Environmental Chemistry

Environmental Chemistry
Part 4 Pedospheric Chemistry
4.1 Soil Structure and Components

4.1
土壤组成
(Soil Components)
4.1.1 土壤化学组成 (Chemical components of soil)
Fig. 4.1 structure of solid, liquid
and gas phases in soil
Soil:multiphase system (solid, liquid, gas)
– lithogenic,biogenic,atmogenic and
hydrogenic components
Solid phase in soil:soil minerals (90%,
including primary and secondary
minerals), organic matters(1%-10%)
and organism
Liquid phase in soil:water and its
solutes
Gas phase in soil:between the matters
in different phases exist holes and
gaps (taking up 35% of the whole
volume of soil) where are filled with air
The ratio of matters in three phases in
soil: mainly in solid phase, the relative
amount of matters in other two phases
depends on the conditions.

Fig 4.2 the general ratio of different components in
silty soil (粉砂土壤)
Most soils contain four basic components: mineral particles, water,
air, and organic matter. Organic matter can be further sub-divided
into humus, roots, and living organisms. The values given above are
for an average soil.

Soil layer
Overburde
n layer (A 0 )
Eluvial
layer (A)
Illuvial
layer(B)
Parent
material
layer (C)
Bedrock
(D)
Typical soil stratification
A 00
A 0
A 1
A 2
A 3
B 1
B 2
B 3
D
C C
C S
C T
Loose litter layer, undecomposed
Dark hemic oranic matter layer
Dark humus layer
Offwhite podzol layer
Transition layer to B layer,
similar to A layer
Transition layer to A layer, similar to B layer
Brown and brick red illuvial layer
Transition layer to C layer
CaCO 3 accumulation layer
CaSO 4 accumulation layer
Gley layer
Fig 4.3 synthetic profiles of natural soil
The most bio-active
layer in soil and most
of the soil organic
matters exist in this
layer, where the
eluviation of metallic
ions and clay particles
is the most significant.
Receive the organic
matters, salts and
clay particles from
the upper layer
Made of
weathered
pedogenic
rock
Unweathered
bedrock
possible specific layer

Fig 4.3b Typical layers
found in a soil profile
Soil Profiles
• Most soils have a
distinctive profile (剖面图)
or sequence of horizontal
layers.
• Generally, these horizons
result from the soil
processes of eluviation (淋
溶作用) and organic
activity.
• Five general layers are
normally presented in a
typical soil: O, A, B, C, and
R horizons (Fig. 4.3b).

• The O horizon
is the topmost layer of most soils.
• It is composed mainly of plant litter at various levels of
decomposition and humus (腐殖质).
• Below it is the A horizon. This layer is composed primarily
of mineral particles, which has two characteristics: it is the
layer in which humus and other organic materials are
mixed with mineral particles, and it is a zone of
translocation from which eluviation (淋溶作用) has
removed finer particles and soluble substances, both of
which may be deposited at a lower layer.
• Thus the A horizon is dark in color and usually light in
texture and porous. The A horizon is commonly
differentiated into a darker upper horizon or organic
accumulation, and

a lower horizon showing loss of material by eluviation.
• The B horizon is a mineral soil layer which is
dominated by illuviation (淀积作用). It receives
material eluviated from the A horizon.
• This layer also has a higher bulk density than the A
horizon due to its enrichment of clay particles. The B
horizon may be colored by oxides of iron and aluminum
or by calcium carbonate illuviated from the A horizon.
• The C horizon is composed of weathered parent
material that has not been yet significantly affected by
the pedogenic processes (成土过程) or translocation
and organic modification.
• The R horizon
consists of unweathered bedrock.

