eChapters

Table 19.1: Reference Guide for Landscape Plants.

Soil testing involves an estimation of its available nutrient status i.e. the portion or amount of nutrient directly available in soil for subsequent uptake by crop plant. This exercise is commonly referred to as soil testing and is used to arrive at optimum fertilizer application ratio. The need for estimation of available nutrient arises because only a small fraction of what the soil contains is the total nutrient content of the soil. Soil test are calibrated by correlating them with crop response and the result from the basis for making fertilizer recommendations.

Estimation of nutrient contents and forms in materials that are involved in nutrient supply and dynamics is a conical step towards planning scientific nutrient management. In this content, both soil and plant testing information comes out of the interpretation of analysis assumes a greater value when their concentrations and amounts can relate to soil fertility, nutrient availability, plant growth, yield and quality of the crop produce.

Determination of pH is actually measurement of hydrogen ions activity in soil-water system. It is defined as negative logarithm of the hydrogen ion activity. Mathematically, it is expressed

as:pH = - log a H⁺

The pH value of a soil is an indication of soil reaction i.e. acidic, neutral or alkaline. The nutrient availability is governed by soil reaction. It is maximum at neutral pH and decreases with increase in acidity or alkalinity. Thus, pH value gives an idea about the availability of nutrients to plants.

Principle

The pH is a measure of hydrogen or hydroxyl ion activity of the soil-water system. It indicates whether the soil is acidic, neutral or alkaline in reaction. Since crop growth suffers much under both very low (strongly acidic) as well as very high pH (alkaline) conditions.

Now a days, most of the pH meters have single combined electrode. Before measuring the pH of the soil, the instrument has to be calibrated with standard buffer solution of known pH. Since, the pH is also affected by the temperature, hence, the pH meter should be adjusted to the temperature of the solution by temperature correction knob.

The method is based on the Stokes law which states that the rate of fall of particles in suspension is directly proportional to their size. Accordingly, larger particle (sand) will settle first, followed by silt and clay. The resistance offered by the liquid to the fall of the particles varies with the radius of the sphere.

Principle

This method is based on the principle of dispersion and sedimentation techniques, employed to a given weight of soil sample. Mechanical stirring disperses macro aggregates while chemical dispersing required for micro aggregates. The mixture of dispersed soil particles in water is called a soil suspension. Sedimentation refers to the settling rates of the dispersed particles in water, which is a function of particle size and is governed by Stoke’s law.

Purpose

The bulk density varies indirectly with total pore space present in the soil and gives a good estimate of porosity of soil. Bulk density is of great importance than particle density in understanding the physical condition of soil.

Soil bulk density is defined as the ratio of the mass of the oven dry soil to its bulk volume.

Method of determination

Different methods are available for bulk density determination which differ how the soil sample is obtained and its volume is determined such as core, clod and excavation methods. There are other methods for the determination of bulk density where different principles are employed, for example, radiation method. Commonly used methods are weighing bottle, core and clod method.

Particle density determination is based on the measurements of weight of solids and their volume. The weight of solids is obtained by weighing a sample of oven dried soil. The volume of solids is determined is determined by immersion. The method outlined here is “Pycnometer method”.

Principle

A given amount of dry soil when immerses in a definitive volume of water, expels air and results in the displacement of equal volume of water. The volume of soil particles is determined by measuring the volume of water displaced in the pycnometer bottle.

Purpose

The determination of water holding capacity in soils is important as it gives an idea of the capacity of soil to hold water for the use by crops. The light soils which do not hold such water require more frequent irrigations than heavy clay soils; well decomposed organic matter increases the water holding capacity. Exchangeable sodium and type of clay mineral also have a marked effect on water holding capacity. Water holding capacity of soils is useful for selection of soils for irrigability classification. It also helps for comparing other properties of soils.

Soil aggregates refers to a group of two or more primary particles which cohere to each other more strongly than surrounding particles. Under field condition, all such units also cohere to some degree with neighboring aggregates and form larger aggregates. Such aggregates form and break easily into smaller aggregates during tillage and by disruptive action of water and air. The aggregates maintaining their identity are those in which the cohesive forces among particles are greater than the disruptive forces. A quantitative characterization of aggregates is done by determining aggregate stability and size distribution of aggregates. The aggregate stability refers to the resistance of soil aggregates to breakdown by water, air and mechanical manipulations.

