Placeholder Image

字幕列表 影片播放

  • Before 1960, the Aral Sea was one of the 4 largest lakes in the world and covered 68,000

  • square kilometers.

  • It was supplied by the waters of the Amu Darya and Syr Darya rivers, and the soil nearby

  • grew a lush variety of plants.

  • But during the Soviet era, fields of cotton and rice took over the region, using most

  • of the waters of these rivers and their tributaries.

  • And by the 1980s, only a trickle of new river water made it to the Aral Sea, so it began

  • to shrink.

  • As the liquid water molecules evaporated, they left behind all the salt that had been

  • dissolved in the water.

  • This salinization covered the newly exposed soil in a salty white crust, which then blocks

  • plants from being able to absorb water and nutrients.

  • In the nearby fields, Aral Sea water would still be pumped in for the crops, but rapid

  • evaporation continued to leave behind a thick crust of salt on the soil.

  • So the sea was being destroyed to grow crops, but the crops couldn't grow because of more salinization.

  • As the Aral Sea continued to shrink and more sediment was exposed, salt and dirt -- along

  • with fertilizers, pesticides, and other pollutants that built up over time -- whirled into massive

  • dust storms and were transported far and wide.

  • All that stuff eventually spread over thousands of square kilometers and endangered valuable

  • cotton and other crops elsewhere in the region.

  • And what once was a fertile area by the Aral Sea looks like a desert with scrubby vegetation

  • and a salty crust coating the land.

  • The destruction of the Aral Sea has been called the worst environmental disaster of the 20th century.

  • And one moral of this real-life parable is that soil is a living, dynamic, and precious

  • substance that's deeply affected by how we manage our land and resources.

  • With the right composition, care, and natural cycling, soil supports entire ecosystems -- so

  • we shouldn't take it for granted.

  • I'm Alizé Carrère and this is Crash Course Geography.

  • Soils bring together all four spheres of physical geography, and understanding soil composition

  • is kind of like baking.

  • Like a rich carrot cake that needs just the right amounts of flour, water, spice, carrot,

  • and cream cheese frosting to make a delicious treat, soils are a complex collection of minerals,

  • organic material, air, and water in just the right proportions for plants to thrive.

  • The flour in our soil cake is the parent material -- rocks that are broken down by plants, animals,

  • wind, and water.

  • The size of these rock particles determines a soil's texture and structure.

  • Like sandstone is very strong, so it makes a chunky, coarse textured soil.

  • All the non-living inorganic soil minerals come from the parent material rocks.

  • So we usually end up with elements commonly found in rocks like silicon, aluminum, oxygen and iron.

  • And we get different compounds of those elements as chemical processes break down the rocks.

  • Unlike the inorganic minerals, the organic material in soil, or humus, comes from living

  • things like leaves and partly decomposed plants and animals.

  • Humus supplies energy and nutrients and influences the color, texture, structure, and chemical

  • properties of a soil and how much water and air it can hold.

  • The organic material is like chunks of carrots and walnuts in our cake.

  • And just like in a cake, the right amount of water and air help create the perfect texture.

  • Let's cut a wedge of our soil cake to see how it all comes together.

  • This slice is a soil profile with layers called soil horizons that each have different properties.

  • On the surface is the O horizon made of some of that humus, like extra chopped walnuts

  • we sprinkle on top of our cake.

  • Soils rich in humus areworkablewhich means they have good porosity or capacity

  • for holding water.

  • Below this is the A horizon, commonly called topsoil, which is like the top layer of our

  • cake with a rich cream cheese frosting slathered on top.

  • It has tons of nutrients and decomposed organic material.

  • [So like if we lived in an ideal world where frosting is nutritious.]

  • The O and A horizons hold a vast hidden world of biodiversity or all different plants, animals,

  • and microbes that exist.

  • In fact, a quarter of our planet's biodiversity is made up of soil organisms in the ground.

  • Small land mammals burrow and redistribute the soil, earthworms aerate soil and improve

  • soil structure, and microscopic organisms break down organic material, hold important

  • nutrients, or bind soil particles together.

  • But this teeming life in soils wouldn't be possible without precipitation, just like

  • a cake would be dry without liquids.

  • Water absorbs minerals in the soil and becomes soil water, carrying nutrients farther down

  • so plants can absorb them with their roots.

  • Any extra soil water that beads and sticks to soil particles is called capillary water,

  • which plants can use during dry periods.

  • But soil water doesn't stop at plant roots -- it can filter down to deeper levels and

  • keep leaching, or depleting, the nutrients from the topsoil.

  • A well-made cake is moist but also light and fluffy, so the spaces between soil particles

  • not filled with water hold soil air, which supplies oxygen and carbon dioxide necessary

  • for life.

  • Below the A horizon are layers of basically all the extras from the topsoil.

  • Certain soils have an E horizon made up of coarse sand and silt.

  • Here, finer clay and iron oxide particles are leached and carried even farther down

  • with the soil water.

  • This process is eluviation, which is where the E comes from.

  • All leached materials from the A and E layers accumulate in the B horizon which is kind

  • of like the storage center for minerals and nutrients that get leached down.

