What is Soil (and Why is it Important)?: Crash Course Geography #17 - Free Educational videos for Students in K-12

What is Soil (and Why is it Important)?: Crash Course Geography #17 - By CrashCourse

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00:0-1	before 1960 the Aral Sea was one of the four
00:02	largest lakes in the world and covered 68,000 square kilometres
00:06	. It was supplied by the waters of the Amu
00:08	Darya and Syr Darya rivers and the soil nearby grew
00:11	a lush variety of plants . But during the soviet
00:13	era , fields of cotton and rice took over the
00:15	region , using most of the waters of these rivers
00:17	and their tributaries , And by the 1980s only a
00:20	trickle of New River water made it to the Aral
00:22	Sea , so it began to shrink as the liquid
00:24	water molecules evaporated , they left behind all the salt
00:27	that had been dissolved in this water . This salinization
00:30	covered the newly exposed soil in a salty white crust
00:33	, which then blocks plants from being able to absorb
00:36	water and nutrients in the nearby fields . Aral sea
00:38	water would still be pumped in for the crops ,
00:41	but rapid evaporation continued to leave behind a thick crust
00:44	of salt on the soil , so the sea was
00:46	being destroyed to grow crops . But the crops couldn't
00:48	grow because of more salinization . As the Aral Sea
00:51	continued to shrink and more sediment was exposed . The
00:54	salt and dirt , along with fertilizers , pesticides and
00:57	other pollutants that built up over time , whirled into
00:59	massive dust storms and were transported far and wide .
01:02	All that stuff eventually spread over thousands of square kilometres
01:06	. An endangered valuable cotton and other crops elsewhere in
01:08	the region and what once was a fertile area by
01:11	the Aral Sea looks like a desert with scrubby vegetation
01:14	and a salty crust coding the land . The destruction
01:16	of the Aral Sea has been called the worst environmental
01:19	disaster of the 20th century . And one moral of
01:21	this real life parable is that the soil is a
01:23	living , dynamic and precious substance that's deeply affected by
01:27	how we manage our land and resources , with the
01:29	right composition care and natural cycling . Soil supports entire
01:33	ecosystems so we shouldn't take it for granted . I'm
01:36	al is a career and this is crash course geography
01:46	, soils bring together all four spheres of physical geography
01:50	and understanding soil composition is kind of like baking like
01:53	a rich carrot cake that needs just the right amount
01:55	of flour , water , spice , carrot and cream
01:58	cheese frosting . To make a delicious treat , soils
02:01	are a complex collection of minerals , organic material ,
02:04	air and water is just the right proportions for plants
02:06	to thrive . The flower in our soil cake is
02:08	the parent material , rocks that are broken down by
02:11	plants , animals , wind and water . The size
02:13	of these rock particles determines the soils texture and structure
02:17	like sandstone is very strong , so it makes a
02:19	chunky coarse textured soil . All the non living in
02:21	organic soil minerals come from the parent material rocks So
02:25	we usually end up with elements commonly found in rocks
02:27	like silicon , aluminum , oxygen , and iron .
02:30	And we get different compounds of those elements as chemical
02:32	processes break down the rocks unlike the inorganic minerals ,
02:35	the organic material in soil or humus comes from living
02:39	things like leaves and partly decomposed plants and animals .
02:42	Human supplies , energy and nutrients and influences the color
02:45	, texture , structure and chemical properties of the soil
02:48	and how much water and air it can hold .
02:49	The organic material is like chunks of carrots and walnuts
02:52	in our cake . And just like in a cake
02:54	, the right amount of water and air helped create
02:56	the perfect texture . Let's cut a wedge of our
02:59	soil cake to see how it all comes together .