The soil aggregates which can stand the disruptive forces of wetting by water are known as water stable aggregates. Therefore, the process of wetting, disruption of dry aggregates and screening of aggregates of different sizes should reasonably compare to the disruptive action of water and mechanical forces of tillage under wet conditions. The vacuum wetting of dry soil largely simulates the process of soil wetting in the surface layers but the immersion wetting is more comparable to the wetting of surface soil by irrigation. Similarly, sieving under water compares more closely with the disruptive action of water and other mechanical forces. The mean weight diameter (MWD) of water stable aggregates is a useful index to characterize the change in structure due to tillage. The mean weight diameter can be calculated using following equation.

Soil Organic Matter (SOM) is the seat of nitrogen in soil and its determination is often carried out as an index of nitrogen availability. The top soil (surface soil) range from 0.5% to 3.0% organic carbon for most upland soils and less than 0.5% are limited to desert soils. Soil organic matter averages about 58% carbon, it follows that soils generally range from about 2 to 6 % organic matter (% O.M. = %C x 1.724, The factor 1.724 = 100/58). There is also a close relationship between carbon and nitrogen in soils. Most organic matter average about 5% nitrogen so that the N: C ratio is 1:11.6. Therefore by multiplying the soil organic matter percentage by 0.05 an approximate value for the soil nitrogen, percentage is obtained. In soil the chief source of some of the nutrients essential for plant growth is organic matter, such nutrients are N, S and boron is also largely derived from organic matter.

The major part of soil nitrogen exists in complex combination in the organic compounds. It becomes available to plants after breakdown to simple form followed by mineralization. The chemical methods for determination of soil nitrogen availability involve either direct or indirect of total nitrogen or measurement of a fraction of easily hydrosable nitrogen by employing solution of dilute acid or alkali. Distillation of alkaline potassium permanganate solution has often been adopted for estimating the ox disable and reactive form of soil nitrogen. The available nitrogen in soil is determined by alkaline permanganate method as per procedure suggested by Subbiah and Asija (1956).

Principle

A known weight of the soil is mixed with alkaline potassium permanganate (KMnO₄) solution and distilled. The organic matter present in soil is oxidized by the nascent oxygen, liberated by potassium permanganate, in the presence of sodium hydroxide and the released ammonia is condensed and absorbed in known volume of a boric acid with mix indicator to form ammonium borate, the excess of which is titrated with a standard sulphuric acid.

The term available phosphorus (P) refers to the inorganic form, occurring in soil solution, which is almost exclusively ‘orthophosphate’. The available P is considered to be a fairly good indicator/measure of the P supplying capacity of a soil. A knowledge of available P content of soils is important for determining its critical limit , based on soil-test-crop response calibration study.

Principle

In soil, phosphorus exists in the form of various types of orthophosphates. A very small fraction of these is available to plants at a given time. Available phosphorus content of soil consists mainly of Ca-P, Al-P and Fe-P. In the neutral and alkaline soils particularly, Ca-P is the dominant fraction. Organic –P fraction is also considerable amount, but is usually not include in the determination of available phosphorus. A large number of extractions, buffer solutions, acids and chelating agents have been suggested for available phosphorus from time to time. However, no single extractant appears to be suitable for all types of soils. Two types of extraction methods are more popularly adopted. Under acidic soil conditions, Bray No.1, which involves soil extraction with a solution consisting of 0.003 N NH₄F and 0.025 N HCl is widely followed. The fluoride complexes Al and Fe in soil, thus releasing some bound p besides the easily acid soluble P (largely Ca-P). This extraction is suitable for soils containing less than 2% calcite or dolomite because in calcareous soils, carbonates quickly neutralize the acid, resulting in less extraction of P.

The total potassium content (K) of a soil varies from 0.05 to 2.5%. The total K is distributed in mineral form (90-98%), fixed non-exchangeable (1-10%) and exchangeable plus water soluble K(1-2). Exchangeable K represents an average of 2-15% of the sum of exchangeable bases and 1-3% of the total capacity of cationic exchange.

