Chemical properties of soil

talking about the soil chemical properties and its objectives ,parts and etc .it also includes soil chemistry,buffer soil,acid soil,properties of acid soil,chemical composition and so on

1. 1

2. TABLE OF CONTENTS
Chemical composition of soil
Ion Exchange
Soil ph
Development & properties of acid soil
Soil buffer capacity
Significant & management of soil ph
Effects of flooding in chemical properties
2

3. SOIL
Soil is the thin layer of organic and inorganic materials that covers the
Earth's rocky surface.
Soil is a mixture of organic matter, minerals, gases, liquids, and organisms
that together support life
Basic soil properties can be divided into three categories.
1.physical properties(Texture,structure,bulkdensity etc)
2.chemical properties(salinity, ph,organicmatter, mineral content etc)
3.biological properties (biomass,biodiversityetc)
3

4. Soil Chemistry: Soil chemistry is the collaboration of various chemical
components that exists among all particles as well as in the soil solution the water
sustained by soil.
Chemical properties with aspects: one of the three components of balanced soil is
chemical composition. Essential elements for chemically balanced soil-
*Both macronurientsand micronutrients need to be in balance.
*The phof soil should match the need of the plants
*The salt content of the soil should be low
*Difficult to alter soil buffer capacity
*cataionexchange capacity(CEC) measures the soil's capacity to hold on to cations.
4

5. *Top four elements:carbon,hydrogen,nitrogen,oxygen
*Macro
Nutrients:phosphorus,sulpher,calcium,potassium,magnesium
*Micro Nutrients or trace
elements:Boron,copper,Iron,Manganese,
zinc, chlrorine.
*Neutral Ph: 6.3-6.8 is ideal for most plant.
Chemical Composition Of the soil:
5

6. Ion exchange:
* Ion exchange is a cyclic process. That means the exchange of
ions on soil surface with ions in soil solution.
* Ion exchange involves cationsand anions that are adsorbed
from solution onto negatively and postivelycharged surfaces,
respec-tively. Such ions are readily replaced or ex-changed by other
ions in the soil solution of similar charge, and thus, are described
by the term, ion exchange.(Foth, 1990)
* There are two types of ions , they are cationsand anions.
Cation exchange is greatherabundance in soil than anion
exchange.
6

7. Cation Exchange Capacity:
* Cation exchange capacity is the value of a soil to indicate how much cationnutriens
have the soil.
* Cation exchange is the interchange between acationin solution and another cationon the sur-
face of any negatively charged material, such as clay colloid or organic colloid.
Assume for purposes of illustration that X
-
represents a negatively charged ex-changer that
has adsorbed a sodium ion (Na), producing NaX. When placed in a solution con-taining
KCI, the following cationexchange reac-tionoccurs:
7

8. In the reaction, K+ in solution replaced or ex-
changed for adsorbed (exchangeable) Na', re-sulting
in putting the adsorbed Na' in solution and leaving
K
+
adsorbed as KX. Cationsare ad-sorbedand
exchanged on a chemically equiva-lent basis, that is,
one K+ replaces one Na', and two K+ are required
to replace or exchange for
one Ca++.
There are many cationin soil but all are not
absorbed in same amount . There is an sequence
That is. :Ca> Mg > K > Na.
Percentage is :
Ca(80%)>Mg(15%)>K(1.8%)>Na(1%)
* CEC is high in clay soil and low in sandy soil
because in clay soil organic matter is high.
8

9. Anion EcxhangeCapacity:
* Anion exchange sites arise from the protonation of hydroxyls on the edges of silicate clays and on the surfaces of
oxidicclays.
* The anion exchange capacity is inversely related to soil pH and is, perhaps, of greatest importance in acid soils dom-
inatedby oxidicclays. Nitrate (N0
3
-
) is very weakly adsorbed in soils and, as a consequence, remains in the soil
solution where it is very sus-ceptibleto leaching and removal from soils.
* In general, plants absorb as many anions as cations. The availability to plants of the anions nitrate, phosphate, and
sulfate is related to miner-alizationfrom organic matter, as well as anion exchange.
Ion exchange capacity VS soil pH:
* CEC and AEC both depend on soil pH.Because pH indicates the soil's character as the soil is acidic or basic.
* To make valid comparisons of CECandAEC between soils and various materials, it is neces-saryto make the
determination of CEC at a common pH.The CEC is positively correlated with pH; therefore, acid soils have a CEC less
than the maximum potential CEC. On the other
hand AEC is negatively correlated with pH.
* The CEC at the soils' current pH is called the effective cationexchange capacity (ECEC).
* The relative proportion of acidic or basic ions on the exchange sites determines the soil's pH value.
* Sandy soils with lower pH are more subject to leaching of nutrients.
9

