Soil and water Analysis:

Theoretical background and practical

application

Dr. Adel Aly

Laboratory of Agricultural and Environmental Chemistry

IAM-Bari

ACLIMAS Training course on “AGRONOMIC AND ENGINEERING ASPECTS OF

ADAPTATION TO CLIMATE CHANGE IN MEDITERRANEAN AGRICULTURE”

Agricultural extension and service Center of the Litani River Authority - Kherbet Kanafar,

Bekaa, Lebanon 2-6 November, 2015

SAMPLING - I

• The sampling consists in taking a small mass

(sample) from a great mass

• The sample must be representative of the whole

mass

• The concept of representativeness is the guidance

for all sampling procedures

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SAMPLING - II

• A representative sample is one that is considered

to be typical of the universe of concern and

whose composition can be used to characterize

the universe with respect to the parameter

measured (Taylor, 1988)

“The analysis cannot be better than the sample”

- an axiom

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SOIL SAMPLING - I

• The purpose of soil sampling is to obtain information

about a particular soil

• Any soil is characterized by several types of variation

and is not a homogeneous mass but a rather

heterogeneous body of materials (called “population”)

• In view of the variability of the soils, it seems

impossible to devise an entirely satisfactory method

for sampling

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SOIL SAMPLING – II

For any soil there are certain characteristics which

describe it. The true value of such characteristics

in the soil is called a parameter. The purpose of

sampling is to estimate these parameters with an

accuracy that will meet our needs at the lowest

possible cost.

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Principles of soil sampling

• Soil volumes, not areas, are sampled

• With the soils, variation and the heterogeneity

are the rule rather than the exception

• The details of sampling procedure should be

determined by the purpose for which the sample

is collected

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Principal sources of variability of

soil chemical and physical analysis

1. Sampling error

2. Sub-sampling error

3. Analytical error

The error due to sampling is generally greater than that due to sub-sampling and analytical procedures

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Main Types of soil sampling

• Systematic sampling

• Unsystematic (random) sampling

• Circle grid sampling

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Systematic sampling

• The systematic sampling plan is an attempt

to provide better coverage of the soil study

area than could be provided with the simple

random sampling plan.

• Systematic sampling collects samples in a

regular pattern (usually a grid or line transect)

over the areas under investigation.

• The samples are collected at regular intervals

in one or more directions

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systematic sampling I

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systematic sampling II

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Unsystematic (random) sampling

• The basis of most sampling plans in

environmental sampling is the concept of

random or probabilistic selection of the

sample to be collected and the subsample

that is to be analyzed.

• In random sampling of a site, each sample

point within the site must have an equal

probability of being selected.

• The same can be said for the selection of

particles within a sample

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“X” unsystematic sampling

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“W” unsystematic sampling

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circle grid sampling

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AUGER - I

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Shovel

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Pickaxe

Sampling by shovel or spoon

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Laboratory sample collection

• Laboratory identification numbers are assigned

to the bulk soil samples

• In the laboratory database, a file corresponds to

each number; the file contain the most important

information about the soil sample

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Laboratory sample preparation

• Laboratory analyses of soil samples are generally

carried out on the air-dry, fine-earth fraction (< 2

mm)

• Generally the > 2 mm fraction is sieved, weighed,

and discarded; it is excluded from chemical and

physical analyses

• Stones and gravel Ø > 2 mm

• Fine earth Ø < 2 mm

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• sample homogenization

• aggregates crushing by pestle and mortar

• sieving to 2 mm

• If it is necessary, reduce the mass of sample

Sample preparation

X

Quartering method

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Fraction > 2 mm

1000sampledryweight

mm2fractiondryweight)kg/g(graveland

Stones

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Humidity (g/kg) = wet weight – dry weight (105 °C)

wet weight X 1000

Correction factor = wet weight (105 °C)

dry weight

Sample preparation - I

• Reduction of the sample by the “quarter”

method

S

S

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Sample preparation - II

• Subdivision of the sample in two aliquots:

1. Moist sample for the analysis of pH, N, etc.

2. Dry sample (air drying) for the analysis of

further parameters

• Moisture content is determined on both aliquots

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Sample preparation- III

• Many analytical determination are performed on

very small portions of sample (< 1 g)

• In order to analyse a representative sample, it is

therefore necessary to further homogenize the

sample to very small particles ( < 0,5 mm)

• The mill could be performed through:

Ball mill

Blade mill

and by means of certified sieves

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manual mortar

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Certified sieve 28

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Automatic ball mill and blade mill

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Ball mill chambers

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Main parameters for evaluation of soil

fertility

• Particle-size analysis

• Electrical conductivity

• Organic Matter

• Exchangeable

potassium, calcium,

magnesium and

sodium

• pH

• Total carbonate

• Nitrogen

• Available phosphorus

• Cation exchange

capacity (CEC)

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Main chemical parameters for evaluation of

water quality for irrigation

• pH

• Electrical

conductivity

• Sodium

• SAR index

• Chloride (Cl-)

• Hardness

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pH

• Soil pH is probably the single most informative

measurement that can be made to determine soil

characteristics. It tells much more about a soil than

merely indicating whether it is acid or basic. For

example, availability of essential nutrients and

toxicity of other elements can be estimated

because of their known relationship with pH.