Principle

It is based on the principle of the equilibrium of the soils with an exchangeable cation mode of the solution of neutral ammonium acetate, in a given soil:solution ratio. During the equilibrium, ammonium ions exchange with the exchangeable K ions of soil.The K content in the equilibrium solution is estimated with a flame photometer.

Sulphur (S) occurs in different forms in soil viz., sulphites, sulphates , sulphides and in organic compounds. Plants absorb S in the form of sulphate(SO₄ ^2-) . The organic forms of S compounds become assimilable, especially following microbiological transformation into SO₄ ^2-. The mineralization of organic sulphur in a soil depends primarily upon the N:S ratio and any SO₄ ^2- formed may be fixed against extraction, particularly if much Fe and Ba is present or if soil is very acidic.

Principle

Beside some amount in the soil solution, available sulphur in mineral soils occurs mainly as adsorbed SO₄ ^2- ions. Both CaCl₂ and phosphate solutions (as monocalcium phosphate) are generally used for replacement of the adsorbed SO₄ ^2- ions. Use of Ca salts have a distinct advantage over those of Na or Na, as Ca prevents deflocculating in heavy textured soils and leads to easy filtration. SO₄ ^2- in the extract can be estimated turbid metrically using a spectrophotometer. Sulphur in the extract can also be measured by colorimetric method using barium chromate but the turbimertic method given below is mainly used in the soil testing laboratories.

Carbonate is a natural constituent of many soils occurring as sparingly soluble, alkaline earth carbonate, chiefly as calcite and dolomite. Most carbonate minerals found in local soils are calcite and account 90% of total soil carbonates. The determination of total carbonate such as CaCO₃in soil is of great interest on account of high useful ess for diagnosing soil status in terms of structure, texture, biological activity or nutrient content. In calcareous soils carbonates exert a major influence on the chemical properties. Also carbonate content is used as a differentiating criterion for some classes at the family level of soil taxonomy.

Principle

A qualitative rapid test for free calcium carbonate is made by adding 10 mL of diluted HCl(1:3) to about 5 g of air dry soil on a watch glass and observing the intensity of effervescence which results from the evolution of CO Based on the . degree of effervescence the amount of calcium carbonate present in the soil can be broadly categorized into “traces or negligible, low, medium, high and very high”. Soils having traces to low amounts of calcium carbonate do not pose any problem. But those showing medium to very high degree of effervescence, a separate quantitative analysis should be carried out using Puri’s rapid titration method.

Calcium (Ca) and magnesium (Mg) are the two most abundant alkaline earth cations in soils. They both occur as soluble ions in soil solution, exchangeable, non-exchangeable and in organic material. Soils from arid regions are richer in Ca (1-2% CaO), while those from humid zones have a low content (0.01-0.02% CaO). On the other hand exchangeable Mg is found in very small quantities in calcareous soils. Sometimes, in acid soils, Mg²⁺ represents the largest quantity of the sum of exchangeable bases.

Principle

The method is based on the principle that calcium, magnesium and a number of other cations form stable complexes with versenate (ethylendiaminetetraacetic acid disodium salt, EDTA) at different pH. A known volume of standard calcium solution is titrated with standard 0.01N EDTA solution using murexide (ammonium purpurate) indicator in the presence of NaOH solution.

The end point is a change of color from orange red to purple at pH 12 when the whole of Ca forms a complex with EDTA. Calcium plus Mg are also present in the solution and can be titrated with 0.01N EDTA using buffer solution and a few drops Eriochrome Black T indicator. The end point is a change of color from red to blue at pH 10. Beyond pH 10, magnesium is not bound strongly to Erichrome Black-T indicator to give a distinct end point.

Principle

Carbonate (CO₃) and bicarbonate(HCO₃) ions are species of the same acid, carbonic acid. Their proportionate content is a function of pH. The CO₃ starts to form as pH rises above 8.4. Carbonate and bicarbonate are generally determined in soil saturation extract by titration with 0.01 N H₂SO₄ to pH 8.3 and 4.5, respectively.