1.
Soil minerals
Soil minerals are formed through the process of physical
and chemical weathering, which are devided into
primary and secondary minerals according to its cause
of formation.
(1)Primary minerals:They are clasts from the process of
physical weathering in different degrees that all kinds of
rocks (mainly magmatite) are subject to. Since there’s no
occurrence of chemical weathering, the chemical
components and crystal structure remain unchanged.
Among all the primary minerals, quarts are most
difficult to be weathered, followed by feldspar. Thus quarts
and feldspar constitute the grain skeleton of soil.
Secondary minerals include pyroxene (辉石类), hornblende
(角闪石类) and mica that are easy to be weathered. They
all together provide a lot of inorganic nutrition to plants.

three steps of rock weathering include: oxidation,
hydrolyzation and acid hydrolyzation.
e.g. olivine (橄榄石)
Fe(II)
is oxidized
to become Fe(III)
Hydrolyzation:
acid hydrolyzation:

Chart 4.1
main types of primary minerals in soil
Primary minerals Properties
silicates
oxides
sulphides
phosphates
Including minerals such as feldspar (e.g. KalSi 3 O 8 ), mica
(e.g. K(Si 3 Al)Al 2 O 10 (OH) 2 ), pyroxene (e.g. (Mg,
Fe) 7 (Si 4 O 11 ) 2 (OH) 2 ) and olivine (e.g. (Mg, Fe) 2 SiO 4 ), most of
which are unstable, easy to be weathered and to release
elements such as sodium, potassium, calcium, magnesium
and iron, etc. that are aborpt by plants and form new
secondary minerals.
Including rutile (TiO 2 ), quarts (SiO 2 ), hematite (Fe 2 O 3 ) which
are highly stable, difficult to be weathered and contribute
little to nutrition for plants.
Mainly in the forms of alkaline compounds, ad. pyrite and
marcasite. These two minerals are isomeric, easy to be
weathered and the main source of sulphur element in soil.
Phosphorite is the most widespread phosphate mineral in
soil, including flurapatite (Ca5(PO4)3F) and (Ca5(PO4)3Cl),
followed by iron phosphates, aluminum phosphates and
other phosphates. It is the main source of phosphorus in
soil.

(2) Secondary minerals:most are new minerals formed
after the process of chemical weathering of primary
minerals, the chemical components and crystal structure
are both changed. They inlude all kinds of simple salts,
secondary oxides and aluminum silicate minerals, etc.
Due to their water solubility, the simple salts of
secondary minerals are easy to be eluviated, thus are
rare in soil but rich in saline soil. Trioxides and
secondary aluminum silicates, commonly known as
secondary clay minerals, are the most tiny soil minerals.
Many of the physical and chemical properties of soil
such as absorbency, dilatability and contractility and
sticking tendency, etc. are related to the clay minerals
contained in the soil, especially to the type and amount
of secondary aluminum silicates.

chart4.2
Secondary
minerals
Simple salts
Trioxides
Secondary
aluminum
silicates
main types of secondary minerals in soil
Examples Properties
Calcite (CaCO3), dolomite (Ca,
Mg(CO3)2), plaster
(CaSO4·10H2O), epsom salt
(MgSO4·7H2O), rock salt (NaCl),
Glauber’s slat (Na2SO4·10H2O),
bischofite (MgCl2·6H2O), etc.
Rubinglimmer (Fe2O3·H2O),
limonite (2Fe2O3·3H2O),
gibbsite (Al2O3·H2O)
Kaolinite (Al2(OH)4Si2O5), turface
(Al2(OH)4Si4O10), ledikite
(Al0.66(OH)2Si3.34O10)
Water soluble salts and easy to
be eluviated. The final product of
chemical weathering of primary
minerals, simple crystal structure.
Commonly seen in the soil of
arid or semi-arid zones.
The product of complete weathering
of silicate minerals, simple crystal
structure. Commonly seen in the
soil of wet and hot tropical or
subtropical zones.
Commonly exist in soil and
varied in types. The product of
weathering of primary silicate
minerals like feldspar. The main
component of soil.

•Ledikite (or hydromica) a type of mineral with a low
rate of weathering, distributed in common soil,
especially in the soil of arid temperate zones. Particle
diameter less than 2 mm,low dilatability, high cation
exchange capacity, potassium-rich (K2 O 4%-7%)
•Turface is the product of further weathering of ledikite,
formed under alkaline conditions from basic rocks,
relatively rich among arid temperate zones. Particle
diameter less than 1 mm, very high cation exchange
capacity. However, the water absorbed in turface are
difficult to be used by plants, thus cracks are commonly
found in soil containing a large amount of turface and
the plants growing on them are easily subject to lack of
water.

