Soil Analysis Methods

Element

Method ID

Method

Method description

pH- Water

CN-TM-S02

Potentiometric

pH is determined by using a high impedance voltimeter. The potential between two electrodes is measured using the voltmeter on a soil suspension of 1:2 (soil :water)

pH is determined by using a high impedance voltimeter. The potential between two electrodes is measured using the voltmeter on a soil suspension of 1:2 (soil :1MKCl)

EC(S) (uS/cm)

CN-TM-S01

Potentiometric

The electrical conductivity indicates the amount of soluble (salt) ions in soil. The determination of electrical conductivity (EC) is made with a conductivity cell by measuring the electrical resistance of a 1:2 soil:water suspension, after air drying the soil sample.

P (ppm) Na (ppm)

Fe (ppm)) K (ppm) Ca (ppm Mg (ppm) Mn (ppm S (ppm) Cu (ppm) ) B (ppm) Zn (ppm) Al (ppm)

CN-TM-S07

Determination of the content of elements after extraction with Mehlich 3 using ICP-OES

Mehlich 3 extract is composed of 0.2 M glacial acetic acid, 0.25 M ammonium nitrate, 0.015 M ammonium fluoride, 0.013 M nitric acid, and 0.001 M ethylene diamine tetraacetic acid (EDTA)

The extracted major and minor elements are the determined using Inductively coupled Plasma Optical emission spectroscopy (ICP-OES) which excites the atoms. When the excited atoms return to low energy position, emission rays (spectrum rays) are released and the emission rays that correspond to the photon wavelength are measured. The element type is determined based on the position of the photon rays, and the content of each element is determined based on the rays' intensity

Total Carbon & Total Nitrogen

CN-TM-S54

Combustion

CN analyzer (Elemental analysis)

Elemental Analysis is a process that analyzes the elemental composition of a compound as weight percent of each element present in the compound. Soil samples are weighed in tin capsules and then dropped in a tube where, in the presence of external oxygen flash combustion occurs at a temperature of 900- 950 °C. At this high temperature both organic and inorganic substances are converted into elemental gases, which, after further reduction, are separated in a chromatographic column and finally detected by a highly sensitive thermal conductivity detector. (TCD) which produces an electrical signal proportional to the concentration of nitrogen and carbon.

Soil Nitrogen (Kjeldahl)

CN-TM-S04

Colorimetric

Kjeldahl digestion

Digestion - the decomposition of nitrogen in organic samples utilizing a concentrated acid solution. This is accomplished by boiling a homogeneous sample in concentrated sulfuric acid. The end result is an ammonium sulfate solution.

The Kjeldahl digestion converts organic N compounds in to the ammonium form, which is then determined colorimetrically on the UV at 655nm.

Organic Carbon (Walkley-Black)

CN-TM-S05

Colorimetric

The determination of soil organic carbon is based on the Walkley-Black chromic acid wet oxidation method. Oxidisable matter in the soil is oxidised by 1 N K2Cr2O7 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. Alternatively the amount of chromic acid (Cr3+) produced is determined by colorimetry at about 600nm.

Soil Texture (% Clay, Silt, Sand)

CN-TM-S06

Hydrometer Method

The hydrometer method of silt and clay measurement relies on the relationship between of particle size or radius and the differential vertical velocities of the particles through a water column, i.e. the sedimentation rate, based on Stoke’s law. Sedimentation rate is dependent upon liquid temperature, viscosity, and the diameter and specific gravity of the falling soil particles.

Soil is dispersed into individual particles throughout a water column and allowed to settle. Sodium Hexametaphosphate is added to aid dispersion. Hydrometer measurements quantify the amount of material remaining in suspension at specific time intervals, which in turn can be related to the amounts of sand, silt and clay in the soil.

P(olsen) (ppm)

Colorimetric

This method determines the relative bioavailability of ortho-phosphate (PO4-P) in soils by extraction using alkaline sodium bicarbonate (pH 8.5) solution and determining the P concentration in the extract colorimetrically (after color development) using UV-Vis spectrophotometer.