Apparatus Required

Burette, pipette, conical flask and mechanical shaker etc

Soluble chloride (Cl) concentration in the extract is determined by silver nitrate titration. This method quantifies the concentration of Cl (meq/L) in the saturation paste extract. Chloride may be determined using anion selective electrode (potentiometric). Plant tolerance to Cl can be related to its concentration in the soil saturation paste extract. The method detection limit is approximately 0.1 meq/L, dependent on the method of analysis, and is generally reproducible within ± 10 %.

Apparatus Required

Burette, pipette, conical flask and mechanical shaker etc

All atoms can absorb light at certain discrete wavelengths corresponding to the energy requirement of the particular atom. When at ground state the atom absorbs light it is transformed into the excited state. It is the same atom containing more energy. This energy is measured in relation to the ground state and a particular excited state say for example in case of Na may be 2.2 eV (electron volts) above the ground state.

Each transition between different electronic energy states is characterized by a different energy and by a different wavelength. These wavelengths are sharply defined and when a range of wavelengths is surveyed, each wavelength shows as a sharp energy maximum (a spectronic line). These characteristic lines distinguish atomic spectra. The lines, which originate in the ground state of atom, are most often of interest in atomic absorption spectroscopy. These are called the resonance lines. The atomic spectrum, characteristic of each element, then comprises a number of discrete lines, some of which are resonance lines. Most of the other lines arise from excited states rather than the ground state. The lines of excited states are not useful generally in atomic absorption analysis as most of the atoms in a practical atomizer are found in the ground state.

Boron occurs as anion in soils and isrequired by plants in very small quantity. Water soluble B makes the estimate of its availability to plants. Total boron in soils varies from 20 to 200 mg kg^-1 and available (water soluble) boron in soils ranges from 0.03 to 12 mg kg^-1 respectively. The threshold value ranging from 0.1 to 0.5 mg kg^-1 (water soluble B) depends upon the soil type, crops, and other factors, below which the response to applied boron may be expected. Its availability is affected by soil pH as under:

A pesticides residue is any substance or mixture of substances in food, agricultural commodities or animal feed resulting from the use of pesticides and includes any specified derivatives such as degradation products and impurities that are considered to be of toxicological significance.

Pesticide residue analysis (PRA) may be defined as the quantitative and qualitative analyses of the representative samples drawn from agricultural field, market and environment for pesticides and their toxic metabolites.

Chromatography is a physical method of separation in which components to be separated is distributed between two phases, one of which is stationary while other moves in a definite direction. By separating the sample into individual components, it is easier to identify (qualitative) and measure the amount (quantitative) of the various sample components. There are numerous chromatographic techniques and corresponding instruments.

Gas Liquid Chromatography (GLC) is the most widely used analytical technique for estimation of volatile and thermo stable type of pesticides with speed, accuracy, reproducibility and sensitivity.

Although Gas Liquid Chromatography (GLC) is widely used technique for pesticide residue analysis, it has some limitations. It can analyze of volatile and thermo stable type of pesticides only because it operate at high temperature. These problems have overcome in HPLC. Although HPLC can analyze both polar as well as non-polar type of compounds but it has less sensitivity than GLC.

Principle

Acetone is used for extraction the most nonionic pesticide residue, and residue are partitioned from acetone to dichloromethane/hexane phase. After removing traces dichloromethane, final extract is made up with acetone. This extract can be determined by GC with selective detectors. Further for organochlorine and synthetic pyrethroids extract should be cleaned up using florisil column. And for determination organophosphate extract should undergo to charcoal-celite cleanup.

For the determination carbamates, the extract is subjected to C-18 solid phase cartridge clean up.

Principle: Acetone is used for extraction the most nonionic pesticide residue, and residue are partitioned from acetone to dichloromethane/hexane phase. After removing traces dichloromethane, final extract is made up with acetone. This extract can be determined by GC with selective detectors FPD-P/NPD without cleanup. Further for organochlorine and synthetic pyrethroids extract should be cleaned up using florisil column. And for determination organophosphate extract should undergo to charcoal-celite cleanup.

Principle

Pesticide residues are extracted from soil using a mixture of acetone/water in a soxhlet apparatus for 7-8 hours. This extract can be determined directly by GLC.

