soils over time Light in color because little Iron is Dissolved and Oxidized Actually quite fertile, given more water If COLD and MOIST -- Acid soils -- Spodosols, under coniferous forest

If VERY COLD -- Ice Freezing and Thawing Stirs Up Soil Climate also influences how mineral component is Organic component -- Plants, burrowing animals, insects, worms, MOSTLY: microorganisms (Bacteria, fungi) Stir up soil, aerate Much work done by bacteria, fungi, worms Input organic material, waste products of animals, decomposed plant material -becomes nutrients for next generation Relief -Slope -- influences how thick mineral deposits, soil can get Aspect -- direction slope is facing -influences how much sunshine, how much soil moisture Parent Material -- source of mineral component Chemical makeup of soil -- depending on source rocks Bedrock underlying soil or Sediment -- transported to that place by water or wind Sonoma County: much volcanics, marine sediments -- broken down by weathering over millions of years -- much clay Soils on Serpentine -- part of Franciscan Formation metamorphic rock with some toxic chemicals --discourages most vegetation Midwest: Loess -- wind deposited Glacial Flour from Ice Age -Very rich in nutrients because rock was ground down mechanically, not chemically leached Floodplain soils -- usually fine grained, rich in nutrients because contain much organic material carried by river Time -- Soils in different environments develop at different rates Parts of soils (e.g. A horizon) develop at different rates

Over Time, Soils develop Horizons -- layers that are distinct from each other A horizon -- often dark gray or brown, or even black Where most organic material is Most interaction with atmosphere and biosphere Rainwater percolates down through A horizon, dissolving some mineral components, leaching down deeper in soil So in A, organic material concentrates, minerals carried away May be O Horizon on top: layers of decaying plant and animal litter B horizon -- Where material leached from A horizon accumulates Water percolates through, some evaporates at this level Leaves behind minerals, sometimes organic matter from A horizon Here clays, calcium carbonate, iron, or other minerals accumulate over time Bt = clay accumulation -- NOT Calcium Carbonate! C horizon -- Small amount of change in parent material R horizon -- Regolith -- broken and weathering bedrock material Because soils gradually change through time, they can be used to estimate how long a particular land surface has been stable, not eroded, not received any sediment input (deposition) This is useful when trying to reconstruct the past history of a landscape -- what kinds of changes occurred at a place in the past For example, on the North side of the Missouri River, there are places where several layers of LOESS have been deposited over the last 300 thousand yrs or so Soils have formed on the tops of some of those layers, then more loess came along and buried the soils Scientists can look at the SOILS that have developed in some of those layers to figure out 1. what the climate was like when those soils were developing,

2. about how long the climate was stable like that 3. what the climate was like before and after that time One of these soils, the Sangamon, was formed during the last interglacial, about 120,000 years ago Soil Texture -- combination of Sand, Silt, and Clay -Texture changes as soil develops -Sand, Silt, Clay - sized particles large to small 2 mm to few micrometers Sandy soil -- perhaps from breakdown of ancient marine sandstone e.g. Sebastopol well drained -- good for some crops water passes through quickly Clayey -- e.g. here around SSU from sediments and soils washing down from Sonoma Mountain not well drained -- water penetrates slowly holds water to roots -- good for some plants, but not others Silty -- in between -- dust, blows in air...