. . Experiment No:
4
Date: 01 - 12 - 21. STUDY OF CHEMICAL PROPERTIES OF SOIL. 

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a) pH, b) Organic matter and c) calcium carbonate

AIM: To estimate the pH of given soil sample.

Soil pH is a measure of acidity or alkalinity of a soil. pH is defined as the negative logarithm (base 10)
of the activity of hydronium ions (H+ or, more precisely, H3O+aq) in a solution. In water, it normally
ranges from -1 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. The term pH
applies to the solution, so the analysis must be conducted on a soil and water mixture. The soil is mixed
with water and allowed to equilibrate, and then the pH is measured. Several factors affect Ph
measurement. Primary among these is the salt concentration of a soil. The soil concentration may vary
with the season or with fertilizer application, and is generally greater immediately following fertilizer
application than before. The nature of the rock base, texture, precipitation, organic content etc influence
the ph of the soil. Soil ph provides information on the potency of toxic substances present in the soil. It
is indicative of the status of microbial communities and its net effect on the neutralization of organic
residue and the immobilization of available nutrients.

Mathematically pH is represented as, log 1/H = - log H+

Chemicals: universal indicator, BaSO4, soil sample, distilled water

Glassware: Test tube, watch glass, beaker, dropper, glass rod.

. . Using universal indicator. 1.
1g air dried soil was taken.. 

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7.
The ph was checking using a colour chart.

1.
Take a small amount of soil in a watch glass.

3.
Cut half an inch of the ph paper & dip in the sample.

4.
Observe the colour change and note down the ph.

The given soil sample showed___green____ colour after the reaction. On matching with the colour
chart, the ph of the soil sample was observed to be ___5.5______.

RESULT: The colour on the pH paper and when pH indicator is added turns to green. The pH is seen to
be 5.5 and 8.5

The soil sample 1 has a pH of 5.5, which is moderately  acidic in nature. If the same sample is tested
with an indicator the pH comes out to be 8.5. Hence, the given soil is not fit for agricultural use, as the
pH of the soil should be between 6.5 to 7.5

1.
http://vlab.amrita.edu/?sub=2&brch=193&sim=1549&cnt=1

2.
http://www.d.umn.edu/~pfarrell/ORGANIC%20MATTER%20LAB.htm

3.
MM. Rai, Principles of soil science, second edition 1981.

4.
Nylec. Brady,The nature and properties of soil, 8th edition, April 1980.

5.
H.D Foth, L.M. Turk,Fundamental of soil science, 1972.

. . b) Organic matter in soil:. AIM: To estimate the amount of organic carbon in the soil sample.. 

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AIM: To estimate the amount of organic carbon in the soil sample.

Estimates of total organic carbon (OC expressed as C) are used to assess the amount of organic
matter in soils. The method measures the amount of carbon in plant and animal remains, including soil
humus but not charcoal or coal. Levels are commonly highest in surface soils but wide variations from
almost zero to above 15% C are possible.

The role of soil organic matter in relation to soil fertility and physical conditions: Sources of plant
nutrients which are released in assimilated forms during microbial degradation. Soil organic matter
contains 5 % Nitrogen, 0.5 % each of Phosphorus and Sulfur. Thereby having the ratio of 10:1. The
organic matter of a soil is estimated by the amount of Carbon present, as this element represents on an
average, 52-58 % of the organic matter and the C: N ratio is 10-15.