Scope: This method is suitable for most OPs, OCs, SPs, and herbicides residue analysis.

Principle

Pesticide residues in milk are derived from the intake of pesticide contaminated feed and fodder.

Scope

This method is suitable for most OPs, OCs, SPs, and herbicides residue analysis.

Scope

This method is suitable for most OPs, OCs, SPs, and herbicides residue analysis.

Apparatus required

• Homogenizer
• Centrifuge
• Nitrogen evaporator

Horticultural plants present a kaleidoscopic scenario from small herbs to very big deep rooted perennial trees. The associated rhizospheres present variable conditions as to supply of nutrients, water and air. The principles governing their physical, chemical and biological properties along with their relationship with plants need to be understood clearly. The plants differ in their requirements at various stages of growth; hence their tissues require constant monitoring of nutrient status for better productivity and quality. As nutrient behavior in soils is governed by soil properties and environmental conditions, measurement of such properties is often required. These include pH, salinity, organic matter, CaCO₃ and texture in drier areas the presence of Na and gypsum (CaSO₄2H₂O) is also of concern.The irrigation water is a source of dissolved salts which are also nutrients and in excess impede water and nutrient relations of soil and plant. It requires a thorough understanding of the principles underlying methods of collection, characterization of water, air and nutrient relations of soil-plant system, monitoring of plant nutrient status, quality of irrigation water and soil and water pollution for effective management of nutrients avoiding pollution.

Soil and plant analysis is a diagnostic instrument for soil fertility and basis for fertilizer recommendation; to know where and where not fertilizer is to be applied. Obtaining accurate and precise values has always being the basis of soil analysis.

Introduction

The objective of soil sampling is to obtain reliable information about a particular soil. The value of laboratory data in soil studies largely depends on effective sampling. Soil sampling is one of the most important aspects in the determination of the properties of the soil. Care in preparation and analysis can overcome the damages of careless or inappropriate sampling in the field (Bates, 1993). Errors arising from sampling are mainly related to spatial variability of the soil. While keeping in mind financial constraints, sampling procedures should be adopted properly to minimize the influence of spatial variability and to maximize the accuracy of mean plot values for the measured soil variables. The obtained soil data should be representative for the whole observation plot, taking into consideration the spatial variability of the soil within the plot.

Importance of soil sampling

Soil sampling and testing are done to provide an estimate of the capacity of the soil to supply adequate amount of nutrients to meet the needs of growing crops and information to its nature and problem. Traditionally, the goal of soil sampling is to develop a representative estimate of a field. A few grams of soil are actually used for the laboratory analysis. That small amount must represent the entire area for which the recommendation is to be made. The samples should be collected in such a way that it would represent the area for which recommendation is to be made.

Introduction

Plant analysis can be defined as the quantitative determination of the concentration of an element or extractable fraction of an element in a sample from a particular part or portion of a crop. From the nutritional standpoint, plant analysis is based on the principle that the concentration of a nutrient within the plant is an integral value of all the factors that have interacted to it. Plant analysis is used as an index of available nutrient element supply. Plant growth or yield are compared with the elemental concentrations contained in the dry matter of the entire plant or plant structures such as leaves, petioles, fruit or grain sampled at different times during their development. Plant analysis gives the overall picture of the nutrient levels within the plant at the time the nutrient was taken. The use of plant analysis is based on the principle that the nutrient level present is as a result of all factors affecting the growth of the plant.

Plant analysis involves the determination of nutrient concentration in diagnostic plant part(s) sampled at recommended growth stage(s) of the crop. In a way plant analysis complements soil analysis. There are reliable sampling criteria and procedures for most of the world’s commercial crops.

Importance

Plant analysis assesses nutrient uptake while soil testing predicts nutrient availability. The two tests are complementary to each other as crop management tools. Plant analysis will detect unseen hidden hunger and confirm visual deficiency symptoms. Toxic levels may also be detected. If it is done early, plant analysis will allow a corrective fertilizer application in the same season.

