Soils

1. Soils In this lecture period, we wish to learn: • What is "soil" and why are soils important and fragile? • What is weathering and what are the factors responsible for weathering? • How are nutrients cycled through soils? • How are human activities influencing the Earth’s soils, and hence productivity? • Definition: soil is a dynamic natural body capable of supporting a vegetative cover

2. Why Are Soils Important? • All our elements came from the dying throes of large stars • Our proteins are built from amino acids (comprised of a NH3 group, a carboxyl group, and a variable CH chain) coming from plant life • Plants need nutrients to help form proteins • Plants synthesize amino acids from sunlight, water, and nutrients derived from interaction with soils Composed primarily of weathered materials, along with water, oxygen and organic materials, soil covers most of the land surface with a fragile mantle.

3. Nutrients Although living tissue is composed of carbon, hydrogen, and oxygen in the approximate proportion of CH2O, as many as 23 other elements are necessary for biochemical reactions and for the growth of structural biomass. • Examples of important nutrients: • Nitrogen: the proteins found in plants and animals contain about 16% by weight of nitrogen. • Phosphorus: part of the important protein ribulose biphosphate carboxylase and is part of ATP - adenosine triphosphate, the universal molecule for energy transformations • Calcium: Is a major structural component of plants and animals • Others: Magnesium, Potassium, Iron, Sulfur, etc. C, N, and S come from the atmosphere; Ca, Mg, K, Fe, and P from rock weathering processes

4. Soil Horizons Typical soil profile developed on a granite bedrock in a temperate region. “O” horizon: at the surface, composed of organic matter or plant litter "A" Horizon: marks the beginning of the mineral soil. Usually darkest in color and contains the most organic content of the mineral soil profile. Minerals in the A Horizon are mostly clays and insoluble minerals. "B" horizon: has relatively little organic material, but contains the leached soluble minerals, including silicate clays, humus, Fe and Al oxides. Material is leached from the O and A horizons, enriching the B horizon. "C" horizon: parent material - slightly broken-up bedrock, typically 1-10 meters below the surface Many other types of soils exist, depending on climate

5. Types of Soils • Soils vary in color, content, pore space, depth, and acidity (pH) • Texture refers to relative amounts of different sizes and types of particles • Five major soil orders are mollisols, alfisols, spodosols, oxisols, and aridisols • Most of world’s crops are grown on grassland mollisols, and on alfisols exposed when deciduous forests are cleared

6. Soil Texture • Soils vary in content of clay (very fine particles), silt (fine particles), sand and gravel (coarser particles) • Soil texture (chart) affects soil porosity • Soil porosity and permeability affect how water, gases and nutrients are available to plants • loams, with roughly equal amounts of sand and silt, with less clay, are best for most crops http://res.agr.ca/CANSIS/

7. Soils of the U.S.

8. World Soils http://hum.amu.edu.pl/~zbzw/glob/glob1.htm Source: FAO

9. Global Vegetation http://hum.amu.edu.pl/~zbzw/glob/glob1.htm

10. Soil’s Goods and Services “below that thin layer comprising the delicate organism known as the soil is a planet as lifeless as the moon” (Jacks and Whyte) • Vegetation requires soil to grow • all nutrients come from soil • soil, along with the vegetation and bacterial life it supports, purifies water and ameliorates runoff • soil is necessary for life

11. Plant Roots "A single plant of winter rye, 50 cm high, was found to have a root system consisting of 143 main roots, 35,600 secondary roots, 2.3 million tertiary roots, and 11.5 million quaternary roots. The root system was found to have a total length of 600 km and a total surface of about 250 square meters“ - Emiliani, 1992. • Plants obtain inorganic minerals from the soil (e.g. NO3) and incorporate their elements into biochemical molecules. • Animals may eat plants and each other, and synthesize new proteins, but the building blocks are the amino acids originally synthesized in plants. There are no vitamin pills in the natural biosphere!

