Short essay on Soil Quality Monitoring

Land and fertile soils are enormously important to mankind. Land or soils not only provide a solid substratum for us to live on but it feeds us also. Through the plants which grow on it, it supports a vast variety of animal life including that of man and contributes about 5.2 x 10¹⁷ k. cals to the total annual productivity of our planet.

Soil is a complex mixture of inorganic and organic constituents. While rock fragments (clay, silt, sand, gravel etc.) form the bulk of soils, its unique properties are actually due to the organic fraction – both living and non-living. Properties like efficient decomposition and mineralization of organic matter, nitrification and gentrification, release of plant nutrients, production of growth promoting substances etc. are all due to the microbial life which thrives in the soil.

Since times immemorial soils has been a sink for various waste materials dumped on it or brought down to it as wet or dry precipitation from the atmosphere. It’s vastness and the remarkable capacity to absorb, bind and breakdown materials added to it has made it an ideal place for disposal of many wastes which may otherwise contaminate the environment. However, problems arise when water percolating through it or air current flowing over it carries the wastes or their decomposition products and contaminate other places.

Soil quality is usually monitored when we have to use it for some purpose. If it has to be used as a foundation for buildings dams, roads, bridges or buildings its porocity, tensile strength and nature of underlying rock etc. are determined. If it has to be used for making bricks, its pH, soluble salt content etc. has to be determined.

Probably the most important singular use of our soils is food production for which soil pH, porocity, organic matter content, the status of available plant nutrients etc. are estimated. In cases where pollution of some sort is suspected the soils may reanalyzed for the determination of pesticides or their residues, salinity and concentration of toxic heavy metals like Ni, Cu, Hg, Pb, Cd, Cr, As etc. Routine analyses of physico-chemical properties of soil generally involve the following parameters:

(1) Temperature:

Soil temperature is usually measured by using a soil thermometer. The soil thermometer is a battery operated device with a bulb or senser which is burried at the desired depth in the soil and the temperature is recorded directly on the dial of the instrument. Soil temperatures near the surface usually show diurnal variation depending on sun’s position.

However, lower down beyond the depth of about one meter the temperature remains almost constant. Intense microbial activity caused by rapid decomposition of organic wastes results in production of plenty of heat due to which higher temperatures may be observed in the sub-surface layers of the soil.

(2) Soil colour:

Soil colour has frequently been regarded as an index of fertility and has served the purpose of naming broad soil groups. The colour of a soil also influences its temperature and is usually due to the presence of salts of iron manganese, lime and organic matter. Colour charts are available for visual comparison of colours with the help of which the colour of soils can be reliably determined.

(3) Soil texture and structure:

Based on the relative percentage of particles of various sizes present in a soil, soils are classified into various types. An experienced soil analyst can judge the soil type simply by feeling it between his thumb and fingers. However, an accurate determination of textural class of a soil is usually done by passing the oven dried soils through sieves of different pore sizes which separates the soil particles of different pore sizes. Later these soil separates are weighed and their relative percentage determined.

While soil texture refers to the proportion of sand silt and clay in the soil, the arrangement or the state of aggregation of the soil particles in a mass of soil is termed as the soil structure. The aggregation of sand, silt and clay into compound particles may be granular, crumb like, platy, blocky, subangular-blocky, prismatic or columner in nature.

Colloidal fraction including sticky material of organic origin and very thin film of water molecules are responsible for the development of aggregates of soil particles which are sometimes referred to as peds. The state of soil structure is important as it influences physical properties of the soil particularly aeriation and water holding capacity.

(4) Soil density and porosity:

The density of soil is usually expressed in terms of particle density and the bulk density. Particle density is the sum total of density of individual particles present in a soil whereas the bulk density represents the weight of a unit volume of soil inclusive of pore spaces. It is the bulk density of a soil which is usually determined as it yields valuable information about the quality of the soil. A known volume of soil (or the soil taken from a pit, 10x10x10 cms or 1000 cubic cms is dug out, oven dried at 110°C and weighed. The density is calculated as follows:

The space occupied by air or water in between soil particles in a given volume of soil is termed as Pore space. The percentage of the volume of soil occupied by pore spaces is referred to as Porosity of the soil. It can be calculated from the observation on bulk density by using the following expression:

(5) Soil Moisture:

Soil moisture is a very important parameter for plant growth. It is usually determined by taking a known weight of moist soil, drying it in oven at 100-110°C followed by its weighing. The difference between the two weights represents the moisture content of the soil.

