Soil, Plant, and Water Testing Laboratory Record

Vocabulary flashcards covering laboratory apparatus, soil sampling and preparation techniques, and the determination of organic carbon by the Walkley and Black method.

pH meter

An electronic device used to measure the acidity or alkalinity (pH) of soil and water.

Electrical Conductivity (EC) meter

An instrument used to measure the ability of a solution to conduct electricity, which indicates the total dissolved salts in soil or water.

Analytical balance

A highly sensitive weighing instrument used for the precise measurement of sample weights in laboratory analysis.

Soil sieve

A mesh screen used to separate soil particles by size and prepare uniform soil samples for laboratory testing.

Mechanical shaker

An automated device used to mix soil suspensions thoroughly to achieve uniform distribution of particles.

Centrifuge

A machine that rotates samples at high speed to separate soil extracts based on density differences.

Spectrophotometer

An optical instrument that measures the amount of light absorbed by a solution to estimate the concentration of various nutrients.

Flame photometer

An instrument used to estimate the concentration of alkali metals, particularly potassium ($K$) and sodium ($Na$), in soil and plant samples.

Kjeldahl/Distillation apparatus

A combined setup used for the digestion and distillation of samples to estimate total nitrogen content.

Volumetric flask

A flat-bottomed flask with a long neck and a precise calibration mark, used to prepare standard solutions of exact concentration.

Soil sampling

The process of collecting a small portion of soil that represents the entire field, essential for analyzing properties such as nutrient content, pH, and texture.

Composite sample

A single representative sample formed by thoroughly mixing multiple individual soil samples collected from $8$ to $10$ spots across a field.

Quartering method

A sample reduction technique where mixed soil is spread on a clean sheet, divided into four equal parts, and opposite quarters are discarded repeatedly to reduce sample size without bias.

Soil Organic Matter (SOM)

The seat of nitrogen in soil consisting of decomposed and undecomposed plant, animal, and microbial residues, serving as an index of soil productivity and containing an average of $58\%$ carbon.

Van Bemmelen factor

The factor ($1.724$, or $\frac{100}{58}$) multiplied by the percentage of organic carbon to calculate the total percentage of soil organic matter.

Dry combustion method

An accurate method of estimating organic carbon where soil pre-treated with $0.5\%$ sulphurous acid ($H_2SO_3$) is ignited with copper oxide in silica tubes, and the evolved $CO_2$ is absorbed in weighed soda lime.

Walkley and Black rapid titration method

A wet digestion method where soil organic matter is oxidized by potassium dichromate ($K_2Cr_2O_7$) and sulphuric acid ($H_2SO_4$), and the unreduced chromic acid is back-titrated with standard ferrous ammonium sulphate.

Mohr's salt

Ferrous ammonium sulphate ($FeSO_4 \cdot (NH_4)_2SO_4 \cdot 6H_2O$), used as a standard $0.5\,\text{N}$ titrant to determine unreduced chromic acid in organic carbon analysis.

Diphenylamine indicator

An indicator used in the Walkley and Black titration that undergoes oxidation to diphenylbenzidine violet and changes to bright green at the titration endpoint.

Average recovery factor

The $77\%$ average recovery value assumed in the Walkley and Black method, requiring organic carbon values to be multiplied by $\frac{100}{77}$ (or $1.3$) to obtain the actual percentage.