03:01	This slice is a soil profile with layers called soil
03:04	horizons that each have different properties on the surface is
03:06	the o horizon made of some of that humus like
03:09	extra chopped walnuts . We Sprinkle on top of our
03:11	cake , soils rich in humus are workable , which
03:14	means they have good porosity or capacity for holding water
03:16	Below . This is the A horizon commonly called topsoil
03:20	, which is like the top layer of our cake
03:21	, with a rich cream cheese frosting slathered on top
03:24	. It has tons of nutrients and decomposed organic material
03:27	. So like if we lived in a world where
03:29	frosting is nutritious , the O . And A horizons
03:31	hold a vast hidden world of biodiversity , or all
03:34	different plants , animals and microbes that exist . In
03:37	fact , a quarter of our planet's biodiversity is made
03:39	up of soil organisms in the ground , small land
03:42	mammals , borough and redistribute the soil earthworms , aerate
03:45	soil and improve soil structure and microscopic organisms break down
03:48	organic material hold important nutrients or bind soil particles together
03:52	. But this teeming Life in soils wouldn't be possible
03:55	without precipitation . Just like a cake would be dry
03:57	without liquids . Water absorbs minerals in the soil and
04:00	become soil water carrying nutrients farther down so plants can
04:04	absorb them with their roots . Any extra soil water
04:06	that feeds and sticks to soil particles is called capillary
04:09	water , which plants can use during dry periods ,
04:11	but soil water doesn't stop at plant roots . It
04:14	can filter down to deeper levels and keep leeching or
04:17	depleting the nutrients from the top soil . A well
04:19	made cake is moist but also light and fluffy ,
04:22	so the spaces between soil particles not filled with water
04:25	. Hold soil air , which supplies oxygen and carbon
04:27	dioxide necessary for life below the A horizon are layers
04:31	of basically all the extras from the topsoil . Certain
04:34	soils have an E horizon made up of course sand
04:36	and silt here , finer clay and iron oxide particles
04:39	are leached and carried even further down with the soil
04:42	water . This process is alleviation , which is where
04:44	the E . Comes from . All leached materials from
04:47	the A . And E . Layers accumulate in the
04:48	B . Horizon , which is kind of like the
04:50	storage center for minerals and nutrients that get leeched down
04:53	. Scientists usually only use the word soil to talk
04:56	about the A . Through B . Horizons , that's
04:58	where plant roots are and the layers actively changed through
05:00	interactions with weather , nutrients , plants and animals .
05:04	But our full soil cake is bigger . The next
05:06	layer is the sea horizon or regular , which comes
05:09	from partly broken down parent material . This layer is
05:11	pretty unaffected by all the stuff happening above and finding
05:14	plant roots or even soil microorganisms here is pretty rare
05:17	and the our horizon is the lowest layer made of
05:19	unbroken parent material or bedrock , it's kind of like
05:22	the plate the cake sits on . If the plate
05:24	were made of hardened flower or something much older than
05:26	the soil , the inorganic minerals and rocks in upper
05:29	layers might have come from breaking down some of the
05:31	bedrock or they could have been carried by streams ,
05:33	glaciers , waves and wind from far away . A
05:36	soil profile is a complicated recipe to develop , but
05:39	I think paul Hollywood would give a star baker and
05:41	maybe even a Hollywood handshake . In a non cake
05:45	ecosystem . A few centimeters of prime farmland soil may
05:48	require 500 years to gather nutrients and build a rich
05:51	topsoil . But from Iowa to china peru to Ethiopia
05:55	and the Middle East to the Americas , the topsoil
05:57	is being worn away faster than new soil can form
05:59	and there are record levels of soil loss happening .
