Soil  Science  Lecture  

Soil  is  the  interface  between  the  geosphere  and  the  biosphere  and  thus  plays  a  unique  role  in  terrestrial  ecosystems  

Ecosystem  Services  Provided  by  Soil  

Soil  provides  habitats  for  many  organisms,  both  macroscopic  and  microscopic.    These  include  bacteria,  fungi,  insects  as  well  as  vertebrates  such  as  gophers.        Soil  also  allows  for  the  cycling  of  nutrients  through  terrestrial  ecosystems.    As  soil  organisms  break  down  decaying  organic  matter,  these  nutrients  are  returned  to  the  soil  where  plants  can  then  take  them  up  again.        Soil  provides  a  substrate  for  plant  growth.    Most  land  plants  require  something  for  their  roots  to  grow  into,  so  most  plants  would  not  be  able  to  exist  without  soil.        Soil  filters  water  as  rainwater  passes  through  it  on  its  way  to  rivers  and  groundwater.    

Formation  of  soil    

Weathering  is  the  primary  process  that  forms  soil.    

Weathering  breaks  down  the  parent  material,  which  is  rock,  into  smaller  particles.    These  small  particles  of  mineral  or  rock  combine  with  decomposing  organic  

material  to  form  soil.    As  a  result,  soil  is  neither  completely  biotic  nor  completely  abiotic.      

There  are  three  types  of  weathering  that  can  contribute  to  soil  formation.    Physical  weathering  consists  of  water  

or  wind  separating  piece  of  the  rock.    It  can  also  occur  when  water  in  a  crack  in  the  rock  freezes,  wedging  the  rock  apart.      

Chemical  weathering  occurs  when  the  rock  reacts  with  surrounding  materials,  such  as  when  a  weak  acid  dissolves  part  of  a  rock.      

Biological  weathering  is  weathering  caused  by  organisms  such  as  tree  roots  wedging  a  rock  apart  or  

lichens  secreting  acids  to  dissolve  a  rock.      

Weathered  material  can  be  transported  by  wind  or  water  or  it  can  remain  in  situ  

Factors  that  influence  Soil  Formation  

There  are  a  number  of  factors  that  can  influence  the  quality  of  soil  that  forms  in  a  particular  location  and  how  quickly  that  soil  forms.  

#1  Climate:  Temperature  and  humidity  will  influence  the  speed  of  weathering.    For  example,  in  very  cold  climates,  there  will  be  little  water  movement,  so  physical  and  chemical  weathering  will  proceed  slowly.    

As  a  result  you  will  have  underdeveloped  soils  that  do  not  contain  a  lot  of  minerals  or  rocks.    They  will  mostly  contain  poorly  decomposed  organic  material  since  biological  activity  is  also  slow  at  these  temperatures.      

#2  Organisms:  Burrowing  animals  can  help  to  mix  soil  and  distribute  organic  and  inorganic  components.    

Plants  can  secrete  acids  that  can  help  to  speed  chemical  weathering.      

#3  Topography:  steeper  slopes  will  lose  soils  due  to  erosion,  so  will  not  have  very  developed  soils,  whereas  valleys  will  accumulate  large  amount  of  soils.    Wind  exposure  or  heavy  rains  on  one  side  of  a  hill/  mountain  could  speed  weathering  as  well.      

#4  Parent  Material:  The  chemical  composition  of  the  parent  material  (ei.  basalt,  limestone,  granite)  

determines  how  quickly  it  will  weather  and  will  determine  the  mineral  content  and  many  other  properties  of  the  soil.      

#5  Time:  The  more  time  has  passed,  the  more  developed  soil  will  become.    Old  soils  with  productive  ecosystems  and  moderate  rainfall  can  become  very  deep  and  fertile  (such  as  grasslands).    However,  a  

soil  with  heavy  rainfall  maybe  less  fertile  due  to  the  leaching  of  nutrients  from  the  soil.      

Regions  Differ  in  Soil  Characteristics    

Many  cultures  throughout  human  history  have  classified  soil  to  communicate  its  value  for  growing  crops.    Current  soil  classification  is  based  on  particular  properties  of  soil.    These  properties  are  so  

specific  that  in  some  cases  they  can  be  used  to  pinpoint  the  exact  location  that  soil  came  from.    Types  of  soil  are  named  with  the  suffix  

–sol.    For  example,  the  top  photo  to  the  right  represents  a  mollisol,  a  fertile,  thick  grassland  soil.    The  bottom  photo  is  an  oxisol,  which  is  a  highly  weathered  and  oxidized  soil  with  low  fertility.    It  can  be  found  

in  a  tropical  rainforest.    While  you  do  not  need  to  know  the  names  of  the  different  types  of  soils,  you  should  know  that  words  ending  in  –sol  are  particular  types  of  soil.    

Soil  Horizons  

One  of  the  ways  that  we  classify  soils  is  by  the  layers,  or  horizons.    Not  all  locations  will  have  all  horizons  

so  the  number  of  horizons  present  and  the  thickness  of  the  layers  in  certain  areas  is  one  way  of  characterizing  soils  

O  horizon:  O  stands  for  organic.    This  layer  is  sometimes  called  humus  and  consists  of  decaying  organic  material.    It  is  most  pronounced  in  forest  ecosystems.      

Below  the  O  horizon,  the  layers  go  in  alphabetical  order  with  some  exceptions.      

The  A  horizon  is  generally  below  the  O  horizon.    The  A  horizon  is  a  zone  where  minerals  and  organic  material  have  

been  mixed  together.    Another  name  for  it  is  topsoil.    In  some  ecosystems,  there  will  be  no  O,  so  the  A  horizon  will  be  at  the  top.      

