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Properties of soils (teach)


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Exposes the elementary science student to the idea there are three major kinds of soil found on earth as well as the very important remains of dead plants and animals called humus. Discusses soil and humus along with as some of the properties of each.

Published in: Education, Technology

Properties of soils (teach)

  1. 1. Properties of the Different Kinds of Soilby Moira Whitehouse PhD
  2. 2. Remember physical properties are thethings we can observe about a substanceusing our five senses.For soil, the two main physical propertiesare color and texture.Color tells us something about the plantnutrients that are found in the soil.Texture, determined by the size ofparticles in the soil, affects the soil’s abilityto hold water and thereby sustain plantgrowth.
  3. 3. Soil ColorThe most obvious property when lookingat soil is its color.Geologist officially recognize over 170different soil colors.But the most common color of soils are shadesof black, brown, redand gray. Red soil
  4. 4. Generally speaking, the darker a soil, themore nutrients it contains.The darker color often indicates an increase indecomposed organic matter known as humus.Gray soil often indicate poor drainage, whilered soil may indicate a lack of nutrients.
  5. 5. infertile red soil black fertile soil Photos courtesy of USDA
  6. 6. Soil texture however, not color, is the singlemost important physical property of the soil. Knowing the soil texture alone will provide information about: • 1) how easily water flows through it • 2) its water holding capacity • 3) how well plants will grow in it
  7. 7. The the size of the particles that make upsoil determines its texture and greatlyaffects the previous three things—howfreely water flows through it, how muchwater it retains and how well plants growin it.Remember the distinguishing characteristicof our three types of soil formed fromweathered rock (sand, silt and clay) is thesize of their particles.
  8. 8. The three types of soil formed from weathered rock Sand Silt Clay largest smallest particles
  9. 9. The size particles that make up each typeof soil determines the size of the poresbetween the particles. soil particle poreThe pores in the soil hold air and water.The larger the particles making up thesoil, the larger the pores between them.
  10. 10. Let’s look at the particlesthat make up sand.• < 2 mm to > 0.05 mm• particles are visible without microscope• rounded or angular in shape Images from Wikipedia Commons Particles under a microscope
  11. 11. Because of the sizeof the particles, sand• feels gritty• does not stick together in a mass unless it is very wet.• has fewernutrients for plants than silt or clay• has pores between sand particles that allow free drainage of water and entry of air• holds little water and is prone to drought
  12. 12. Sandy soilSandy soil has large particles with largespaces or pores between them.Therefore, water drains through sandy soilquickly. Sandy soils do not hold or retainwater very well. As a result, it is not goodfor growing most plants.Humus added to sandy soil acts like asponge, absorbing and holding moisture andany nutrients dissolved in it.
  13. 13. Silt• particles < 0.05 mm to > 0.002 mm• particles not visible without a microscope• erosion by glaciers often responsible for formation of silt
  14. 14. • feels floury powdery -- smooth like silly putty when wet.• wet silt does not stick together and cannot mold it into different shapes.• smaller particles than sand -- retains more water for plants and have slower drainage than sand.• easily washed away by flowing water – highly erosive.• has more plant nutrients than sand.
  15. 15. • < 0.002 mmClay • particles are flat plates or tiny flakes • when mixedwith water the small particles of clay donot settle Clay particles under a microscope
  16. 16. • clay is very powdery when dry and very sticky and slippery when wet. Wet clay can can be molded readily into any shape or a rod. • can be easily formed into long ribbons • swells when you add water and shrinks and become hard when the if water evaporates
  17. 17. • pore spaces between particles are very small.• water and air move very slowly through clay• tremendous ability to hold water.
  18. 18. Clay soilClay soils has small particles and small spaces orpores between them.Clay soil tends to stick together causing water tofill up the air spaces. Since moisture does not drain from this soil well, clay soil is often too wet for plant roots to absorb oxygen. As a result, they rot. Adding humus to clay soils discourages the small particles from sticking so tightly together, resulting in larger spaces that drain water more easily and hold more air.
  19. 19. The three different kinds of soil formed fromweathered rock have different sized particlesand pore spaces.
  20. 20. Which kind of soil would water drain through more quickly? Which kind of soil would retain or hold more water? Pore space in Pore space in sandy soils clay soils
  21. 21. Determining Soil Texture - Feel Method• Wet a sample of the soil in your hand• Try to roll it into a ribbon.• If it makes a ribbon, it contains clay.• The longer the ribbon the more clay in the soil.• If it does not roll into a ribbon, it is sand or silt• Gritty feel indicates sand.• Smooth floury feel indicates silt.
  22. 22. Moving water, as in streams andrivers, carries soil. When the water slowsdown, it drops or deposits the soil.Sand and clay behave differently from oneanother when mixed with water that ismoving and when the water stops moving.To demonstrate this, you can add sand andclay to water in a plastic water bottle andshake it up.
  23. 23. Add sand or clay to a plastic bottle half fullof water, shake up the bottles and observehow long it takes for the particles to settle.
  24. 24. sand claySand settles rapidly to the bottom of the bottlewhen you quit shaking it whereas clay settlesmore slowly. It stays suspended in the water.
  25. 25. Observe the difference in the clarity of the waterof the sand and water mixture to the clay andwater mixture. Which is clearer? sand clay
  26. 26. Now what about theproperties of humus--the soil formed whendead plants andanimals decay. Free clip are for educational useIt is a dark brown or black (color).It feels crumbly and loose when dry andspongy when wet (texture).
  27. 27. When dead plants andanimals decay leavingbehind the humus, itaccumulates in thesecond layer of soil (A).Humus holds more water than sand or siltbut less than clay.Water does drain through humus quickly.
  28. 28. Humus contains the minerals that were partof the bodies of the dead plants and animals.It contains nutrients (minerals suchas, nitrates, phosphates, potassium, copper, zinc dissolved in water) that plantsneed to be healthy. Without thesenutrients plants will not flourish.In short, humus brings soil to life.
  29. 29. Comparing the different soils—sand, silt, clayand humus.First on their ability to hold water:Clay soils hold more water than sand, silt orhumus.Humus hold more water than sand or silt.For growing most things, clay holds too muchwater whereas sand, silt and humus hold toolittle water.
  30. 30. Second, compared by the amount of nutrients(minerals), each type of soil holds.Clay has more nutrients (minerals) thansandy soils due mainly to a processknown as leaching.When water drains through sandy soils, itoften dissolves the minerals in the rock andcarries them along with it. This condition iscalled leaching.When nutrients leach out of the soil, theyare no longer available for plants to use.
  31. 31. Humus soils have lots of nutrients forplants.Adding humus to infertile soil increases itsfertility.
  32. 32. Loam is a mixture of clay, silt, sand and humusand is the best soil for growing plants.Because loam is a mixtureof four kinds of soil, itholds the proper amountof water and provides allthe nutrients plants need. AlsoLoam is formed in needsnature when the dead humusplants and animals areleft to rot and mix inclay, sand or silt.