12. Hubbard Brook Experimental Forest

13. Sources of Nutrients The pool of nutrients held in the soil and vegetation is many times larger than the annual receipt of nutrients from the atmosphere and rock weathering Life husbands nutrients effectively on land, storing much of the total in the humus

14. Net Primary Production Net Primary Production (NPP) = photosynthesis - respiration NPP can be measured in units of energy or in tons of carbon. Most often we think in terms of the amount of carbon produced by life. NPP (global) = 60 x 1015 g C /yr Measurement of NPP in ecosystems is not easy, but methods are well developed for forests, grasslands and the oceans. The direct harvesting of plants for food, fuel and shelter accounts for about 6% of the total.

15. Now, back to soils Chimney Rock

16. Igneous Rock Weathering b) Chemical weathering (decomposition) is due to water, oxygen, and acids that results from biochemical activity. Most intense in wet, hot areas, this results in plant nutrients in solution. Together, physical and chemical weathering transform the igneous rock into sand and clay particles and dissolved salts. a) Physical weathering (disintegration). As rocks heat and cool they expand and contract, cracking apart. Water, by erosion and freeze-thaw cycles, and abrasion by wind-blown sand or glacial ice masses, grind down rocks.

17. Feldspar a general term for a group of aluminosilicate minerals containing sodium, calcium, or potassium and having a framework structure. the most common mineral in the Earth's crust In humid tropics many granite boulders can be kicked into a pile of grains - because the feldspar grains, which originally form an interlocking crystal network, weather to a loosely adhering clay called kaolinite. K Al Si3 O8 ---> Al2Si2O5(OH)4 (feldspar) (water) (kaolinite) Notice this equation does not balance exactly - the extra potassium (K) and silicon (Si) not appearing on the right hand side are carried away in solution. We say that the potassium and silicon is leached away and that the feldspar is hydrated (water consumed). This process is accelerated by carbonic acid!

18. Rock Weathering Igneous rocks + acid volatiles = sedimentary rocks + salty oceans • Due to interaction of the atmosphere with the Earth’s exposed crust • Volcanic gases (e.g., C, S) dissolved in water produce acids that react with surface minerals. • Later, oxygen in the atmosphere reacted with reduced materials. • With the advent of land plants, soil minerals have been exposed to high concentrations of carbon dioxide maintained in soil pores as a result of decomposition and the metabolic activities of roots. • The reaction of this carbon dioxide with soil water produces Carbonic acid (H2CO3), which determines the rate of rock weathering in most ecosystems. • CO2 + H2O ---> H2CO3 • (gas) (liquid) (solution) • • Acid rain, produced by human effluents of nitrogen and sulphur • gases will increase the rate of rock weathering in downwind areas.

19. Rates of Weathering a) A 3000-year old Eqyptian Obelisk b) Same Obelisk 100 years after moving to New York Weathering rates are a strong function of local climate

20. How Fast Does a Rock Decay? • Iron Nails rust quickly in soil (years). • Iron nails rust more slowly in drier environments (100's years) • Aluminum cans decay very slowly • Glass decays even more slowly. • Plastic is considered non biodegradable. • Soils aid rock decay, as do melt/freeze cycles and bacteria Soils are both a factor in weathering and a consequence of it. The production of soil is a positive feedback process. Rates of Weathering of clean rock surfaces (m/1000yr) Rock Cold Climate Warm, Humid Climate basalt 10 100 granite 1 10 marble 20 200 Bottom line: average rate of formation of topsoil is 10-20 millimeters per thousand years

21. Acceleration of Weathering Weathering proceeds via the interaction of the moist atmosphere (or soil) with bare rock. The greater the surface area of rock, the faster the weathering. The total surface area (for the same original volume of rock) increases as the number of particles increases. As a boulder breaks into smaller ones along fractures and joints, much more of the bulk's surface becomes available for chemical decay. This is a positive feedback mechanism

22. Products of Weathering Reactions Igneous rocks + acid volatiles = sedimentary rocks + salty oceans The products of rock weathering are carried to the ocean where they accumulate as dissolved salts or in deposits of sedimentary rock. Rock weathering is crucial for the release of biochemical elements that have no gaseous form: Ca, K, Fe, and P.