Soil organic matter represents the remains of roots, plant material, and soil organisms in various stages
of decomposition and synthesis, and is variable in composition. Though occurring in small amounts in
soil, organic matter has a major influence on soil aggregation, nutrient reserve and its availability,
moisture retention, and biological activity. The term ‘Organic Matter in Soil’ embraces the whole known
mineral fraction of soil and consists essentially of a series of products, which range from decayed plant
and animal tissues to fairly amorphous brown to black material normally defined as ‘soil
humus’. Organic matter also contains living and dead microbial tissues, microbially synthesized
compound and derivatives of these materials produced as a result of microbial decay. The soil
productivity is determined primarily by organic matter as they provide food micro organisms. It also
provides a storage house for the exchangeable and available cations. A mixture of complex compound
having an average composition given below:
Chemical elements                           humus in soil (%)
Carbon                                                    50
Oxygen                                                   35
Nitrogen                                                   5
Hydrogen                                                 5
Ash                                                           5

. . The determination of soil organic carbon is based on the Walkley-Black chromic acid wet oxidation
method. The Oxdisable matter in the soil is oxidized by 1NK₂Cr₂O₇ solution. The reaction is assisted by
the heat generated when two volumes of H₂SO₄ are mixed with one volume of the dichromate. The
remaining dichromate is titrated with ferrous sulphate. The titre is inversely related to the amount of C
present in the soil sample.. 

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The determination of soil organic carbon is based on the Walkley-Black chromic acid wet oxidation
method. The Oxdisable matter in the soil is oxidized by 1NK₂Cr₂O₇ solution. The reaction is assisted by
the heat generated when two volumes of H₂SO₄ are mixed with one volume of the dichromate. The
remaining dichromate is titrated with ferrous sulphate. The titre is inversely related to the amount of C
present in the soil sample.

Potassium dichromate acts as an oxidizing agent in presence of a strong acid like H₂SO₄ .it oxidizes the
excess of organic carbon amount of K₂Cr₂O₇ solution and is then titrated against 1N ferrous sulphate
(FAS) using di phenyl amine as indicator.

Chemicals: 1N K2Cr2O7 , H2SO4, D/W, H3PO4, DPA indicator, 1N FAS

Glassware: conical flask, burette, pipette, beaker, measuring cylinder

1)
Take 0.5 g of soil sample in conical flask.

2)
Add 5ml of 1N K₂Cr₂O₇ and 10ml of conc H₂SO₄.

3)
Mixed well and allowed to stand for 30 min.

4)
Then 10ml of D/W and 5 ml of H₃PO₄ and 6-8 drops of DPA indicator was added.

5)
The solution was titrated against 1NFAS,till purple colour turned to green

6)
A blank was prepared with D/W with all the chemicals.

Solution in conical flask: Soil Sample + 1N K2Cr2O7 + H2SO4 + D/W +  H3PO4

. . Difference
8.5 cm3
17.0cm3
17.5cm3. FOR SOIL
Burette reading
1
2
3. 

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Two molecules of K₂Cr₂O ₇ are required to produce 2[O]molecules which oxidizes 1 atom of carbon

i.e 8 e.q of K₂Cr₂O₇= 1 e.q of O₂
1 ml of 1N K₂Cr₂O ₇ = ½ e.q of O₂ = 8mg of O₂
In CO₂, 12g of C = 32 of O₂
……(1)

3g of carbon = 8g of O₂
or 3ml of C = 8mg of O₂…… (2)

From (1) & (2) we get 1 ml of 1N K₂Cr₂O₇= 3mg of carbon
Amount of organic carbon =[(X-Y)x 3mg of C x 100/2×W]×1.3, where X= blank titration reading and
Y=Sample Reading and W= weight of the soil used

Amount of organic matter = amt of organic carbon x 1.72

The soil sample contains___1.74____ g% of organic matter

REFERENCE:
1)
Book “Environmental science”Author – Balram pane,Edition first (1987),Pg no-131,132

2)
Book “Practical methods on ecology and environmentalscience” Author – R.K Trivedi and
D.KGoel,Publisher – Environmental publishing

3)
http://www.d.umn.edu/~pfarrell/ORGANIC%20MATTER%20LAB.htm

4)
http://faculty.fortlewis.edu/shuler_p/soils/Lab%20PDF%20files/Soils%20Lab%209.pdf

organic carbon fin

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