eChapters

1. The smallest size soil separate is

a. Sand      b. Silt

c. Clay        d. Loam

 1. A mineral that forms an original component of rock is called

a. Primary         b. Secondary

c. Essential         d. Accessory

Soil is a mass of large variability and thus its sampling is the most challenging problem for any soil analyst. Practically, soil sampling is a two step process which includes (i) collection of soil from homogeneous unit and (ii) processing which in turn includes drying, grinding, sieving, mixing, partitioning and storing.

1.1. Sources of Error in Soil Chemical Analysis

Different sources of error involved starting from soil sampling to chemical analysis of a sample can be divided into three main groups:

1. Sampling error : This error results from variability among the different samples drawn from same soil unit with respect to the objective of sampling.
2. Sub-sampling error : This error is due to variability introduced among sub samples of the same soil sample.
3. Analytical error : This error introduced due to variability from one chemical determination to another on same sub sample.

In natural condition primary soil particles cohere to each other to form a cluster or an aggregate through the cementing action of various agents like organic matter, oxides of iron and aluminium, carbonates, etc. Aggregate analysis tells one about the structural status, pore geometry which in turn influences the movement of air and water in soil, amount of water stable secondary particles (aggregate) as well as the extent of finer particles aggregated to the coarser particles and the erodibility status of surface soil against wind and water.

Methods of Analysis

Among the several methods of aggregate analysis the commonly used methods practiced in agriculture and related fields are wet sieving and dry sieving method of aggregate analysis.

Soil consistency is a term used to designate the manifestation of different physical forces acting within the soil at various moisture levels. The consistency limits, also known as ‘Atterberg Limits’ (Atterberg, 1911; 12) are defined by the water content of the soil to create specified degree of consistency. These limits are – (i) Liquid limit or upper plastic limit, (ii) Plastic limit or lower plastic limit and (iii) Sticky limit.

7.1. Determination of Liquid Limit or Upper Plastic Limit

Equipments and Materials

Liquid limit device (Casagrande, 1932) and accessories, spatula, dish, oven, moisture box, balance, desiccator

8.1. Law of Mass Action

Gulberg and Waage, the two Norwegian chemists in the year 1864 stated the law of mass action in this form: ‘The velocity of any chemical reaction at any instant at a given temperature is proportional to the active mass of each of the reactants at that instant present in the system, the active mass in a homogeneous system is defined as the number of gram molecule of the substance present per unit volume i.e., the molar volume’.

Mathematical Formulation of the Law of Mass Action

Let us take a simple case of a reversible reaction at constant temperature represented by the equation:

9.1. Balance and Weighing

9.1.1. Principle of Functioning of a Balance

It is one of the important tools used in the laboratory and one should have thorough knowledge about its assembly, use and care, otherwise it may be the initial step for introducing error in any physical or chemical analysis. Every balance has its maximum load limit and sensitivity depending upon its construction.

Essentially balance may be regarded as a rigid beam, BC having a central fulcrum, O and two arms of equal length, d₁, d₂ (Fig.9.1). The both ends of beam have prism edges upon which the balance pans are suspended by means of suitable suspension.

10.1. Determination of Soil pH

The soil pH value is the measure of hydrogen or hydroxyl ion activity in the soil solution and it indicates whether the soil is acidic, neutral or alkaline in reaction. The pH is the most important property of soil as it controls the availability of plant nutrients, microbial activity and physical condition of soil. Since crop growth is affected both under very low (strongly acidic) and very high (strongly alkaline) soil pH, reclamation of these soils are necessary. Soil pH in fact indicates the extent of active acidity (activity of hydrogen ion in soil solution) of soil. Depending upon the purpose of measurement and soil condition, soil pH is measured in several soil-water or soil-salt solution ratios. The saturated paste is used for identifying specific soil problems like degree of acidity or alkalinity. For making fertilizer recommendation, 1: 2 soil-water suspension is generally adopted. To mask the variability in salt content of the soil, the pH is measured in soil-CaCl₂ suspension (1:2 soil-0.1 M CaCl₂ solution). The soil pH value is independent of dilution over a wide range of soil-salt solution ratio, thus more reproducible than those obtained with soil-water suspension.

