Physical properties of soil

Many physical characteristics of the soil are those which may be determined by sight or feel. They can be measured against some kind of a scale, such as size, volume, strength, intensity, or others. The physical characteristics of soil are significant when considered singly as well as when they are in certain combinations.

E.g. the amount of pore space in a soil is related to soil texture (the size of soil particles) and soil structure (the way in which soil particles stick together).

(a) Soil Texture

This refers to size distribution of soil particles. Soil is made up of various particles of different sizes and on the basis of this, soil can be classified into different classes like sand, silt and clay.

The particles that make up a soil sample include Gravel, Sand, Silt and Clay. However, depending on the proportion of the various particles in a soil, it can be described as sandy, clayey or loamy soils.

Light and heavy are terms often used by farmers to describe the texture of soils. A light soil is one which is easy to cultivate and contains a large proportion of coarse particles. A heavy soil has a large proportion of very fine particles.

The most common soil texture are sand, silt, loam, silty-clay-loam, silty-loam, sandy-clay, sandy-loams, silty-clay, clay-loam, and clay.

To develop an understanding of proportion of the constituents and names of types of soil, the following examples are given.

Microscopic structure of various soils

(i) Sandy-Loam: 50% sand, 25% silt and 15% clay.
(ii) Silty-Loam: 54% silt, 28% sand and 18% clay.
(iii) Clay-Loam: 34% silt, 30% clay and 25% sand.
(iv) Sandy: 85% or more of sand.

Importance of soil texture

It is used in the evaluation of soil ability to supply mineral nutrients.
It supports soil micro-organisms essential for plant growth.
It determines the type of crop to grow on the land.
It enables the farmer to know the type of soil in his farmland.

(b) Soil structure

The term soil structure refers to the arrangement of the soil particles. These may occur as individual fragments or as groups or, in a mixture of both. The degree to which the soil particles is also included in a description of structure. In coarse-textured soils, the particles of sand exist and function largely as individual grains, but in fine-textured soils the particles of clay are held together by the various colloidal materials to form aggregates or groups. The units of different shapes is regarded as structure.

The common types of soil structure include plate-like, prism-like, block-like and spheroidal.

Soil structural types

(A) Spheroidal
(B) Platy (Plate-like)
(C) Block-like
(D) Columnar
(E) Prismatic

Importance of soil structure

It determines the level of fertility of the soil.
A good soil structure supports aeration.
It also prevents erosion and water-logging.
A good soil structure promotes the activities of soil micro-organisms.

SIMPLE EXPERIMENTS WITH SOILS

A. Experiment to show that soil is made up of different sized particles

The layers shown in the measuring cylinder are:

Organic materials
Clay in suspension
Silt
Fine sand
Coarse sand

Observation and results

Distinct layers of different soil particles can now be seen and actually measured in depth with a ruler. The heavy/coarse gravels have settled first. This is followed in succession by sand, silt, and clay. The humus or organic materials remain floating in the water or settle on top of the clay.

Conclusion

Garden soil is a mixture of particles of different sizes.il is made up of different sized particles.

Apparatus: Water, garden soil, 250 cm³ measuring cylinder, sodium carbonate.

Procedure: Put about 50 g of garden soil into a 250 cm³ measuring cylinder and add about 4 times its volume of water with some quantity of sodium carbonate in it, to aid in the dispersion of the particles. Cover the cylinder mouth with your hand and shake vigorously for about three to five minutes. Allow the cylinder to stand undisturbed for a day or two.

Separation of soil particles

B. Experiment to compare the porosity and water-holding capacity of three soil types

Apparatus: Funnels, measuring cylinders, cotton wool, water, dry sand, dry clay, dry loam of equal quantity with a high percentage of organic matter, stop-clock.

To compare the porosity and water-holding capacity of different soil types

Procedure: Grind all the soil samples except the sandy one into fine particles. Equal volumes or masses of the dry sand, clay and loam are placed in separate funnels plugged with cotton wool. Tap the funnels carefully and persistently on the bench until all visible air spaces are filled and stand each funnel in the open end of each 100 cm³ measuring cylinder as shown in fig. above.

Then quickly pour 50 cm³ of water into the funnel containing each soil sample. With a stop-clock, note the time taken for the first drop of water to drip through into the measuring cylinder in each case.

Allow the water to drain through the soils until no water drips into the measuring jars. If necessary allow the apparatus to stand overnight and then allow the apparatus to measure the volume of water which has passed through in each case. By subtraction find the volume of retained water in all cases.

Conclusion

The rates of porosity vary from one soil type to another. It is seen from results obtained that different soil types have different water-holding capacities. Loamy soils with a high percentage of organic matter may have good water-retaining properties, followed by clayey soils, while sandy soils have very poor water-retaining capacities.

CC. Analysis of the mechanical composition of soil — Sieve method

Apparatus: A sample of soil, a piece of paper or cardboard, a tower of sieves of various meshes with the widest mesh at the top and the finest at the bottom, scale balance.

Procedure: Dry some soil by leaving it spread out on paper exposed to the air for some days, until it is thoroughly loose in texture. Shake and rub the soil through a tower or sieves of various meshes with the widest mesh at the top. Weigh the contents of the various sections, thus obtaining a quantitative analysis of the soil as shown below.

(i) Clay: less than 0.002 mm diameter.

(ii) Silt: 0.002–0.02 mm diameter.

(iii) Fine sand: 0.02–0.2 mm diameter.

(iv) Coarse sand: 0.2–2 mm diameter.

(v) Gravel: over 2 mm diameter.

Conclusion: Soils are made up of various size fractions.