Below  the  A  horizon  is  the  B  horizon,  also  called  the  subsoil.    

This  layer  contains  mostly  inorganic  material  and  is  where  nutrients  will  accumulate.    The  B  horizon  is  present  in  all  soils.      

The  C  horizon  is  below  the  B  horizon  and  is  the  least  weathered  layer  of  soil.    It  consists  of  pieces  of  the  parent  

material.    

The  R  horizon  is  below  the  C  horizon  and  is  bedrock.    R  stands  for  Rock.      

There  are  some  soils  that  can  have  extra  horizons  because  of  particular  properties  of  the  soil.    The  E  horizon  is  a  zone  of  leaching  that  can  sometimes  be  found  above  

the  B  horizon  in  acidic  soils.    (E  stands  for  eluviation)This  is  a  zone  of  leaching.    Due  to  the  soil’s  acidity,  material  is  more  likely  to  dissolve  and  be  transported  down  to  the  B  horizon.      

Properties  of  Soil  

In  addition  to  looking  at  the  horizons  present  in  certain  areas,  soils  can  be  described  and  classified  based  on  a  number  of  physical  and  chemical  properties.    This  can  tell  us  its  suitability  for  agriculture  and  help  

in  remediating  the  soil  to  rehabilitate  damaged  ecosystems  

Soil  Texture  

Soil  texture  is  determined  by  the  amount  of  sand,  silt  and  clay.    These  three  types  of  particles  differ  in  both  size  and  composition  and  determine  many  other  properties  of  soil.  Sand  is  the  largest  particle,  

followed  by  silt  and  then  clay.    Soil  scientists  can  use  a  soil  triangle  to  classify  soils  into  groups.    Try  the  following  examples:  

What  is  a  soil  called  that  is  40%  sand,  40%  silt  and  20%  clay?  Loam  (very  fertile  soil)  

What  is  a  soil  called  that  is  60%  sand,  10%  silt,  30%  clay?      Sandy  clay  loam  

What  is  a  soil  called  that  is  33%  sand,  33%  silt,  33%  clay?    Clay  loam  

Porosity  &  Permeability  

Both porosity and permeability are properties that are directly determined by the texture of the soil. Porosity is the amount of space in between the grains of sand, silt and clay that

make up the soil. Porosity is important because it determines the ease with which water, oxygen, and nitrogen can work their way down between soil particles to the root zones of plants. It also determines how quickly & easily water can soak in and become groundwater in an aquifer.

Permeability  describes  how  easily  water  passes  through  soils.    It  is  influenced  by  the  porosity  and  can  be  measured  with  a  percolation  test.    If  the  permeability  is  too  low,  the  soil  can  become  water-­‐logged,  but  

if  it  is  too  high,  the  soil  can’t  retain  enough  water  for  plant  growth.  As  a  result  the  best  soils  for  plant  growth  have  medium-­‐size  pores  or  a  mixture  of  pore  sizes.  

Chemical  Properties  of  Soil  

Plants  need  three  main  nutrients  from  the  soil:  nitrogen,  potassium  and  phosphorus.    On  fertilizer  labels  you  will  see  three  numbers  indicating  the  ratios  of  these  nutrients.    Deficiencies  in  one  or  more  of  these  

nutrients  can  limit  plant  growth,  so  soil  scientists  and  farmers  will  monitor  the  levels  of  these  three  nutrients  and  add  them  in  the  form  of  organic  or  chemical  fertilizers  if  necessary.    

Soils  can  have  a  variety  of  pH  levels  which  can  influence  the  types  of  plants  that  can  grow  there  and  the  

nutrients  available  to  those  plants.    As  I  mentioned  previously,  highly  acidic  soils  can  exhibit  leaching  of  minerals  from  the  O  and  A  horizons  so  they  will  not  be  available  to  plants.      

Soil  Erosion  

Environmental  Scientists  study  these  properties  of  soil  because  it  tells  us  about  the  health  of  the  ecosystem.    In  many  ecosystems  

the  soil  is  being  eroded  or  degraded  which  will  have  future  ramifications  on  the  health  of  the  ecosystem.      

Erosion  can  be  measured  by  inserting  fixed  pins  (see  picture)  for  a  certain  amount  of  time  and  then  measuring  the  amount  of  soil  

loss.    This  technique  has  also  been  useful  in  quantifying  strategies  to  reduce  erosion.      

According  to  these  measurements,  5-­‐7  million  ha  (12-­‐17  million  acres)  of  productive  cropland  are  lost  annually  worldwide.        

Causes  of  Soil  Degredation  

 Over  the  past  50  years,  soil  degradation  has  reduced  

global  grain  production  by  13%  

Currently  70%  of  the  world’s  rangeland  is  classified  as  degraded  

We  will  address  these  causes  later  in  the  unit  as  we  talk  about  specific  ways  that  we  use  the  land  

Promoting  Soil  Conservation  

Legal  measures  can  be  used  to  prevent  the  degradation  and  loss  of  soils.    The  US  government  enacted  the  Food  Security  Act  of  1985  to  preserve  soil.    It  authorizes  farmers  to  receive  price  supports  and  other  

benefits  if  they  adopt  soil  conservation  practices.  The  Conservation  Reserve  Program  (1985)  is  similar  in  that  farmers  are  paid  to  place  highly  erodible  land  into  conservation  reserves.    Essentially,  they  are  paid  to  stop  farming  on  some  of  their  land  and  plant  trees  and  grasses.    This  program  saves  an  estimated  771  

million  tons  of  topsoil  per  year  while  generating  income  for  farmers  and  promoting  biodiversity  by  providing  habitats  for  native  wildlife.