  • Scientists usually only use the word "soil" to talk about the A through B horizons.

  • That's where plant roots are and the layers actively change through interactions with

  • weather, nutrients, plants, and animals.

  • But our full soil cake is bigger.

  • The next layer is the C horizon or Regolith, which comes from partly broken down parent

  • material.

  • This layer is pretty unaffected by all the stuff happening above, and finding plant roots

  • or even soil microorganisms here is pretty rare.

  • And the R horizon is the lowest layer, made of unbroken parent material or bedrock.

  • It's kinda like the plate the cake sits on, if the plate were made of hardened flour or

  • something much older than the soil.

  • The inorganic minerals and rocks in upper layers might've come from breaking down

  • some of the bedrock, or they could've been carried by

  • streams, glaciers, waves, and wind from far away.

  • A soil profile is a complicated recipe to develop, [but I think Paul Hollywood would

  • give usstar baker.”

  • And maybe even the Hollywood Handshake.

  • In a non-cake ecosystem, a few centimeters of prime farmland soil may require 500 years

  • to gather nutrients and build a rich topsoil.

  • But from Iowa to China, Peru to Ethiopia, and the Middle East to the Americas the topsoil

  • is being worn away faster than new soil can form, and there are record levels of soil

  • loss happening as of 2021.

  • Plants can't grow and entire economies are changing, like from the salinization of the

  • Aral Sea.

  • It's a hard problem that we've given soil scientists who are kind of like doctors, and

  • look after soils to prevent such disasters from happening again.

  • They map and analyze soil types, determine their suitability for different uses, and

  • lead conservation efforts using science from all sorts of fields like physics, mineralogy,

  • hydrology, climatology and more.

  • To best understand the way to protect soils, understanding the different characteristics

  • of soil across Earth can help.

  • Like if we could walk along the 20 degree meridian, we'd see many specific soil forming

  • processes as we moved between climates.

  • We'd start in the shady cool of the Congo rainforest under a dense canopy of tall trees.

  • In this rainforest climate, rocks break down rapidly and minerals are

  • decomposed as part of a chemical process called laterization.

  • We call the soil that forms laterite, meaningbrick-like,” because it's mostly a

  • hardened B horizon made of iron-rich clay mixed with quartz and other minerals.

  • In fact, it's so hard it's used as building material.

  • Moving north, we find ourselves in the tropical grasslands, which transition into semi-desert

  • scrub, and then the true desert of the Sahara as there's less and less moisture.

  • In climates like these where there's not a lot of moisture for trees to grow but grasses

  • thrive, the soil forms through calcification.

  • Over thousands of years, calcium carbonate leaches down to the B horizon and creates

  • a hard layer called caliche.

  • And more calcification happens as grasses draw up calcium from the A horizon and return

  • it to the soil when they die.

  • Crossing the Mediterranean Sea and heading into the Alps, the topography or the shape

  • of the land influences soil development.

  • Like on steep slopes, water quickly flows downhill without absorbing into the soil.

  • And because of increased erosion, soils also have less time to develop.

  • The Sun also plays an interesting role in the highland climates of the mountains.

  • South-facing slopes in the Northern Hemisphere that receive the Sun's rays at a steeper

  • angle are warmer, and their soils are drier.

  • After crossing the Alps, we reach the coniferous forests of northern Europe.

  • Here the soil forms from podzolization which is a word that comes from the Russian word

  • podzolwhich meansashy.”

  • As pine needles decompose, they make the soil more acidic, which leaches out aluminum and

  • iron compounds from the A horizon.

  • The remaining silica gives the E horizon a distinctive ash-gray color.

  • That was just one little stretch of one meridian, but even there no two soils are alike.

  • And their development and distribution depends on spatial factors like climate, vegetation,

  • topography, parent material, and time.

  • No matter where we are, soils are the foundation of life on Earth, from the local ecosystems

  • of plants and animals, to the crops we grow and food we eat.

  • Good fertile soils are like gold we seek out at any risk -- like planting crops in the

  • shadows of volcanoes or in the flood zone on the banks of rivers.

  • The UN food and agriculture organization celebrates December 5th as World Soil Day because soil

  • should be celebrated, but also because our soils are at risk.

  • Soils are a bridge between all four of Earth's physical geography systems, but especially

  • the biosphere and the lithosphere, or Earth's solid realm which forms a platform for plant,

  • animal and human life.

  • The lithosphere is shaped by internal and external processes that build it up and wear

  • it down, and we'll start exploring that next time when we look at how rocks and minerals

  • are formed.

  • Many maps and borders represent modern geopolitical divisions that have often been decided without

  • the consultation, permission, or recognition of the land's original inhabitants.

  • Many geographical place names also don't reflect the Indigenous or Aboriginal peoples languages.

  • So we at Crash Course want to acknowledge these peoples' traditional and ongoing relationship

  • with that land and all the physical and human geographical elements of it.

  • We encourage you to learn about the history of the place you call home through resources

  • like native-land.ca and by engaging with your local Indigenous and Aboriginal nations through

  • the websites and resources they provide.