•
•
Kaolinite is highly weathered, commonly seen in the
soil of steaming zones, relatively rich among the soil
that are formed on the residual parent material of
granite. Particle diameter about 0.1-5.0 mm, low
cation exchange capacity. Soil rich in kaolinite has
fine permeability of water, from which plants can get
more water for their growth. But plants ofen suffer
from nutrient deficiency due to the weak nutrient
supplying and preserving capability of kaolinite .
The difference in soil properties between ledikite,
turface and kaolinite is closely related to their
respective crystal structures. (refer to the textbook
p203-205)

2.
Organic matter
• Any of a class of carbonaceous organic matters. It
commonly takes up less than 10% of total weight of
the solid soil. However, it’s an important part of soil
and the main symbol of soil formation and has big
influences on soil properties.
• Organic matters mainly come from the residual of
animals, plants and microorganism and are divided
into two series: non-humic substance and humus

(1)
Non-humic
substance:
It’s the simple product of decomposition of biorelict and
generally includes the following organic matters:
nonnitrogenous organic matter such as simple monose,
organic acid, amylose, resin, fat, waxiness and lignine,
etc.;nitrogenous organic matters mainly including protein,
which are largely contained in plant residue, animals
and microorganism in soil; ash content matters,
remaining inorganic matters after the combustion of
plants.

(2) Humus:
It’s dark, formless macromolecule organic matters
that are difficult to decompose and sophisticated in
structure, which are the product of complicated
transformation from animal residue under the effect
of soil microorganism.
Including humic acid, fulvic acid and humin,etc. , it’s a
special type of organic matter that is none of the
existing organic matters. Humus is divided to different
levels and series according to the dissolving behavior
in acid, alkaline and alcohol. There are recently more
research about humic acid (HA), fulvic acid (FA) and
humin.

•
Classification of soil humus
Fig. 4.4 Classification of soil humus

•
Soil humus is a type of ampholytoid (两性胶体)
but in most cases is negatively charged (带负电).
Carboxyls and phenyls on the particle surface as well
as the protonation of amidocyanogen (胺基) are the
main source of electrical charge (电荷).
e.g.
• The degree of ionic disassociation or protonation
mentioned above varies with pH of the medium. Thus
the electricity humus bears tends to vary and depends
on pH.

•
The components of soil humus is of great
importance to environment. These macromolecule
compounds help with the conservation of soil moisture
and heat. They have large superficial area and thus
present strong adsorption to pollutants by forming loose
congeries (疏松的聚集体) through hydrogen bond.
Besides, soil humus contains many kinds of
functional groups such as carboxyl, phenolic hydroxyl
group, alcoholic hydroxyl group, carbonyl and methoxyl,
etc. and thus present strong tendency of complexation
and chelation to metallic ion, which is advantageous
when these materials are nutrients to plants and
otherwise not.

4.1.2 soil moisture and soil solution
•
soil moisture is an important part of soil that mainly
comes from precipitation and irrigation.
• Groundwater serves as important source of soil
moisture where the groundwater level is as 2~3m as
close to the land surface.
• Besides, water vapor in the air can become soil
moisture through condensation.

Part of the precipitation goes back to the atmosphere
through vaporization after reaching the ground while
the rest seeps into the soil.
Overland runoff (地表径流) comes into being when
precipitation overtakes the infiltration capacity of soil
and water accumulates on the land surface.
Water seeping into the soil can enlarge the reservation of
water between the pores and gaps in soil while at the
same time flows slowly into deeper soil.
The root system of plants absorb water from soil and
leave the surplus water which further seep to the
underground and serve as new supply for
underground water source. The whole process is
called water circulation.

From the prospective of function relationship, water in
different states possesses energy different in amount.
The contact between water and dry soil makes the
extending water cover the surface of soil particles in a
water-membrane form, the process of which will reduce
the activity and energy of water particles.
The strong adhesive force of soil surface can hold water
on the surface of soil particles which is called
absorbed water that almost remains immobile and
contributes little to plant growth.

Those surplus water particles unaffected by soil
absorption can stay on the water membranes by
cohesion(内聚力)(hydrogen bond between water
particles).
The outer layer of membrane water, called
cohesion water or capillary water, is at a higher
level of energy and easier to transport
compared to absorbed water.
2/3 of the water in the water membrane in soil is thought
to be effective to plant growth.