Three types of water movement within the soil are recognized. They are

1. Saturated flow
2. Unsaturated flow
3. Water vapour movement

Saturated flow

This occurs when the soil pores are completely filled with water. This water moves at water potentials larger than 33 kPa. Saturated flow is water flow caused by gravity’s pull. It begins with infiltration, which is water movement into soil when rain or irrigation water is on the soil surface. When the soil profile is wetted, the movement of more water flowing through the wetted soil is termed percolation.

Hydraulic conductivity can be expressed mathematically as

Soil air, its composition, gaseous exchange

Soil air is a continuation of the atmospheric air. It is in constant motion from the soil pores into the atmosphere and from the atmosphere into the pore space. The circulation of air in the soil and renewal of component gases like oxygen and carbon dioxide is known as soil aeration. Soil aeration is essential for the respiration and survival of soil organisms and plant roots.

Composition of soil air

Soil air contains gases like nitrogen, oxygen, carbon dioxide, water vapour and others. The composition of soil air is different from atmospheric air. Soil air contains more carbon dioxide and less oxygen than atmospheric air. It also contains more water vapour than atmospheric air. The nitrogen content of soil air is almost equal to atmospheric air. (Table below)

Measurement of soil aeration

Oxygen Diffusion Rate (ODR) - the rate at which O₂ in soil air is replenished. ODR decreases with soil depth.

Measurement of ODR in soil

Lemon and Erickson (1952) devised a method for measuring oxygen diffusion in the soil system with the help of platinum micro electrode

Principle

When a certain potential is applied across the platinum electrode and a reference electrode inserted in the soil, oxygen is reduced at the platinum surface. The electric current flowing between two electrodes is proportional to the rate of oxygen reduction. The later, in turn, is related to the rate of oxygen diffusion to the electrode. The oxygen diffusion rate (ODR) is calculated from the measured electric current with the following equation.

In nature there are nearly 300 nuclei, consisting of different elements and their isotopes. Isotopes are nuclei having the same number of protons but different number of neutrons. These techniques are now being used in almost all the areas of plant physiology, soil chemistry and plant biochemistry. In agriculture, radioisotopes are used in the nutritional studies of major and minor elements, plant metabolism, mechanism of photosynthesis studies, and uptake of nutrients and ions mobility in soil. In applying this technique, a minute quantity of radioisotope element is usually mixed with ordinary element of the same kind, the whole batch is termed “tagged” and their role in chemical reactions is traced. The radio labeled fertilizer has been used to study the uptake, retention and utilization of fertilizers.

Radioisotopes are used for determining the function of fertilizer in different plants. The tracer technique is used to study the rate and direction of movement of an element in a plant. For this a radioisotope of that element is injected in the ground near the plant. After a few days the plant is laid on a photographic paper to produce an autoradiograph. The dark areas in the radiograph show the positions reached by the element. This technique gives valuable information regarding the optimum season for fertilizing crops and for poisoning weeds. Use of N¹⁵ in organic fertilizer studies has significantly advanced the understanding of N release from organic materials. Crop residue or green manure studies are relatively simple. These techniques have been extremely useful in determining how residue quality parameters affect mineralization values. Because of the attention focused on nutrient cycling and management studies in recent years, a number of new P isotope techniques have been developed.

Microorganisms being minute and microscopic are ubiquitous. Besides supporting the growth of various biological systems, soil and soil microbes serve as a best medium for plant growth. Soil fauna & flora convert complex organic nutrients into simpler inorganic forms which are readily absorbed by plants.

Soil microbes as nitogen fixers-Symbiotic (Associative): The organisms involved are Rhizobium, Bradyrhizobium in legumes(aerobic): Azospirillum, Azotobacter (grasses), Actinonycetes- Frantkia (with Casuarinas, Alder).

Soil microbes as biocontrol agents: Several ecofriendly bioformulations of microbial origin are used in agriculture for the effective management of plant diseases, insect pests, weeds etc. eg: Trichoderma sp and Gleocladium sp are used for biological control of seed and soil borne diseases. Fungal genera Entomophthora, Beauveria, Metarrhizium and protozoa- Maltesia grandis. Malameba locustiae etc.- are used in the management of insect pests. Nuclear polyhydrosis virus (NPV) is used for the control of Heliothis/American boll worm. Bacteria like Bacillus thuringiensis, Pseudomonas are used in cotton against Angular leaf spot and bollworms.