After a heavy rain the soil becomes saturated with water. It displaces air in the pore spaces and fills it completely. At this point the soil is said to be at its Maximum water retentive capacity. If the supply of water is terminated, there is a rapid movement of water downwards under the pull of gravity. Within twenty-four hours or so all water which can move down to underground water table under the influence of gravitational force is removed.

The soil is now left with only Capillary water – water retained in the soil in the form of fine film over soil particles. Movement of this water in any direction is almost negligible. However, this water is available to plants and the quantity thus retained in the soil is referred to as Field water retaining capacity. As the soil dries further or plants take up water through their roots, water present in deeper layers is drawn upward.

A point may be reached when the water present in the deeper layers is no longer able to replenish this stock and plants start wilting or drying up. The amount of water present in the soil at this point is termed as Critical moisture or Wilting co-efficient. Some water is there in the solid but it not available to plants as it is tightly bound to the soil particles. If plants have to grow more water has to be supplied to the soil.

If we leave some soil, which is oven dried for twenty-four hours at a temperature of 100-110°C, in an atmosphere saturated with moisture it absorbs some moisture. This water forms a fine film which is tightly held over the soil, particles.

The water present at this point is termed as hygroscopic water. Pollution of the soil presence of waste material of organic or inorganic nature over the soil surface or within the pore spaces of the soil, change the water holding capacity of the soil which in turn affects plant growth and activity of microbial life which in it’s own turn disrupts normal functioning of the soil.

(6) Salinity:

The term salinity refers to the concentration of soluble salts in the soil. A higher concentration of chlorides and sulphates of alkaline bases are usually present in saline soils. The salinity of a soil is generally a consequence of poor drainage and rapid evaporation of alkaline soil solutions. Salts accumulate in the soil because more salts are added to the soil than can be removed from it by plant uptake or leaching.

Such conditions often result due to persistent application of irrigation water having a high salt content. River waters which receive sewage and industrial effluents show a high salt concentration as organic matter is decomposed by microbial activity but soluble salts persist. Persistent use of such water for irrigation purposes is injurious to the soil in the long run.

Electrical conductivity of the soil solution is a convenient parameter for estimation of salinity of a soil. Salts present in a sample of soil are dissolved by shaking it with a known amount of distilled water. The electrical conductance of clear supernatant solution is measured with the help of Conductivity meter. The conductivity of soil solution rise with rise in the salt concentration of the soil sample.

(7) Soil pH or Hydrogen ion concentration:

Hydrogen ion concentration is a very important singular property which provides valuable information about the chemical nature of a soil. The pH of the soil is dependent upon the relative amounts of absorbed hydrogen and cautions on the surface of soil particles. When hydrogen ions predominate the soil becomes acidic. If the reverse is true alkalinity results.

A very simple method of determination of pH of a soil sample is by means of Universal pH indicator solution. There are certain dyes which change colour at specific pH. By mixing a number of such dyes different colours can be obtained at different pH values. This mixture of dyes is known as Universal pH indicator.

The soil sample is shaken with equal amount of Barium sulphate and about 20 ml of distilled water in a test tube. When the particulates settle down the clear supernatent is drained out and a few drops of universal indicator solution is added. The colour obtained is compared with the standard colour chart to determine the pH.

The pH of a soil solution can also be determined by electrometric method which uses electrical potential developed across 2 glass membrane of glass electrode when immersed in the solution. Commercial pH meters are available in the markets which give direct pH readings. To determine the pH of a solution pH meters have to standardize by using a series of buffer solutions of known pH values.

One of the greatest influences of pH on plant growth is its effect on the nutrient availability. Availability of some ions increases with rise in pH while others become locked with the soil particles. With lowering of pH the reverse may happen. More nitrogen is available to plants between pH 6 and pH 8.5 than at lower or higher pH values. Availability of phosphorus rises with rise in pH beyond 6. It diminishes after 7.5. At pH below 6 availability of K, S, Mo, Ca, Mg is low.

However, the availability of B, Fe, Cu, Mn, Zn is higher at pH below 6. At the extremes of pH scale the balance of available nutrients like Al, Fe, Mn, Zn, Cu etc. become excessively soluble. Their release in large amounts may make the soil toxic to plants. Hydrogen and Hydroxyl ions are themselves toxic for the growth of plants at higher concentrations.

