Soil Chemical Analyses
ASL AVAILABLE SOIL ANALYSIS
- Electrical Conductivity (EC)
Total Kjeldahl Nitrogen
Available Phosphorus_Olsen P Method
- Available Phosphorus_Bray P Method
- Organic Carbon (Walkley and Black Method)
- Active Iron and Manganese Determination
- Available Zinc and Copper
Total Exchangeable Bases (TEB)
Cation Exchange Capacity
- Exchange Aluminum and Acidity (KCl method)
- Particle Size Analysis
Available Boron (Hot Water Method)
- Pipet Method
- Hydrometer Method
ABSTRACT OF ANALYTICAL METHODS FOR SOIL
Soil samples received at field moisture
are air dried for 3-4 days at 35 to 40 C. These are then ground to pass a 2 mm
stainless steel sieve to produce "fine earth" for the determination of moisture,
pH, electrical conductivity, exchangeable bases, CEC, available P and particle
size. Powdered sample (60 mesh) is used for the analysis of Kjeldahl N, Org C
and total elements.
Ten g of fine earth is dried at 105oC
for at least 12-24 hrs, cooled and weighed. The loss in weight expressed as %
represents moisture content.
pH and Electrical Conductivity (1:1 soil to
Twenty g of fine earth is placed in a
container and 20 ml of distilled water added. The suspension is shaken for 30
min and allowed to settle. Electrical conductivity and pH of the solution are
Note: It is important to measure
conductivity before taking the pH reading, as the potassium chloride from the
reference electrode can increase the EC.
One hundred mg of air-dried, finely
ground (60 mesh) sample is digested with 1 g of K2SO4 and
Se (100:2 ratio) and 2 mL of conc. H2SO4 in a digestion tube at 390oC
for 2-1/2 hr. The digest is cooled and made up to 20 mL. After shaking the
residue is allowed to settle. Nitrogen is determined colorimetrically as
indo-phenol blue on a Technicon AutoAnalyzer at 625 nm.
Using the N analysis figure and assuming
a C/N ratio of 10, enough soil sample (estimated to contain 10-15 mg C, but not
exceeding 2 g soil), is allowed to react with 10 mL of N K2Cr2O7
and 20 mL conc. H2SO4 in a 125 mL Erlenmeyer flask. After
cooling, 80 mL of 0.4N BaCl2 solution is added to the mixture, the
contents mixed by swirling and the solids allowed to settle. Organic C is
estimated from the intensity of the green color (measured at 625 nm) which is
due to chromic sulfate that resulted from the reduction of the dichromate ions.
Solutions of sucrose containing 5, 10, 15, 20, 25, and 30 mg C/2 mL are used for
Five g of soil mixed with enough quartz
sand to facilitate percolation are leached with 10 portions of 20 ml of neutral
normal NH4OAc into a 250 mL volumetric flask. The leachate is made up
to 250 mL and analyzed for Na, K, Ca and Mg by AAS. The leached soil is saved
for the determination of Cation Exchange Capacity.
Cation Exchange Capacity
The leached soil from the exchangeable
bases extraction is washed with 4 portions of 25 mL of 80% industrial ethanol.
The washings are discarded. The adsorbed NH4+ ions are replaced by leaching with
4 portions of 20 mL each of N KCl (pH 2.5). The leachate is made up to 100 mL
and analyzed for NH4+-N as indophenol blue.
Five g soil is shaken for 30 min with
100 mL of extracting solution (0.5 M NaHCO3 at pH 8.2) and
polyacrylamide as precipitating agent. Phosphorus is determined in the filtrate
colorimetrically as reduced phosphomolybdate at 625 nm using ascorbic acid as
the reducing agent.
Bray 2 Method
2.5 g of soil is shaken for 1 min with
50 mL of extracting solution containing 0.03N NH4F and 0.1N HCl).
Phosphorus is determined colorimetrically as in Olsen Method above.
Particle Size Analysis (Texture Analysis)
Twelve g of air-dried, 2 mm soil sample
is pretreated first with 20 ml hydrogen peroxide and dried at 80oC.
The soil is further treated with 20 ml hydrogen peroxide and then dried at 100oC.
The soil is then cooled and weighed. The mineral soil is shaken overnight with
100 mL water and 10 mL of solution containing sodium metaphosphate and Na2CO3.
The mixture is quantitatively transferred to a 250 mL graduated cylinder and
enough water added to make up the total volume to 200 mL and shaken. Twenty-five
mL of the suspension is pipetted from a depth of 6 cm, evaporated to dryness and
the weight taken. This weight representing 1/8 of the clay fraction, is
multiplied by 8 to get the corrected weight. The rest of the slurry is washed
through a 50 sieve and the fraction retained on the sieve quantitatively
recovered, dried and weighed. This represents the sand fraction. Percent clay
and % sand are calculated on the basis of the weight of the mineral soil. The
silt content (%) is determined by difference.
Forty g of air-dried, 2-mm soil sample
in a 500 ml plastic bottle is mixed with 100 mL Calgon solution and 100 ml
deionized water and shaken for 1 hr using a reciprocating shaker. The soil
suspension is transferred quantitatively to a graduated cylinder and make up to
1-L with deionized water
(include a blank consisting of 100 mL
Calgon solution and 900 mL H2O). The mixture is allowed to stand overnight.
Using a plunger, the suspension is stirred vigorously for 1 minute. After
plunging, take hydrometer reading for RSILT+CLAY
exactly 40 seconds after the completion of stirring. Add a couple of drops of
amyl alcohol if the surface of the suspension is covered with foam. Repeat the
process determining hydrometer reading on a blank solution and record as RSC.
Remove the hydrometer carefully and rinse with deionized water. Let the cylinder
stand undisturbed. After 6 hours, determine the temperature of the suspension at
about 5 cm depth. Using the temperature correction values in Table 1.1 determine
the settling time for the clay fraction. Based on time after initiation of
settling, reinsert the hydrometer carefully and record as RCLAY. Repeat the
process determining hydrometer reading on a blank solution and record as RC.
Determine oven dry soil moisture on a 20 ± 0.1 g sample of
Table 1. The influence of suspension temperature on the
hydrometer determination of
soil clay (<2 um) based on a particle density of 2.65
g cm-3 and a solution density of
0.5 g L-1. (Source: Gee and Bauder (1986))
Temperature °C Settling time for clay (hours
For every 1°C above 20°C, a 0.36 graduation is added to the
hydrometer reading, and for every 1°C below 20°C, a 0.36 graduation is
The correct hydrometer readings are obtained by correcting
for temperature and
subtracting the blank reading.
Sand % = (oven dry soil weight) - ( RSILT+CLAY - RSC ) x
(oven dry soil weight)
Clay % = ( RCLAY - RC) x 100/(oven dry soil weight)
Silt % = 100 - (Sand % + Clay)