Week 1: What is soil?

for this first week's of lecture

material we're really just going to kind

of introduce what soils are why they're

important and then what we're going to

cover for the rest of the course so it's

going to be a pretty short lecture video

for this week um there's some content

also on canvas for you to read over um

but it's really just about getting the

general idea of what soil science is so

we'll Begin by talking about what is the

definition of soil there's a lot of

different definitions out there that all

kind of have similar themes and

different pieces to them and some some

include whether or not it can support

plant growth or not support plant growth

but in general they're kind of all the

same so looking right here at the first

one the soil science uh Society of

America it defines it as the

unconsolidated mineral or organic matter

on the surface of the Earth now there's

a lot more to it there but that that's

the big key pieces to it so

unconsolidated mean it's not one piece

it's a bunch of granules it's a bunch of

broken up components right like if you

think about a rock that is one

consolidated collection of one or more

minerals into a single item soils aren't

that they're a unconsolidated mixture a

broken up bunch of minerals and organic

matter all

together so also mentioned already

minerals and or organic matter so

minerals are inorganic coming from the

rocks that the the soils come from

organic matter comes from living things

typically broken down plant material and

we're talking about

soil now if we look at the the book on

soil taxonomy definition here which we

will use throughout the course um it

defines it as soil is a natural body

comprised of solids minerals and organic

matter liquids gases that occur on the

land's surface so a natural body meaning

it is some natural collection of things

mostly minerals and organic matter

that's about half of what soil is then

you also have liquids gases and they all

kind of occur on the surface of the

Earth that's very similar to the

previous definition just worded a little

differently so what is soil soil is

basically five things and they they're

similar to what the soil tonomy book

defines as the components of soil first

it's minerals so minerals derived from

the parent material usually we're

talking about rocks in that case and

rocks are one or more minerals combined

into one Consolidated mass and so the

mineral minerals from soil or minerals

found in soil come from that material

usually

so whatever the minerals were in that

parent material that rock are now part

of the soil it's also organic matter

organic matter is going to give a lot of

important plant nutrients to soil and so

dead dying plants no I guess they have

to be dead plants at this time but and

the root masses that extend down below

them the decomposition of that process

and how it breaks down and put releases

nutrients in the soil all of that is

organic matter so you can think about it

as the inorganic solids the minerals and

the organic solids the living things or

the previously living things organic

matter now there are also living things

in soil so the

organisms bacteria are super super

important to so the function of soils

plants of course some definitions of

soil require that they can grow plants

so they will definitely be present in

the soil so you have microorganisms like

the bacteria you have plants you have

fungi that are going to be very

important for de decomposing organic

matter and also helping plants interact

with the soil um you also have large

animals stuff like Gophers that dig into

the soil and kind of mix the soil all up

together and we're going to have a whole

week on soil biology so we'll talk a lot

more about that soil also has a lot of

gases inside of it um soil is a big

repository of CO2 carbon dioxide from

the atmosphere it has a bunch of other

gases depending on the soil specifics um

if you ever walked across like a wetland

or something you kind of stepped in mud

and it you get like this really like

stinky smell from it that's usually

methane gas being released from anerobic

decomposition in the soil and then you

also have water water is super important

to the soil function particularly for

plants and it's about um I think it's

like 1/3 water roughly for uh that's

found inside the soil and we'll talk a

lot more about water and soil

interactions now when we talk about soil

we're usually talking about like the

grains of soil so when you look at soil

maybe a some sort of mineral soil very

low

Organics it's usually compis of what we

call three different particle sizes

sand silt and Clay we're going to talk a

lot more about this during our soil

texture week but the difference in soils

that you probably are familiar with

whether or not they make a ball you can

throw make a mud ball with them or how

easy water drains through them is really

dictated by their particle sizes so

soils broken down not only by the five

components but the size of different

components within there particularly

soil particle

sizes soil when you see soil from the

top down view like we are always are

seeing it it often looks like it's some

homogeneous mixture of particles like

whatever you see on the surface it

probably extends several feet below

maybe even more so deping on where

you're at but soils actually aren't that

they're very much heterogeneous so soils

are going to change as they move down um

into the the Earth right so we'll talk a

lot more about soil profiles later on

and you'll learn tons about this and we

talk about soil taxonomy but you can see

here at the top where we have these

plants you have this top layer here

called the a layer then you have this

this layer where looks a little bit

lighter the E layer you have this layer

that's that's in the B's the B Horizon

two different versions of you can see a

darker version here and a lighter