• Formation of water solution is the result of exchange
of materials and energy between the soil components
in three phases thus it has complicated components.
Commonly seen solutes include inorganic colloid,
inorganic slats, organic compounds, complexing
compounds and soluble gases, etc. Properties of soil
such as osmotic potential (渗透势) and nutrient are
crucial to organism living in soil.

• Different types of soil presents different ability to hold
water. Water is easily lost in sandy soil due to the
loose structure, relatively large pores and gaps of the
soil; clay soil holds water well for the compactness
and relatively small pores and gaps of the soil. Soil
water is both the main source of plant nutrients and
the medium for the transport to other environmental
elements such as hydrosphere and biosphere.

•
•
4.1.3 soil air
Mixture of different types of gas within the soil
pores and gaps is generally defined as soil air.
Soil air mainly comes from the atmosphere,
whose components are similar to that of the
atmosphere, mainly containing
O 2 , N 2 , CO 2 and
moisture, etc. Besides, there are certain special
components in soil air that can not be found in
atmosphere such as H 2 S, NH 3 , H 2 , CH 4 , NO 2 and
CO, etc., which is the result of biological and
chemical reactions in soil. On the other hand, some
alcohols and other volatile materials can enter soil
through vaporization.

• Soil air is different from air in that firstly, soil air is
discontinuous and exists in pores and gaps
separated by solid soil, with different components;
secondly, the moisture of soil air and content of
CO 2 are higher while the oxygen content is lower
than that of air. e.g. the CO 2 content (weight
fraction) is only 0.02%~0.03% or at most 0.05%
in air while it is 0.15%~0.65% in soil air.
• Soil air is an important component of soil that
plays a crucial role in the microorganism activity,
transformation of nutrients in soil and plant growth.
Thus, the conditions of soil air is an important
factor of the fertility of soil.

4.1.4 microorganism in soil
• There are many types of microorganism in soil such
as bacteria, fungi, rayungus (放线菌), algae and
protozoa (原生动物), etc. microorganism is also in
large amount, with 100 million to tens of billion units
in just a gram of soil or hundreds of thousands of
kilograms of microorganism in an acre of plough
layer of soil. The more fertile the soil, the more
microorganism. The functions of microorganism in
soil are as below:

• (1) decomposition of organic matters Only through
the action of microorganism can the litters of crops
and organic fertilizers entering the soil decay,
decompose and are released as nutrients and used
by new crops; also form humus and improve the
physical and chemical properties of soil.
• (2) decomposition of minerals
•
e.g. phosphor bacteria can release phosphorus
from phosphorus ores and potassium bacteria can
release potassium from potassium ores.

• (3) fixation of nitrogen Nitrogen takes up 80% of air but
this large amount of nitrogen can not be directly used by
plants.
• There exists a type of microorganism called azotobacter
(固氮菌) in soil that can take in the nitrogen in air when
they are alive.
• This kind of fixed nitrogen can then be used by plants after
they are dead and decompose. Azotobacter is divided into
rhizobium (根瘤菌) and autotrophic azotobacter (自生固氮
菌), the former grows inside the root nodule (根瘤) of
legumina (豆科植物), helping explain why the growing of
beans can fertilize the fields in that the nitrogen fixation by
rhizobium raise the amount of nitrogen in soil; the latter
can fix nitrogen just by living independently in soil.

Besides, there also exists some microorganism
that would do harm to soil.
e.g. denitrifying bacteria can reduce nitrates to
nitrogen and release it to the air, thus bringing
loss of nitrogen to soil.
The application of deep ploughing, additional
use of organic fertilizer, spreading of lime to
overacid soil, reasonable irrigation and drain,
etc. can promote the propagation of
microorganism that are beneficial to the soil
and make them fully play the role of fertilization.