Frequent depositions of acids (acid rains), pollution by acidic or alkaline effluents discharged by industrial establishments, dumping of acidic or basic wastes on soils which give rise acidic or basic leachates etc. disturb the balance of available nutrients and turn the soil toxic for plant growth.

(8) Organic matter content of soils:

Organic matter plays a very important role in a soil. It is food for the numerous micro organisms which make a soil, soil in its true sense. The unique properties of soil, its capacity to decompose and mineralize dead plant and animal bodies, excreta and exudates of plants and animals, recycle nutrients, embibe a generous quantity of water, produce growth promoting substances etc. are all due to the activity of these microbes. In absence of organic matter content the soil life diminishes and the soil becomes a dead heap of rock fragments – sand silt and clay.

Organic matter content of a soil may be crudely estimated by heating a known weight of dry soil to the point at which organic matter is completely oxidized. The weight of the residue is determined after cooling it in a dessicator. The difference between the two weights gives an estimate of the organic matter content of the soil sample.

A more accurate method is to oxidise the organic matter of the soil sample with a known volume of strong oxidizing agent of known concentration. After the oxidation of organic matter is complete the excess amount of oxidizing agent is determined. From the quantity of oxidizing solution used for the oxidation of organic matter of the sample the amount of organic matter present may be determined. This is titrimetric method which employs a strong solution of acidic potassium dichromate (K₂Cr₂O₇ + H₂SO₄) of known strength the excess amount of which may be determined by titration with Ferrous ammonium sulphate using a suitable indicator.

A much better method of estimation of organic matter content of a soil sample involves oxidation of organic carbon by K₂S₂O_g. The oxidation is promoted by free radical HO generated by strong Ultraviolet light. A small sample is introduced in an oxidizing chamber along with H₃PO₄ and K₂S₂O₈ and is subjected to strong ultraviolet radiations at a wave length of 185 nm. Carbon dioxide produced as a result of oxidation is carried to a Gas-chromatographic detector and is measured accurately. It is from the quantity of carbon dioxide produced that a fairly accurate estimate of organic matter can be computed.

Organic matter in the soil is decomposed and mineralized by a variety of micro-organisms each of which performs its own task in a co-ordinated way. A diverse microbial community develops in the soil which depends largely on the nature and amount of organic matter supplied to it. The diversity in microbial composition is necessary as it enables the soil to carry out its diverse functions in a proper and balanced way. The pollution of soil usually suppresses the diversity of the microbial community and thus disturbs the vital functions of the soil.

(9) Presence of toxic ions and heavy metals:

Almost all soils contain toxic ions and heavy metals in small amounts. However, their concentration can at best be expressed in traces and their absorption by plants rarely exceeds the permissible limits. Usually the pollution of soil caused by industrial discharges and leachates from dumps of solid wastes, results in rise in concentration of toxic ions and heavy metals.

Important heavy metals which can cause problems are Pb, Hg, Cd, Cu, Ni, As, CN and F etc. These toxicants may be absorbed by plants growing on the contaminated soils, bio- accumulated and may contaminate the food which comes from such plants resulting in damages at higher trophic level. The estimation of these toxic ion and heavy metals involves advanced analytic; techniques which depend on their chemical, optical, spectral or nuclear properties which are dealt with separately in this book.

(10) Presence of pesticides or pesticide residues:

Extensive use of pesticides in domestic establishments, industries, agriculture and forests usually contributes significant amount of toxic materials in waste waters and soils. Many of these pesticides or their toxic residues persist in the environment for long duration of time and damage living organisms. They may be bio-accumulated and passed on to higher trophic levels. The determination of pesticides or other toxic residues, therefore, is often desired.

Pesticides or their toxic residues may be extracted repeatedly with some suitable solvent, such as 15% ethyl ether and 85% hexane. The combined extract is evaporated to a small volume in a steam bath. A small sample of the concentrated extract is introduced into gas-chromatographic column at 180°C with the help of a microsyring. A mixture of Ar and CH₄ is used as the carrier gas which flows at a rate of about 60 ml per minute. The pesticides are vapourized and are carried along the gas-flow in the column which separates the different fractions.

An electron capture detector at the end of the column detects the toxicants. The flowchart is compared with the results of a similar chart run on a known or standard sample from which precise quantitative as well as qualitative determinations are made.