version there you have another B layer

right in here and then a c layer so as

you can see as you go into the soil more

the soil changes right and that has a

lot to do with where the organic matter

is at so organic matter is on top of the

soil and so so the top of the soil

typically has more organic matter the a

horizon is typically where that organic

matter sits um unless it's pure organic

matter which sits on top of the soil we

call the O Horizon which isn't present

in this picture now the interesting

thing is when soil is exposed to water

it's typically moving from the top of

the soil the surface from like rain or

irrigation and water is trickling down

to the soil surface and what that means

is the water kind of changes the way

things behave as they as it goes down

it's also true oxygen is more saturated

at the top top of the soil than at the

bottom right and so you have this

downward Force generally that's

affecting soils and this change in

environmental conditions at the bottom

from the top that cause soils to be

heterogeneous not to have the same

makeup so and different soils have

different compositions some have very

few Horizons like this some have lots

some these Horizons get all perturbated

due to different geological or

biological or um activities that occur

but we will talk a lot more about this

but start to recognize that soils are

very very complex and they they include

a lot of different um geological

biological um physiolog or phys physical

functions that kind of dictate what they

are and what they are is dictated by a

very complex history right soils develop

very very slowly there's a little quote

from the earth policy Institute but that

a layer on the top this layer up here

that's called what we call Top Soil so

when you if you ever heard the term top

soil that we're talking about that a

layer and they estimate that it takes

for about 1 inch of top soil to be

formed it takes about 500 to 1,000 years

now that's going to change depend on a

bunch of environmental factors

particularly the the warmer it is and

the wetter it is the faster soil will

develop and the drier it is and the

cooler it is the the slower soil will

develops so it's not necessarily exactly

in that range but either way it takes a

long time one inch of soil saying about

that big may take a thousand years to

develop now

that's really important because we are

kind of really mucking about with the

soil on the globe now and I'll talk

about that in a second when you look at

soil when we get to taxonomy we're going

to talk a lot about soil orders there's

basically 12 generic soil types that we

will kind of talk about and they vary

from how developed they are so very

undeveloped like something like an inol

which might not have any soil horizons

it's very very early deposits of soil to

highly develop ones that have been

around for a very long time highly what

we call weathered they've been broken

down a lot something like an oxisol in

the tropics we have a bunch of ones that

very in between there um some with

really high organic matter some with

really low organic matter and we'll talk

all about that you can kind of get an

idea of how diverse soils are by looking

at these 12 soil horizons or soil um

orders now those soil orders I'm talking

about are the names of soils and those

are like the broadest classification of

soil naming we have what soil tax what

we call Soil taxonomy which includes

these very complex highly specific

highly informative names that can be

applied to soils in any different region

of the globe there's different

classification systems that we that can

be used each country kind of has

different ones we kind of have several

ones even in the US um but what we're

learning they're kind of all similar to

what we will go over in this class but

you can see here here's a map of the

soil orders so these 12 classifying soil

orders here and here's a map of how they

occur across the US now we will learn

that soil orders for sure and we're

going to go a little bit deeper in the

taxonomy than that and we're going to

focus more on California because there's

a lot of different taxonomic

classifications of soil but if you look

around you can see it's pretty diverse

over here we have altools are really

common in the South Southeast um pretty

highly weathered soils right and then

over here in the greens we have

different mols which are very common to

Grasslands High organic matter areas and

then you look at California and it's

super super diverse extremely diverse

now you're looking around where we are

alpha stalls become very common and so

we'll be talking a lot about Alpha sols

um when we talk about soil orders and

specifically those that are around us

it's hard to recognize that soils are

very diverse there's names to soils

there's ways of classifying soils and

we're going to slowly break that down

across across the

course now soil has a bunch of different

roles that it plays in basically the

global environment right one of the

biggest ones is that it supports plant

growth if soils did not support plant

growth we probably wouldn't care about

it that much primarily when we talk

about soils it tends to go towards

agriculture a discussion about

agriculture and that is super important

that soils are important to every

environmental mechanism around right

it's important to ecology it's important

to our forest it's important to our Blue

Oaks it's important to our Valley

grasslands but it's also super important

to Agriculture and all of those are

defined by the growth of plant

so soil's ability to support plant

growth is critical and that's why we

primarily study it it also has a lot of

gas exchange with the uh the atmosphere

so soil is an

increasingly we wec we find more

information to show how dramatic the

carbon sequestration is the ability of

uh soils to sequester carbon inside of