10. Nutrients:
Soil is made of many components. Among them organic matter is a important part for
soil's nutrient. In organic matter there stand some cationsand anions theses cationsare
mainly known as nutrients. Of the cations4 cationis very important as nutrient
because these cationsare need in plant growth, food and many other physical habits.
The sodium
may be ab-sorbedby plants, may substitute for some po-tassium, but sodium is not
considered an essen-tialelement.
10

11. Soil pH:
Determination of Soil pH:
Soil pH is commonly determined by:
(1) mixing one part of soil with two parts of distilled water or neutral salt solution,
(2) occasional mixing over a period of 30 minutes to allow soil and water. to approach
an equilibrium condition, and,
(3) measuring the pH of the soil-water suspension using a pH meter.
The common range of soil pH is 4 to 10.
•The pH of a soil is one of the most important properties
involved in plant growth.
•Soil pH is usually measured in a standard suspension of
1:2.5(weight to volume) of soil in distilled water, or in
dilute solution of calcium chloride(0.01M).
•The pH is a negative logarithm of Hydrogen Ion
activity and Soil pH is a measure of soil’s alkalinity
or acidity
11

12. Sources of alkalinity:
Carbonate hydrolysis:
It is the hydrolysis of calcium carbonate produces OH
-
, which contributes to alkalinity in soils:
CaCO
3+H
2O=Ca
2+
+HCO
3
-
+OH
-
Calcium carbonate is only slightly soluble, and this reaction can produce a soil pH as high as
8.3,assuming equilibrium with atmospheric carbon dioxide.
In calcareous soil, carbonate hydrolysis controls soil pH.
When a soil contains Na
2CO
3,the pH may be as high as 10 or more, which is caused by the greater
solubility of
Na
2CO
3 and greater production of OH
-
by hydrolysis in a similar manner.
Mineral Weathering :
The weathering of many primary minerals, however, contributes to alkalinity.
For example, the hydrolysis of anorthite, (calcium feldspar), produces a moderately strong base:
3CaAl
2Si
2O
8 + 6H
2O =HAl
4Si
6O
10(OH)
2 +3CA(OH)
2
 12

13. The generalized weathering reaction, in which M represents metal ions such as calcium,
magnesium, potassium, or sodium, is M-silicate mineral + H
2O =H-silicate mineral +M
+
+OH
-
Sources of acidity:
The following processes contribute to acidity in the soil:
•In all soils respiration by roots and other soil organisms produces carbon dioxide that reacts
with water to form carbonic acid (H
2CO
3). This is a weak acid, which contributes H
+
to the
soil solution.
Acidity is produced when organic matter is mineralized, because organic acids are formed
and the mineralized nitrogen and sulfur are oxidized to nitric and sulfuric acid,respectively.
Natural or normal precipitation reacts with carbon dioxide of the atmosphere and the
carbonic acid formed gives natural precipitation a pH of about 5.6. The results of these
reactions add acidity continuously to soils.
13

14. •Leaching of the basic cations(Ca, Mg, K and Na) results in their replacement by Al
3+
and H
+
, and
soils then become acid. This effect is brought about by hydrolysis when H
+
ions would be generated
through the following reaction :
Al
3+
+ H
→
← Al(OH)
2
+
+ H
+
HYDROXY-ALUMINIUM
Positively charged hydroxy-aluminiumspecies may undergo further hydrolysis to produce additional
H
+
ions, leading to higher soil acidity.
Al(OH)
2
+
+ 2H
2O
→
← Al(OH)
3 + 2H
+
GIBBSITE
•Again, soils of acidic character are generally rich in calcium and/or magnesium, because calcium
and magnesium may precipitate as carbonate, increasing the buffering capacity of the solution.
Ca
2+
+ CO
2+H
2O
→
←CaCO
3+2H
+
A general pattern, however, is that water dominated soils (soils of humid regions) have low pH-values,
because their content of organic and carbonic acids is often subject to replenishing and recharge by
rain fall.
Under arid conditions, however, minerals which are salts of weak acids and strong bases would
dominate the system, producing higher levels of alkalinity and causing the soil pH to raise to values
between 9 and 10 or even more. soil pH-values above 10 may indicate contamination with strong
bases such as Na, OH, Ca(OH)
2
14