11.1. Determination of Electrical Conductivity of Soil Solution

The conductivity or more precisely the specific conductance of a soil at a given soil-water ratio is the reciprocal of the specific resistance offered by the solution at 25ºC between the electrodes of 1 sq cm cross section kept 1 cm apart. Thus, the electrical conductivity of any solution depends on the amount of soluble salt present in it. Knowledge on the extent of salinity is important for management of saline soil including the choice of the crop suitable for that soil. Soluble salt content in saturation extract is more reliable index of salinity level than in soil: water ratio of 1: 2 or 1: 2.5 because it simulates the extent of salinity experienced by crop in field moisture condition.

Salinity class of soil is based on the electrical conductivity (EC) of saturation extract at 25ºC. However, EC of water extract from 1:2 soil-water suspension can be transformed into the EC of saturation extract following the formula (except for soil containing gypsum).

13.1 Exchange Capacity

Soil colloidal particles carry both negative and positive charges on their surface which attract and hold equivalent amount of oppositely charged ions to their surface. The total number of negative charges present on a given mass of soil or capacity of a given mass of soil to exchange cations or total number of cation held by a given mass of soil is termed as cation exchange capacity (CEC). Similarly, the capacity of a soil to retain anions is known as anion exchange capacity (AEC). The ion exchange capacity of a soil depends upon the type and amount of colloidal particle (clay and organic matter) present in the soil. Ion exchange capacity (CEC / AEC) is expressed as milliequivalent ion (cation / anion, respectively) per 100 g of soil (meq / 100g). It is also expressed as centimoles of charge (positive / negative) per kg of soil [cmol (±) / kg]. Meq / 100g = cmol (±) / kg of soil.

Phosphorus in soil present both in inorganic form and organic form. The inorganic P can be divided into different pools. The major active inorganic forms are: P bound to aluminium (Al-P), iron (Fe-P), calcium (Ca-P) and silicate minerals and relatively less active forms are occluded and reductant soluble forms of P. On the other hand the principal organic P compounds present in soil are (i) inositol phosphate, (ii) phospholipids, (iii) nucleic acid and other unidentified esters and phosphoproteins.

15.1. Determination of Available Soil P

The availability of soil P to plant varies greatly depending upon the reaction, minerological composition and amount of the soil colloid. Very little P is present in the soil solution except in recently fertilized or sodic soils. The type of phosphate ions present in the soil solution depends on the soil pH. In soils of neutral to slightly alkaline pH, HPO₄^2- ion is the dominant form. In slightly or moderately acidic soils, both H₂PO₄^- and HPO₄^2- ions prevail. At strongly acidic soil H₂PO₄^- ion tends to dominate, while above pH 9.0, the PO₄^3- ion becomes more important than H₂PO₄^-.

18.1. Determination of Available Micronutrient Cations (Zn, Cu, Fe and Mn) in Soil

The available micronutrient cations (Zn, Cu, Fe and Mn) can be estimated in a single extraction with diethylene triamine pentaacetic acid (DTPA) as proposed by Lindsay and Norvell (1978). DTPA has the excellent property to combine with free metal ions in the solution forming soluble complexes. The stability constants of metal-DTPA complexes make DTPA as a most suitable extractant for simultaneous complex formation with Zn, Cu, Fe and Mn. In calcareous soil DTPA extractant may result overestimation due to excess dissolution of CaCO_3, thus releases occluded micronutrient cations which are not plant available. To avoid this excessive dissolution of CaCO₃, DTPA is buffered at pH 7.3 using triethanol amine (TEA). At this pH major part (3/4^th) of TEA is present as HTEA⁺ (protonation) and exchanges for Ca and Mg ion from the soil exchange site. This increases concentration of Ca and Mg in the solution phase and suppresses dissolution of CaCO₃. The DTPA has the capacity to complex each of the micronutrient cations 10 times of its atomic weight and it ranges from 550 to 650 ppm depending on the micronutrient cation.

The amount of cation in the extract is determined on an atomic absorption spectrophotometer (AAS). The experimental condition such as shaking time, DTPA concentration, pH and temperature during shaking influence the amount of Zn, Cu, Fe and Mn extracted. Increase in shaking time and temperature markedly influence the extractability of these cations.