• It is the first measurement that is made in

laboratory

• On the basis of the pH value, the analyst

sometimes will choose the method of analysis (for

example, “available phosphorous”)

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Determination of soil pH (procedure)

• equipment : pH meter

• suspension soil/water or soil/dilute neutral salt (0.01M CaCl2 o 1M KCl)

• ratio soil to H2O (or salt solution) 1 :1 or 1: 2.5

• place 10 or 20 g of soil in a 50 or 100 ml beaker

• Add water or dilute neutral salt solution

• Swirl and let stand for an hour

• Insert the electrodes into the suspension (in the clear supernatant or in the sedimented soil) or the entire suspension may be stirred during the determination

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Particle-size analysis

• The aim of particle-size analysis is to split (in % or in g/kg) the elemental particles forming the fine earth in dimensional and conventional classes

• There are different classification systems, each of which uses specific dimensional classes

• Once a classification system is chosen, it is possible, by particle-size analysis, to determine the textural class of a soil

• The main classification systems are : – IUSS (ex ISSS) (International Union of Soil Society)

– USDA (United States Department of Agriculture)

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• The analysis may be performed with or without the removal of cementing agents

• The main cementing agents are:

– Organic matter

– Carbonate

– Iron oxides

– Soluble salts

• In any case, the sample is subjected to a dispersion procedure by dispersing agent (NaPO3)6

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The particle-size analysis is based on Stokes’ law

which describes the settling rate of a spherical

particle in a fluid.

• η = fluid viscosity

• h = distance covered

• g = acceleration due to gravity

• ρs = particle density

• ρl = fluid density

• η = viscosity fluid

• X = particle diameter

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X)(gv

2

ls

2

lsX)(g

h18t

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• The most utilized methods are:

1. Pipette

2. Hydrometer

• Both the methods quantify the particles that did

not covered the distance h at fixed time

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Salinity

• The term salinity refers to the soluble plus

readily dissolvable salts in the soil

• Generally, in laboratory it is determined using an

aqueous extract of soil sample

• The soluble salt content can be evaluated from

measurement either of electrical conductivity

(EC) or of residue-weight upon evaporation to

dryness (TDS, total dissolved solids)

• The EC is the most utilized; it is determined on:

– an aqueous extract at fixed soil/water ratio (i.e. 1:2)

– an aqueous extract of so-called “saturated soil-paste”

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• The EC is measured in dS/m at 25°C by

• The saturation extract is better related to soil-water content under field conditions

• The negative effect of high amount of salts in the soil are: – Water stress caused by high osmotic potential of

soil liquid phase (mainly)

– Toxicity of the salts (secondly)

• For the compost, the EC is generally determined on an aqueous extract with ratio 1:10

• For the irrigation water the EC is directly measured on the sample

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Total Carbonate

• Inorganic carbonates in soil occurs predominantly

as calcite (CaCO3) and dolomite ( CaMg(CO3)2 )

but in arid regions sodium carbonate and

magnesium carbonate are also common.

• The carbonate influences:

texture and aggregate stability of the soil

availability of phosphorus, iron and other

elements

pH

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• Soil carbonate is usually quantified by reaction

with an acid, in a closed system (called

“calcimeter”), to form CO2

CaCO3 + 2 H+ → Ca2+ + CO2 + H2O

• At constant temperature, the increase in

pressure is linearly related to the quantity of

carbonate presents in the sample

• The total carbonate is expressed as g/kg

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• In the field, it is possible to do an estimation of

carbonate amount, by adding a few drops of

diluted HCl to some grams of soil

• If the effervescence is:

slight, the amounts is none or very low

strong, the amount is medium

violent, the amount is high

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Dietrich-Fruhling Calcimeter

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Organic Matter

The organic matter is probably the most important component of soil that influences the fertility

Main effects of soil organic matter

• Source of nutrients

• Structure of soil

• Nutrient holding capacity (C.E.C.)

• Water holding capacity

• Biological fertility

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Determination of organic carbon

The most widely method used is “dichromate oxidation techniques”. It consists of an oxidation of organic carbon by treatment with a hot mixture of K2Cr2O7 and H2SO4 according to the equation:

2 Cr2O72- + 3 C + 16 H+ → 4 Cr3+ + 3 CO2 + 8 H2O

After the reaction, the excess of Cr2O72- is titrated

with FeSO4 according to:

2 Cr2O72- + 6 Fe2+ + 14 H+ → 4 Cr3+ + 6 Fe3+ + 7 H2O

The titration occurs in presence of H3PO4 and diphenylamine (indicator)

Alternatively, in the titration, can be used o-phenanthroline without H3PO4

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The main methods adopted are:

• Walkley – Black (heat of dilution) for

soils in which the amount of organic

carbon does not exceed 8-10%

• Springer-Klee (external heat) for soils

with organic carbon content above 8-

10% and for organic fertilizers

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Total Nitrogen - I

• The routine method used for determination of

total N is the “Kjeldahl method” (1883) which is

essentially a wet oxidation procedure

• This procedure do not recover NO3-N, NO2-N

and some minor organic form; however, in

standard condition, all these form are present in

very small amount

• In Kjeldahl method, organic N in the sample is

converted to NH4+-N by digestion with

concentrated H2SO4 in presence of substances

promoting this conversion (1° step)

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• Then, the NH4+-N is determined from the amount of

NH3 liberated by distillation of the digest with alkali

(2°step)

• The speed and the completeness of conversion is

increased by adding salts to raise the temperature

of digestion (K2SO4), catalysts such as Cu (CuSO4)

and strong oxidizing agents such H2O2

• The NH3 liberated by distillation is collected into

boric acid (H3BO3) and then titrated with standard

H2SO4 or HCl

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• Neither the volume nor the strength of the

H3BO3 needs to be known accurately

because the ammonium borate formed is

titrated back to H3BO3

NH3 + H3BO3 → NH4+ + H2BO3

-

H+ + H2BO3- → H3BO3

• The total nitrogen is expressed as g/kg

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Total Nitrogen - II

• The total N can be also measured by “automatic

analyser” using a dry oxidation procedure

(Dumas method)

• This procedure recovers all N forms and

consists in a combustion at high temperature

(950 -1100 °C) with production of NOx

• Then, NOx are reduced to N2 and carried (by a

stream of He) to thermal cell for N measurement

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Soil phosphorus

• Phosphorus is a very important element for plant

nutrition

• It influences many biochemistry reaction in the

plants

• A want of P causes a poor fructification

• Phosphorus exists in soil as organic and

inorganic forms

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• Only a fraction of total phosphorus is available for

the plants

• For agronomic evaluation it is important to know

the “available phosphorus”

• To evaluate P availability, numerous soil tests

(availability indices) have been developed

• As availability indices, the main methods utilized

are:

Olsen, for neutral or alkaline soils (pH > 6)

Bray – Kurtz, for acid soils (pH < 6)

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• Olsen procedure utilizes a NaHCO3 solution as extractant

• Bray-Kurtz procedure uses a NH4F solution as extractant

• The extracted P is generally determined by colorimetric methods (blue Mo /SnCl2 or blue Mo/ascorbic acid) in which the blue intensity varies with the P concentration

• The blue intensity is measured by spectrophotometer UV-VIS using standard P solutions

• The P is expressed as mg/kg

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0

0.1

0.2

0.3

0 2 4 6 8 10 12 14

[ C ]

ABS

y = m x y = 0,018 x x = y/0,018

Calibration curve

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Exchangeable cations

• In the arid and semi-arid regions the main

exchangeable cations are:

– Ca

– K

– Mg

– Na

• K and Mg are very important for plant nutrition

• Ca and Na in excess can cause negative effects

• They are expressed as mg/kg or cmoli(+)/kg

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• The exchangeable cations are extracted by:

• NH4OAC solution (soils pH < 7)

• BaCl2 + TEA (Triethanolamine) solution buffered

at pH = 8.2 (soil pH > 7 )

• Cations in the extract can be determined by:

– Flame photometry (only Ca, K, Na)

– Atomic absorption

– ICP (Inductively Coupled Plasma)

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Heavy metals

• Cadmium (Cd)

• Nickel (Ni)

• Lead (Pb)

• Copper (Cu)

• Zinc (Zn)

• Mercury (Hg)

• Chromium (VI)

• Arsenic (As)

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Main chemical parameters for evaluation of

water quality for irrigation

• pH

• Electrical conductivity

• Sodium

• SAR index

• Chloride (Cl-)

• Hardness

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SAR INDEX

2

MgCa

NaSAR

22

2

MgCa

NaNadjSAR

22

X

Sodium

Adsorption

Ratio

New adjusted

Sodium

Adsorption

Ratio

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• Na, Ca, Mg

– Flame photometry (only Ca, Na)

– Atomic absorption

– ICP (Inductively Coupled Plasma)

– Titration (only Ca and Mg)

• Chloride (Cl-)

– Titration

– Ionic chromatography

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FAO limits for the evaluation

of water quality for irrigation

Restriction on use

None Slight to moderate Severe

Salinity ECw dS/m 25°C < 0.7 0.7 – 3.0 >3.0

Infiltration None Slight to moderate Severe

SAR 0-3 ECw >0.7 0.7-0.2 < 0.2

SAR 3-6 ECw > 1.2 1.2-0.3 < 0.3

SAR 6-12 ECw > 1.9 1.9-0.5 < 0.5

SAR 12-20 ECw > 2.9 2.9-1.3 < 1.3

SAR 20-40 ECw 5.0 5.0-2.9 < 2.9

Ion toxicity None Slight to moderate Severe

Na

Surface irrigation SAR < 3 3 - 9 > 9

Sprinkler irrigation me/l < 3 > 3

Cl -

Surface irrigation mg/l < 142 142 - 345 > 345

Sprinkler irrigation mg/l < 106 > 106

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