23. Soil Erosion • Main forces are wind and flowing water • “rivers are the gutters down which flow the ruins of continents” • sheet erosion occurs when surface water moves across the landscape in a wide, even flow. • in rill erosion, surface water forms small rivulets that flow at high velocity through miniature valleys. • in gully erosion, the rivulets join together to form larger paths of high velocity flow.

24. Erosion by Wind High winds can blow away loose soils from flat or hilly terrain. During May 1934, the entire eastern half of the U.S. was blanketed by a massive cloud of topsoil blown off the Great Plains. Ships 200 miles out in the Atlantic Ocean received noticeable deposits of Great Plains top soil.

25. Erosion by Water Soil erosion occurs both incrementally, as a result of many small rainfall events, and more dramatically, as a result of large but relatively rare storms. Erosion due to small, common events may appear insignificant on the field, but the cumulative impact can be equally severe. Water and soil splashed during a raindrop impact. Source:http://www.eci.ox.ac.uk/ld/ ldintro.htm

26. Soil Erosion and Renewal • Soil, especially topsoil or A-Horizon, is classified as a renewable resource • Provided: average rate of erosion is not greater than average rate of renewal • renewal or soil generation rate = 1 inch per 500 years (range: 220 - 1,000 yrs) • annual erosion rate is 18 - 100 x greater • we cope (for awhile) with fertilizers

27. Soil Erosion Worldwide Accelerated erosion is a result of humankind's unwise actions, such as overgrazing or unsuitable cultivation practices, which leave the land vulnerable during times of erosive rainfall or windstorms. Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm

28. Erosion’s Impacts - “On-site” Erosion's most serious impact may well be its threat to the sustain-ability of agricultural productivity, which results from the the 'on-site' damage which it causes. Crops are particularly reliant on the upper horizons of the soil, which are the most vulnerable to erosion by water and wind. Eritrea, NE Africa. Source: http://www.eci.ox.ac.uk/ld/ ldintro.htm

29. Erosion’s Impacts - “Off-site” ‘Off-site' impacts occur when eroded sediment and agricultural chemicals (which move with the eroded sediment) move into, and pollute, downstream watercourses and water bodies. Where inputs of agricultural chemicals are high - as in the more affluent nations - costs of removing such pollutants from drinking water can be considerable. Sediment plume in the Yangtse River Delta, China. NOAA image. Source:http://www.eci.ox.ac.uk/ld/ ldintro.htm

30. Soil Erosion in Affluent Countries The damaging on-site effects of erosion, in terms of decreased agricultural productivity, are well known in the developing countries of Africa and Asia. In erosion-prone areas of the more affluent countries, productivity may be maintained in the short to medium term by increased fertilizer input. The effects of erosion thus receive less attention by farmers in developed countries. Farming in Mediaeval times. Source:http://www.eci.ox.ac.uk/ld/ ldintro.htm

31. Dust Bowl A consequence of ploughing the prairies and removing the tall grasses

32. The Dust Bowl • U.S.: • 1930’s Dust bowl caused by plowing the prairies (previously the topsoil was retained by long-root grasses) • Soil Conservation Service (SCS) established in 1935 • Great Plains: lost one-third of its topsoil in 150 years • Iowa: lost half of its topsoil • California: topsoil eroding 80 times faster than it can be regenerated • Each day: topsoil eroded would fill a line of dump trucks 3,500 miles long! Costs estimated a $125B/yr! • Global: • topsoil is eroding faster than it is replaced over one-third of the world’s croplands • Each year we must feed 90 million more people, with ~24 billion metric tons less topsoil

33. Soil Conservation • Various practices serve to conserve soil, mainly by keeping the soil covered with vegetation • conservation-tillage: disturb soil less, plow in spring rather than fall, leave cover plants • contour farming and terracing on gently and more severely sloping land • strip cropping alternates crop and cover crop • gully reclamation and windbreaks • a long-term study of Coon Creek in Wisconsin found soil erosion was reduced to 6% of its dust bowl peak, based on sediment accumulation