19.1. Plant Available Fraction of Nickel, Cadmium, Lead and Chromium

The DTPA method was originally developed to extract Mn, Cu, Fe and Zn from slightly acidic to alkaline soils (Lindsay and Norvell, 1978), but in some cases it can be used for other metals also such as Nickel (Ni), Cadmium (Cd), Lead (Pb) and Chromium (Cr). Among them Ni is considered as an essential micronutrient. The DTPA is used to extract soil metals by forming chelates with free metals in solution, which lowers their ionic activities so that additional quantities of metal are released from the soil until equilibrium is attained. Triehanolamine (TEA) is used to buffer the extracting solution at pH 7.3 in an attempt to optimize the extraction of various metals. Calcium chloride is included in the extracting solution to minimize dissolution of CaCO₃.

Crop production broadly depends on the fertility of the soil where a crop is raised. There are several factors like the kind and quality of seed, climate, water supply, and plant protection measures etc. that affect the crop production. Among these factors, the fertility status of the soil largely determines the ultimate yield even if all these factors are optimum for crop production. A clear distinction should be made between soil fertility and soil productivity. Soil fertility deals with the inherent capacity of soil to provide nutrients in adequate amounts and in proper balance, for the growth of specified plants when other growth factors such as light, water and temperature, and the physical condition of the soil are favourable. Soil productivity is concerned with the capability of soil to produce a specified plant or plant parts or a sequence of plants under well defined and specified systems of management inputs and environmental conditions, viz. climatic conditions. It does not mean that a fertile soil is always productive. A fertile soil, under a bad management system, may be unproductive for a crop. In this book, plant nutrient elements and their available forms, functions and deficiency symptoms are briefly described for the better understanding of soil testing and fertilizer recommendation.

Soil testing is a useful tool that can help to ensure the efficient use of applied plant nutrients. Soil tests provide a means for assessing the fertility status of a soil, but soil tests do not provide a direct measure of the actual quantity of plant available nutrients in the soil. Instead, soil tests measure the quantity of a nutrient element that is extractable from a soil by a particular chemical extracting solution. The measured quantity of extractable nutrient in a soil is then used to predict the crop yield response to application of the nutrient as fertilizer, manure, or other amendment. As soil test levels increase for a particular nutrient, the expected crop yield response to additions of that nutrient decreases. A soil-testing programme has four phases as follows:

1.Collection of soil samples,

2.Chemical analysis of soil samples,

3.Calibration and interpretation of the results of chemical analysis, and

4.Nutrient/Fertilizer Recommendation.

3.1 Important Soil Physical Properties

3.1.1 Analysis of Soil Texture

Soil texture refers to the relative proportions of sand, silt, and clay on weight basis. Soil texture was recognized from very early times by experienced farmers from the feel of moist soil placed between the thumb and forefinger. The size groups arbitrarily classified by the International Society of Soil Science (ISSS) into clay (< 0.002 mm), silt (0.002 0.02 mm), fine sand (0.02-0.2 mm), and coarse sand (0.2 2.0 mm). Each soil textural name specifies the weight percent of each fraction falls within certain defined limits. The determination of the percentage of the soil separate present in a sample is called particle-size analysis.

Each type of particle present makes its contribution to the nature of the soil as a whole. Sandy soils are usually quite permeable to air, water, and roots, but they have two important limitations. One is their relatively low water-holding capacity; the second is that they are poor storehouses for plant nutrients. These limitations of sandy soils can, of course, be overcome if both fertilizer and irrigation water are available, but the costs are large.

Clay not only have a large surface area per gram, it also has electrically charged particles. These charges give clay the capacity to attract plant nutrient ions to its surface in forms available to plants, but sand lacks this capacity. Clay holds much more water than sands because they have a large surface area to be coated with water. One important problem with clay soils is their stickiness. When wet, they stick to ploughs, boots, or whatever they contact. Loam and silt loam soils are highly desirable for most uses. They have enough clay to store adequate amount of water and plant nutrients for optimum plant growth but not so much clay as to cause poor aeration or to make working with them difficult.