06:02	As of 2021 plants can't grow . An entire economies
06:05	are changing like from the salinization of the Aral sea
06:08	, It's a hard problem that we've given soil scientists
06:10	who are kind of like doctors and look after soils
06:13	to prevent such disasters from happening again . They map
06:16	and analyze soil types , determine their suitability for different
06:19	uses and lead conservation efforts using science from all sorts
06:22	of fields like physics , mineralogy , hydrology , climatology
06:26	and more to best understand the way to protect soils
06:29	. Understanding the different characteristics of soil across earth can
06:32	help . Like if we could walk along the 20
06:34	degree meridian , we'd see many specific soil forming processes
06:38	as we moved between climates . We'd start in the
06:40	shady cool of the congo rainforest under a dense canopy
06:43	of tall trees . In this rainforest climate rocks break
06:46	down rapidly and minerals are decomposed as part of a
06:49	chemical process called lateralization . We call the soil that
06:52	forms laterally meaning brick like because it's mostly a hardened
06:55	B horizon made of iron rich clay mixed with courts
06:57	and other minerals . In fact , it's so hard
07:00	, it's used as building material . Moving north ,
07:02	we find ourselves in the tropical grasslands which transitioned into
07:05	semi desert scrub and then the true desert of the
07:08	Sahara as there's less and less moisture in climates like
07:11	these when there's not a lot of moisture for trees
07:13	to grow . But grasses thrive . The soils form
07:15	through calcification over thousands of years , calcium carbonate leaches
07:20	down to the B . Horizon and creates a hard
07:22	layer called colucci . And more calcification happens as grasses
07:26	drop calcium from a horizon and return it to the
07:28	soil when they die , crossing the mediterranean sea and
07:31	heading into the alps . The topography or shape of
07:33	the land influences soil development . Like on steep slopes
07:37	, water quickly flows downhill without absorbing into the soil
07:40	and because of increased erosion , soils also have less
07:42	time to develop . The sun also plays an interesting
07:45	role in the highland climates of the mountains , south
07:48	facing slopes in the northern hemisphere that received the sun's
07:50	rays at a steeper angle are warmer and their soils
07:53	are drier . After crossing the alps , we reach
07:55	the coniferous forests of northern europe . Here , the
07:58	soil forms from pods realization , which is a word
08:00	that comes from the Russian word puzzle , which means
08:03	ascii . As pine needles decompose , they make the
08:05	soil more acidic , which leaches out aluminum and iron
08:08	compounds from the A horizon . The remaining silica gives
08:11	the horizon a distinctive ash gray color that was just
08:14	one little stretch of one meridian , but even there
08:17	no to , soils are alike and their development and
08:19	distribution depends on spatial factors like climate , vegetation ,
08:23	topography , parent material and time . No matter where
08:26	we are , soils are the foundation of life on
08:28	earth . From the local ecosystems of plants and animals
08:31	to the crops we grow and food , we eat
08:34	good , fertile soils are like gold . We seek
08:36	out at any risk like planting crops in the shadows
08:38	of volcanoes or in the flood zone on the banks
08:40	of rivers . The U . N . Food and
08:42	Agriculture Organization celebrates december five as world soil day because
08:46	soil should be celebrated . But also because our soils
08:49	are at risk , soils are a bridge between all
08:51	four of earth's physical geography systems but especially the biosphere
08:55	and the little sphere or earth solid realm which forms
08:58	a platform for plant , animal and human life .
09:01	The little sphere is shaped by internal and external processes
09:04	that build it up and wear it down . And
09:06	we'll start exploring that next time when we look at
09:08	how rocks and minerals are formed . Many maps and
09:11	borders represent modern geopolitical divisions that have often been decided
09:14	without the consultation permission or recognition of the land's original
09:18	inhabitants . Many geographical place names also don't reflect the
09:21	indigenous or aboriginal peoples languages . So we , at
09:24	Crash course want to acknowledge these people's traditional and ongoing
09:27	relationship with that land and all the physical and human
09:29	geographical elements of it . We encourage you to learn
09:32	more about the history of the place you call home
09:34	through resources like native land dot C . A .
09:36	And by engaging with your local indigenous and aboriginal nations
09:39	through the websites and resources . They provide thanks for
09:42	watching this episode of Crash Course Geography , which was
09:45	filmed at the Team Sandoval Pierre studio and was made
09:47	with the help of all these next people . If
09:50	you'd like to keep Crash Course free for everyone forever
09:52	, you can join our community on Patreon

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