4.2
Particle-Size and Texture Classification
Systems of Soil
4.2.1
Particle-Size Classification Systems of Soil Minerals
It is the
classification of
soil minerals
according to the
size of the
particles. Soil
minerals of the
same
classification are
similar in
components and
properties,
otherwise not.
International standard for classification of soil particles

Standard for classification of soil
particles in our country
There are mainly two classification systems
country:
of soil particles in our
1、USSR 卡庆斯基制 classification of soil particles (concise system)
0.01mm is the dividing line for classification of soil particles. Soil
particles with diameter over 0.01mm are called physical sand
grains while less than 0.01mm are called physical clay grains.
2、Commonly used classification standard now in our country:
established in 1995;
8 classes: 2~1mm very coarse sand;1~0.5mm coarse sand;
0.5~0.25mm medium sand;0.25~0.10mm fine sand;0.10~
0.05mm very fine sand;0.05~0.02mm coarse silt;0.02~
0.002mm fine coarse silt;less than 0.002mm clay particles

4.2.2
•
Mineral Components and Physicochemical
Properties of Soil Particles
Mineral Components
The weatherability (抵抗风化的能力) of each type
of mineral is different, thus the distribution of them in
each particle classification differs.
• Quarts has strong weatherability and thus commonly is in the
form of coarse soil particle.
• Mica and hornblende are weatherable and thus commonly in the
form of relatively tiny soil particles.

•
Chemical components
• The smaller the soil particles, the more nutrients they
contain (Ca, Mg, P, K, etc.)

•
Physicochemical properties and fertility
1)
Pebble and gravel, sand, silt, clay
pebble and gravel
mammocks (岩石碎块) with the diameter over 1mm;
commonly seen in the soil of some mountain areas and river
beaches. Soil with lots of pebbles and gravels usually has big
pores and gaps, where the loss of water and nutrients is quite
easy.
2)sand
mainly are primary minerals such as quarts,
feldspar, mica and hornblende, etc., particularly quarts;
diameter about 1-0.05mm; commonly seen in the soil of fluvial
plains. Soil with lots of sands also has big pores and gaps, thus
has high air ventilation and water permeability, low ascending
height of capillary water (

3) silt
also called dust sand or facing sand (面砂) the mixture of
primary minerals such as feldspar, mica and hornblended, etc.
(especially chrome mica 白云母) and and secondary minerals such
as secondary quarts, kaolinite, ferric oxide hydrates and alumina
hydrates (especially secondary quarts); diameter about 0.05-0.5mm;
largely contained in loess (黄土); with intermediate physical and
chemical properties between those of sand and clay: weak
constipating and cementing properties, strong dispersity, fine
moisture retention and fertility preserving capability.
4) clay
mainly secondary minerals; diameter less than0.001mm. Soil
with lots of clay is abundant in nutrients and has strong constipating
property; it also has fine moisture retention and fertility
preserving capability but bad air ventilation and water permeability.

4.2.3 classification of soil textures and
characteristics
•
•
•
•
soil textures:the presented coarseness of the
mixture of soil particles of different particle sizes;
Classification of soil textures:classified by the
relative percentage of the amount of each size class
of particles in soil
Stanard for classification of soil textures:
international system, U.S. system and U.S.S.R.
system.
三级分类法:by the percentage of sand, silt and clay,
divided into sand soil, silt soil, clay loam (粘壤土) and
clay soil, four 4 classes and 12 grades (international
system and U.S. system )

表 4.6
国际制

Standard for classification of soil textures
in our country
表 4.7

Relationship between soil textures and its
properties
Soil textures can in certain degree reflect the
chemical components of soil miners while the physical
properties of soil are also closely related to the size of
soil particles which affects the conditions of soil pores
and gaps and thus has big influence on soil moisture,
air, transfer of heat and transformation of nutrients. Soil
with different textures present different properties.

•
表 4.8
Clay loam (壤土) combines the advantages of sand soil and clay
soil while eliminates their disadvantages and thus has perfect
textures.

Graphic model of soil types
Fig 4.5 Ternary graph of soil textures
Sandy clay砂质粘土
Silty
clay
粉质粘土
Loam 壤土
Sandy loam 砂质壤土
Silt loam 粉砂质壤土
Clay loam 粘壤土
Sandy clay loam
土
砂质粘壤
Silty clay loam 粉质粘壤土
Loamy sand 壤质砂土
• Source for graphic: USDA Soil Survey Manual, Handbook # 18,
1993, page 138 Graphic modified for instructional use.