15. In soils with a pH of 7, there is a balance in the processes or reactions that produce
alkalinity and acidity. On the central U.S. Great Plains this occurs with about 26
inches (65 cm) of annual precipitation (near the Iowa-Nebraska border) and the
dominant upland soils are quite fertile for agricultural crops.
The relationship between annual precipitation, the leaching of carbonates and soil
pH has shown in the following figure :
FIGURE:GENERAL RELATIONSHIP OF
UPLAND SOILSBETWEENANNUAL
PRECIPITATION,DEPTHOF LEACHING
OF CARBONATES,AND PH OF THE
SURFACE SOIL IN THE CENTRAL
UNITED STATES. (ADAPTED FROM
JENNY, 1941.)
15

16. Development and properties of acid soils
Role of Aluminum
-After leaching has removed the carbonates, a progressive loss of XMg,XCa,XKand XNa
occurs as leaching continues.
-Subsequent hydrolysis of the released Al3+ produces H+
Al3+ H2O = Al(OH)2 + H
Hydroxy-aluminum from the preceding reaction hydrolyzes to produce additional H+
Al(OH)2 +2H2O= Al(OH)3 + 2H+
Acid Rain Effects
-Acid rain is a environmental problem caused due to repidindustrialization. Acid
rain has become invisible threat to rivers,lakesand forests.
-The basic component of acid rain are nitric acid and sulphuricacid.
-Soils that contain Limestone and Calcium Carbonate can neutralize the acids.
-This causes natural,orordinary precipitionto be acidieand have a pH of about 5.6
-Leaching pushes the inosdeeper in the soil so the plants roots can't reach them.
16

17. Soil buffer capacity
In chemistry, buffer capacity is the amount of acid or base a buffered solution can soak up
before it’spH will start to change in significantl. The buffer capacity of a soil is important
in determining how it’spH will change
Features
•Various minerals in soil help to buffer Against changes in PH when an acid or
base is Added
•At high ph, calcium, magnesium and potassium oxides to getherwith carbonates
help to buffer pH changes at acidic pH, aluminum oxides and iron hydroxides
act as buffering agentss
Functions
•A higher buffer capacity means that the soil can absorb more acid and \or base
without a significant change in pH
•In general, clay soils have higher organic matter content tends to increase
buffering capacity.. 17

18. •Buffering capacity is important because it helps to stabilize the pH
•Changes in pH can affect plants in variety of ways.
•Especially by diminishing the fraction of nutrients in soil that are available to the
plants.
•Increasing uptake of undesirable minerals like aluminum
Importance
Fig:BufferingCapacity of Soils
18

19. Significance of Soil ph:
The associated chemical or biological environment of a cer-tainpH is the most important factor.
Some soil organisms have a rather limited tolerance to variations in pH, but other organisms can
tolerate a wide pH range.
Perhaps the greatest general influence of pH on plant growth is its effect on the availability of
nutrients for plants.
* Nitrogen availability is maximum between pH 6 and 8, because this is the most favorable range
for the soil microbes that mineralize the nitrogen in organic matter and those organisms that fix
nitro-gen symbiotically.
* High phosphorus availability at high pH-above 8.5-is due to sodium phos-phatesthat have high
solubility.
* Maximum phosphorus availablityis in the range 7.5 to 6.5. Below pH 6.5, increasing acidity is
associated with increasing iron and aluminum in solution and the formation of relatively insoluble
iron and aluminum phosphates.
* Iron and manganese availability increase with increasing acidity
because of their increased sol-ubility.
-Earthworms are inhibited by high soil acidity. Earthworms then inhabited the soil un-der the
markings, and the soil remaining on the tennis court remained acid and uninhabitatedby earthworms.
19