inside of the soil out of the atmosphere

to be a huge actual carbon sink um it

does some other things too especially

primarily through plants but moving so

moving um different nutrients through

plants into the atmosphere and back now

it's a huge habitat for animals so whe

that be microorganisms or Gophers swirls

a lot of our ecosystem Engineers ones

that build substrate for other organisms

tend to be burrowers that dig into the

soil soil is also super important for

water so water moves through soil on top

of soil water soils kind of can slow

down water movement and so allow it to

retain some in water it also acts as a

filter it's a basically a gigantic

filter that as water moves through it it

often gets cleaned and filtered out it

can have the opposite effect as well but

it soil can act as a water filter it

also helps with ground water in very

much the same way it's very important

for nutrient cycling if we're talking

about the nitrogen cycle for example

nitrogen in the atmosphere is not usable

by plants and so there's microbes in the

soil that's that survive only in the

soil environment that can take M

nitrogen from the air fix it into a

usable form of nitrogen and then give it

to the plants and so Thor plays pivotal

roles in a variety of different nutrient

cycles and then from a more

anthropocentric perspective from looking

at it from a human perspective

essentially it has it's a medium for us

to grow our societies on right all of

our cities are built on soil all of our

infrastructure is built on soil soil is

super important to our societies it's

one of the reasons why another big area

where you might study soil is if you're

going down an engineering path you need

to understand how the soil will behave

and move if you're going to build on top

of it and so you also need to understand

soil physics to be an

engineer now when we're talking about

supporting of ecosystems here's a little

diagram of it you can see we're looking

inside of a soil profile here and you

can see it's very abundant the how

abundant life actually is in it we have

a bunch of microorganisms down here

right we have primary producers so ones

that produce energy we have a bunch of

primary consumers in the soil something

like an earthworm or a bunch of of

microbes that digest that the organic

material we have things that eat those

the secondary consumers different types

of beetles maybe some gophers and things

like

that and then we have tertiary consumers

like a bird that might eat those as well

but all of this is founded on the fact

that soil is there now if you've taken a

biology class we often describe the

trophic pyramid as primary producers

things that produce energy primary

consumers things that consume the

primary producers secondary consum

consume the primary and tertiary consume

the secondary and that is that it's true

but what we don't usually point out is

that the primary producers are almost

entirely supported by soil now that's

not always the case say in a marine

environment soil doesn't really matter

much there but if we're talking about

terrestrial environments almost all

plants not entirely all but almost all

are supported by soil so if you had that

Tropic pyramid underneath primary

consume or producers should actually be

like a soil layer soil without soil the

primary producers couldn't be there

either so they support a lot of

microbial life and interactions with

plants so we have a plant here you can

see that's undergoing photosynthesis it

acts as a medium for the roots to grow

and acquire nutrients right they're

getting water they're getting other

micronutrients from the soil they

produce chemical gradients for the

plants to be able to get nutrients from

them you can see here the plant puming

out hydrogen ions you get some nutrients

inside it supports these interaction

with nematodes some some good some bad

it acts as a a repository for micro rizy

that can uh facilitate the transfer of

nutrients between fungi and plants and a

bunch of bacterial symbiance that we

really don't know a whole lot about yet

but we know that they're important and

that they change depending on what soil

type they're there what plants there the

anerobic or aerobic conditions of it um

but overall just trying to paint a

picture that there's a lot of life and

things going on underneath the soil top

the

surface now talking about ecosystem

Services those those are services that

ecosystems provide us that we typically

don't pay for but that we should

consider when making decisions right so

some so ecosystem Services provided by

soils they provide habitat like we've

talked about already habitat is super

important um for not just living animals

that are there but can also be a

commodity it can be um ecotourism that

that's it's providing it could be um

birding or any other type of situation

that you want to go out and look at it

helps with air quality and comp position

so it removes CO2 from the atmosphere

typically it can put it back out

depending on the how degraded the soil

is but typically sequesters it and

decreases carbon dioxide levels it can

regulate temperature particularly if

it's somewhat of a wet soil so water has

a high specific heat capacity so it

tends to regulate temperature pretty

well especially if so if the soil is

pretty wet it can help help with

temperature regulation it helps with

nutrient cycling cycling that we've

already talked about water filtration um

now what's really important or really

interesting is the ability of soil

particularly microbes in soil to act as

a bio a mechanism for biom remediation

removing toxins from the environment say

for example if you had an oil spill an

oil spilled on the surface of the soil

if you give it a proper water proper

proper um temperature so it's not too

cold not too dry um there's bacteria in

soil that will digest oil that's called

