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How to determine the Parameters of Soil? (9 methods)

2. Soil organic matter:

A reasonable estimate of soil organic matter can be obtained by loss of weight on ignition. About 5g of oven dried soil is taken, weighed carefully, and then heated red hot in a crucible for about half an hour.

It is cooled in a desiccator, is reweighed and heated again. This procedure is continued until weight of soil becomes constant. The loss in weight, expressed as a percentage, represents the amount of oxidizable organic matter present.

3. Soil texture and composition:

For studying the texture and composition of soil following two methods are used:

(A) Samples of soils are collected from various areas-forests, fields and gardens by the help of a soil augur which enables one to extract soil from known depths and hori­zons. Each type of soil sample is seen under hand-lens and small amount of it, in dry as well as wet or moist state, is squeezed between thumb and fingers. The texture of sample soil can be identified by following characteristics:

Examination of soil and as felt between thumb and fingersTexture
Individual grains may be seen or felt (gritty), from a cast when moist soil is squeezedSandy
Gritty, form a cast but fall apart when dry soil is squeezed, moist soil forms cast that does not breakSandy loam
Mellow with a gritty feel, when dry soil is squeezed forms a cast requiring careful handling, moist one forms cast that does not breakLoam
Dry appearing cloddy, with soft feeling, forming cast (dry as well as moist) that does not break, moist soil does not form ribbonsSilt Loam
Breaking into clods or lumps, hard when dry, if moist soil is pinched forming thin ribbon which breaks readilyClay Loam
Forms very hard clods or lumps when dry, sticky when wet, if moist soil squeezed, it forms a long, flexible ribbonClay

(B) In another standard method, the soil is passed through a series of sieves with different size (diameter) of their meshes (pores), i.e., soil is passed through sieves with decreasing aperture sizes which separate different components of soil, as shown in following table:

Diameter (mm) of the particleTexture
Below 0.002Clay
0.002-0.02Silt
0.02-0.20Fine sand
0.20 -2.00Coarse sand

4. Soil colour:

The soil sample is spread uniformly over a cardboard sheet. The particles of soil are now matched with chips of a number of colours in the Munsell’s soil colour chart and colours of each are noted on the chart.

5. Soil temperature:

Soil temperature is measured by a special type of thermometer-soil thermometer, which has a ver­tical arm with bulb at one end, and a dial with deflection needle on the other end. The bulb of thermometer is buried at different depths, i.e., 1”, 6”, 12”, 18” of the soil. The temperature may be read on the dial in °C values.

Highly accurate temperature measurements of micro locations like soil, leaf litter, tree wood, back, leaf, etc., are taken with resis­tance thermometers or thermostats and microprobes that are buried in the material or object. Thermistor is based on the principle of thermocouple.

The microprobe of a thermostat con­tains thermocouple in glass globule which is lowered at different depths in soil or object. The leads (cable) of a thermostat are projecting above the surface. To take a reading, the wires are attached to the terminals of a potentiometer, on which temperature is read directly.

6. Soil moisture:

Soil moisture content is determined in percentage by comparing the weight of the soil samples at the time of collection with their weight in oven-dry condition. In this method, a soil sample of known weight is kept in a hot-air oven at a temperature of 105°—110°C until the weight of sample becomes constant (i.e., usually 24 hours).

Then, the soil sample is cooled in the desiccators and weighed. The loss in weight from fresh to dry represents the moisture content or actual amount of water lost. The loss in weight expressed as a percentage is the percent moisture content. It is found as follows:

Fresh weight – Oven dry weight (O.D.) = Moisture content (M.C.)

Percent moisture content (P.M.C.) = M.C. ? 100 / O.D.

7. Carbonate content:

For measuring the carbonate con­tents in different soil samples, a little amount of each type of soil is taken in different test tubes and to each tube few drops of hydro­chloric acid (HC1) is added and the degree of effervescence is noted in each test tube.

The degree of effervescence produced in all cases may be divided into different groups as +, ++, +++, ++++, or as A, B, C, D. The sample showing maximum degree of effer­vescence (i.e., D) indicates the maximum amount of carbonate content present.

8. Nitrate content:

For measuring the nitrate contents in different soil samples, a clear solution ol each type of soil in water (soil 1 part: water 5 parts) is prepared. A small amount of each solution is taken on white spot plate and to each is added 2-3 drops of diphenylamine and 2 drops of conc. H2So4 (0.2%). The appearance of blue colour indicates the presence of nitrate in soil sample. The degree of intensity of the blue clout is noted and divided into four arbitrary groups, as done for carbonate content.

9. pH value:

For field test pH of soil is recorded with the help of universal indicator. For pH determination, small amount of each type of soil sample is taken in different test tubes and to each equal amount of barium sulphate and 15 ml of water are added. The solution is kept for some time till a clear supernatant fluid is obtained. Now few drops of this fluid are taken on a por­celain tile and an equal amount of universal indicator is added to it.

The colour produced is matcnea with the colour chart for different values pasted on the indicator bottle and pH value of each is noted. Besides such a colonmeteric determination of pH an electronic pH-meter is also used for this purpose.