20. Management of soil pH:
There are two approaches to assure that plants will grow without serious inhibition from unfa-vorablesoil
pH: (1) plants can be selected that grow well at the existing soil pH, or (2) the pH of the soil can be
altered to suit the needs of the plants. Most soil pH changes are directed toward reduced soil acidity and
increased pH by liming.
From the management of soil pH is known to the need of the environment we have to increasing or
decreasing the level of soil pH.
Lime requirement:
The lime requirement is the amount of liming material required to produce a specific pH or to reduce
exchangeable and soil solution .
The lime requirement is the amount needed to increase or adjust pH to a more favorable value,
*Consider a moderately weathered soil with a pH of 5.0 for the growth of soybeans, which have a pH
preference of 6 to 7. The lime requirement is the amount of lime required to adjust the pH to a more
favorable value for the crop being grown; for soybeans it would be about 6.5.
* If the soils pH decreases and have 2-3 then there can't grow any crop, for this the soil get acidic. In the
soil we mixed lime . Lime increases the soil pH because lime is a basic component, after that the soils
pH fixed at 6-8 . In this pH level all types of plandand organisms can live. Thatswhy lime is used in the
acidic soil.
20

21. Liming equation:
Generally, a minor amount comes from XH unless there is a high organic matter content. Below
pH 5.5, the hydro-lysisis of exchangeable aluminum and, between 5.5 and 7, mainly from hydroxy-
aluminum hydro-lysis. The dominant aluminum form at pH 7 is AI(OH)
3, which has very low
solubility and, for all practical purposes, is inert. Lime hydrolyzes in the soil to form OH
-
:
The hydroxyls neutralize the H
+
that are produced from aluminum hydrolysis, gradually the ex-changeable
aluminum and hydroxy-aluminum are converted to AI(OH)
3, and the pH increases.
The overall reaction representing the neutraliza-tionof Al-derived soil acidity is
Exchangeable and hydroxyaluminum are precipi-tatedas insoluble aluminum hydroxide, and the amount
of XCais increased. The reduction
in acidic cationsand increase in basic cationsre-sultsin an increase in soil pH.Liming can be viewed as
reversing the processes that produced the soil acidity.
21

22. Soil Acidulation
The addition of acid sphagnum peat to soil may have some acidifying
effect. However, significant and dependable increases in soil acidity are
prob-ably best achieved through the use of sulfur. Sul-fur is slowly
converted to sulfuric acid by soil microbes, and the soil slowly becomes
more acid over a period of several months or a year. As with lime, the
amount of sulfur required varies with the pH change desired and the soil
CEC. Sulfur is used to change soil pH in large agricultural fields in arid
regions, but it is less common a practice than the use of lime in humid
regions. Sulfur is commonly used in nurseries and gardens.
22

23. EFFECTS OF FLOODING ON CHEMICAL PROPERTIES
In well-aerated soils, the soil atmosphere contains an abundant supply of oxygen for
microbial respiration and organic matter decomposition.
Oxygen serves as the dominant electron acceptor in the transformation of organic
substrates to carbon dioxide and water. When soils are flooded naturally, or artificially
as in rice production, a unique oxidation-reduction profile is created. A thin, oxidized
surface soil layer results from the diffusion of oxygen from the water into the soil
surface. This layer is underlain by a much thicker layer that is reduced .Oxygen is
deficient in the reduced zone, which initiates a series of reactions that drastically alter
the soil's chemical environment.
23

24. Changes in Soil pH
From the reactions just cited, there is a consumption of H + and an increase in hydroxyls. In most flooded rice fields,
iron is the most abundant electron acceptor (equation 3), and iron reduction
tends to increase soil pH.The production of carbon dioxide, and the accompanying formation of carbonic acid, have
an acidifying effect. Alkaline soils tend to be low in available iron, minimally weathered, and the production of
carbonic acid lowers soil pH.In acid soils the abundance of soluble or available iron results in a net increase in pH
due to hydroxyl production. As a consequence, flooding of most rice fields causes the pH to move toward neutrality,
as shown in Figure 11.15. There is an increase in nutrient availability, and soils generally do not need lime if they are
flooded and puddled for rice production. The increase in pH of highly acid soils is sufficient to reduce the threat of
aluminum toxicity
FIGURE 11.15 Flooding soil increases
pH of acid soils and decreases the
pH of alkaline soils. The pH changes
are related to the soil's content of iron
and organic matter. (Data from F. N.
Ponnamperuma, 1976
24

25. 25
At a glance
Chemical composition of soil
Ion Exchange
Soil ph
Development & properties of acid soil
Soil buffer capacity
Significant & management of soil ph
Effects of flooding in chemical properties
25

26. 26