biom remediation and they can do it with

a lot of other stuff um plants can also

be biomediators Fung can be bi

remediators and that term typically

applies to toxins that humans have

developed and put on top of the soil but

the same is kind of true for

decomposition soil there soil supplies

the microbes and the fungi required to

decompose any organic

matter now there's a bunch of other

things that soil can do that we're going

to kind of talk about through the

class but one of the big really

important ones is that soils provide

food if we're talking about plants we're

talking about food we all eat plants

even if you eat a cow eating the plants

that the cow ate and

so it's pivotal pivotal for us to be

able to grow food in soil now there are

increasingly hydroponic systems they're

not nearly at the size and scale that

would provide even a fraction of the

food that we actually need globally and

so stores are really important for

allowing food production and we'll talk

a lot about food production throughout

the

class now we've talked a lot about what

why STS are good but we're going to kind

of transition to why STS are kind of

suffering on a global scale soils are

kind of eroding away becoming less

nutrient and there's a lot of

speculation as to how bad the problem

actually is and it's still kind of

unclear despite how much we know about

soils there's still a whole lot to learn

so we what we do know is that soil

quality is degrading globally and a lot

of that has to do with agriculture so

agriculture takes a lot of nutrients out

in the form of plant material right it

also leads to erosion so when you till

soil when you like cult when you take

blades and you dig up the top layer of

soil it does a lot of harmful impact

harmful things to the soil it one it

kind of mixes up the layers a little bit

and so it allows for higher nutrient

loss it also allows for wind and water

to sweep that that soil away and as we

irrigate soil we're also taking adding

extra water onto the top which leeches

nutrients down and out of the soil or

into nearby

waterways and so one of the things that

we're seeing we'll talk more about this

particular into the course is that

conventional agricultural practices

particularly tilling which is the

largest impact but also adding things to

soil um is having a huge impact on soil

Health soil is being lost extremely

quickly meaning being eroded away and if

you remember from the very beginning of

this video soil develops very very

slowly so if it develops very slowly and

it's being eroded away we're losing soil

quite rapidly on a global scale food is

also having less nutrients in it because

the the soil itself is less nutrient

rich and so if the plants are being

grown in less nutrient-rich soil the

plants themselves are also suffering as

well fewer crops um and in some areas

where it's really drastic even

desertification where it turns into more

of an aid desert

environment okay so here's a little

diagram of kind of showing that process

out so we have a natural ecosystem here

with a typical soil pattern beneath it

when you take that natural ecosystem you

cut down the trees you start tilling the

land um you change the soil so you can

see here they're showing that erosion

that could be from again tilling and

wind just blowing it away you can also

be from adding water and water carrying

carrying the top soil away but you get a

loss of soil through it's called

erosion now that also means we lose a

lot of organic matter underneath all

this this plant material is usually a

pretty thick organic matter layer and we

tend to low reduce that when we U change

the environment so organic matter tends

to leave now with organic matter we also

tend to lose greenhouse gases so stuff

like CO2 so less CO2 is being held in in

the

soil then we tend to apply a lot of

products to soil a lot of fertilizer

which can lead to urif um sometimes

salts in different in different ways

that could be through water or

purposeful in different other impacts um

but increased salt least to decreased

plant growth and um decrease life in the

soil overall so we tend to get a lot of

biodiversity more acidified soils um

which leads to more nutrient leeching

and higher unification and we'll talk

about this process I kind of wanted to

map that out for you here early

on okay so that's kind of an intro to

what we will be talking about in the

course like a quick snapshot of

different topics we we'll kind of cover

in general and this isn't a weekly

layout but it's kind of the big

categories of what we're going to be

talking about soil formation how do

soils form how do the soil horizons

formed how do we classify different

soils what are the physical and chemical

properties of soil

soils chemistry is actually really

important to soil so we'll be talking

about quite a bit of chemistry now there

is no chemistry prere for this class and

so I'm always framing it in a way to

where it should be understandable by

somebody without a chemistry background

but I know chemistry can be tough so

please don't hesitate to come to office

hours if you need any assistance with

any of that

taxonomy um so talking about the soil

orders and sub orders and great groups

and different levels of how we classify

soils soil biology soils and water soils

and nutrients and then soil kind of

surveying and mapping um and kind of

what that looks like on a global scale

how do we map out where soils go how do

we dictate all of that or how do we

class map out where soils are and then

dictate all the different series names

and classifications of it okay that

that'll be the the lecture for this

first week um I hope you enjoy the the

first week's material as we start the

semester don't hesitate to reach out

just because it's an online course you

can still come to office hours um in

person or online on Zoom um but other

than that let me know if you have any

questions