4.1 Sampling and Preparation for Analysis

The determination of nutritional need of crops is an important aspect of nutrient management of any plant analysis programme. The need of fruit plants for mineral nutrients differ from those of annual crop species in a number of ways, many of which are related to the perennial nature of tree crops. In perennial tree crops, there is a need to supply nutrients both for ultimate fruit production, the fruits and the vegetative organs, which persist from year to year. The need for leaf analysis in perennial horticultural crops and forest trees has proved its superiority over soil diagnostic methods. This is essentially because of their deeper root system and the fact that nutrients supplied in one year may have its effect on both the nutrition and crop production in following years. However, it is largely being used not as an alternative method to soil testing, but as a supplement to soil testing. Results of foliar analysis are usually interpreted using the critical nutrient level (CNL) approach. Accurate interpretation of foliar analysis using the CNL approach is possible only when sampling is restricted to the same growth stage at which standard reference values have been established (Beaufils, 1971, 1973). The general purposes of plant analyses are:

5.1 Sampling

There are different sources of water such as rain, streams, canals, lakes, tanks, ponds, large reservoirs, shallow and deep wells, shallow and deep tube wells, as well as sewage effluents and sea water. The composition of the different sources of water vary with the season, rate of flow, the extent of effluents from the industrial units and sewage effluents from the municipal area during rainy season, the river tank and pond water becomes salty and muddy due to soil erosion. The depth of ground water also varies with the seasons. The sources of water available to the farmer for irrigation of crops are generally few such as canal, shallow and deep wells, pumping sets, tube wells and runoff water collected in excavations along roads, natural drains and adjoining areas under impeded drainage. The composition of water also varies with the season. Whatever may be sources of water, the water sample is taken along with the soil sample to soil testing laboratory for its analysis by way of assessing the suitability of water for irrigation.

For drawing maximum benefit the samples taken be representative of the water in bulk and it should be collected in a clean vessel. One to three litres capacity plastic bottles duly washed with a detergent are satisfactory. However, before filling with water samples the bottles must be rinsed with water samples three times. The information such as farmers name, owner of pumping sets, date and month of sampling depth of ground water, crops and soils being irrigated, any specific agricultural or human problems encountered to be briefly recorded along with the name of village, block, tehsil and district. The water sample should be properly labeled and submitted to soil testing laboratory for its analysis.

India is recovering from the euphoria of the Green Revolution and battling the residual effects of the extensively used chemical fertilizers and pesticides in the country’s soils. The decade from 1980 to 1990 alone saw the area under pesticides in India, increased a whopping 20-fold, from six million hectares to 125 million hectares. After a high annual consumption of nearly 75000 MT reached in the early ’90s, interventions in the form of Integrated Pest Management (IPM) practices have only now started to show a declining trend in the use of pesticides in India. Interestingly, India’s consumption of pesticides per hectare is low (0.288 kg/ha) when compared with world averages—0.5 kg/ha against Korea’s 6.60 kg/ha and Japan’s 12.0 kg/ha. According to the pesticides industry statistics, India spends US $3/ha on pesticides compared with US $24/ha spent by Philippines, US $255/ha spent by South Korea and US $633/ha by Japan. Yet, despite a comparatively low use of pesticides in India, the contamination of food products in the country is alarming. About 20 per cent of Indian food products contain pesticide residues above tolerance level compared to only 2 per cent globally. No detectable residues are found in only 49 per cent Indian food products compared to 80 per cent globally. A pesticide is any substance or mixture of substances intended for preventing, destroying, repelling or mitigating any pest. Though often misunderstood to refer only to insecticides, the term pesticide also applies to herbicides, fungicides and various other substances used to control pests.

Pesticides and their metabolites have received particular attention in the last few years in environmental trace organic analysis because they are regularly detected in food products, surface/ground waters and soil strata throughout the world as consequence of their widespread use for agricultural and nonagricultural purposes. So it is the need of the hour to study the levels of pesticides residues in plants, water and soil and their fate in environment after its degradation.

It would be necessary to work out tolerance limits of pesticides residues in/on crops, soil and water and the efforts, therefore, are directed towards various residue analytical methods like TLC, GLC and HPLC etc. which are suitable for the purpose.

The analytical work in a soil-testing laboratory mainly involves standard chemical methods, suitably modified to permit handling of a large number of soil samples with the required degree of accuracy and speed. Many of the analytical operations are carried out more conveniently with help of common as well as sophisticated instruments.

The various instrumental measurements followed in soil and water-testing work in the laboratory can be broadly divided into two groups:

1) Determination of pH and electrical conductivity

2) Colorimetric, spectrophotometric