Degradation of pesticides in soil: Soil receives different toxic chemicals in various forms and causes adverse effects on beneficial soil micro flora / micro fauna, plants, animals and human beings. Various microbes present in soil act as the scavengers of these harmful chemicals in soil. The pesticides/chemicals reaching the soil are acted upon by several physical, chemical and biological forces exerted by microbes in the soil and they are degraded into non-toxic substances and thereby minimize the damage caused by the pesticides to the ecosystem. For example, bacterial genera like Pseudomonas, Clostridium, Bacillus, Thiobacillus, Achromobacter etc. and fungal genera like Trichoderma, Penicillium, Aspergillus, Rhizopus, and Fusarium play important role in the degradation of the toxic chemicals / pesticides in soil.

Biodegradation of hydrocarbons: Natural hydrocarbons in soil like waxes, paraffins, oils etc are degraded by fungi, bacteria and actinomycetes. E.g. ethane (C2 H6) is metabolized and degraded by Mycobacteria, Nocardia, Streptomyces Pseudomonas, Flavobacterium and several fungi.

There are four major tracts where salt affected soils are commonly met within India. These are:

1. The Semi-arid Indo-Gangetic alluvial tracts (mainly in Punjab, Haryana, Uttar Pradesh and a part of Bihar)
2. The arid tracts of Rajasthan and Gujarat.
3. The arid and semi arid tracts of southern states, particularly of the irrigated rigor (Vertisols) soils.
4. The coastal alluvium

Earlier it was estimated that about 7 million hectares of land have been affected by salinity/sodicity conditions in India. The area under these soils increased and reached to the level 10 million hectares at present.

Chemistry of vegetable crops

Vegetables play an important role in human nutrition supplying some of the materials, which other food materials are lacking. Vegetables are important in neutralizing the acidity formed during digestion. They also serve as roughages in human nutrition and thus helping digestion.

They are the important sources of minerals like Ca, P and Fe. They also form an important source of vitamins, especially vitamin A. Green and yellow vegetables like carrot, turnip, spinach, beans, etc. contain appreciable amounts of vitamin A. Direct seed of beans, peas, and legumes contain appreciable amounts of proteins. Vegetables also contain sufficient amounts of vitamin C and also appreciable quantities of thiamine, niacin and folic acid. Tomatoes and potatoes contain fairly high amounts of vitamin C.

Leaf analysis and its importance

Improved fertilizer management for crops is important in view of today’s need to reduce production costs, conserve natural resources, and minimize possible negative environmental impacts. These goals can be achieved through optimum management of the fertilizer applied. Understanding the crop nutrient requirements and using soil testing to predict fertilizer needs are keys to fertilizer management efficiency.

Plant tissue testing is another tool for use in achieving a high degree of precision in fertilizer management. Timely tissue testing can help diagnose suspected nutrient problems or can simply assist in learning more about fertilizer management efficiency. Leaf analysis (also called stem leaf analysis, tissue analysis or foliar analysis) is the most precise method of monitoring plant nutrient levels. While soil analysis reveals the levels of essential soil nutrients, leaf analysis shows the grower exactly what the plant has successfully absorbed. Leaf analysis is especially helpful in detecting nutrient deficiencies before they affect plant health and yield.

The crop growth and productivity is conditioned by many factors of which, the nutrient status (Content) of plant parts such as leaf, stem, etc play a critical role. Moreover the leaf and stem are considered as the indicator parts of plants for assessing the nutrients content of plant. Each crop plant requires the essential element at a specific concentration at different growth stages and it is known as ‘critical level’. When the nutrient content of plant depletes below the critical level the plants may exhibit some symptoms. The requirement or otherwise the availability of nutrients can be assessed by i) plant diagnosis ii) soil analysis and iii) plant analysis by two methods a) by qualitative test and b) by quantitative estimation. Based on the plant or soil tests, the required nutrients can be applied for crops to sustain the growth and rectify the deficiency disorders. The rapid tissue test would pave way for rectifying the nutritional problems for quick recovery, however the quantitative estimation of both plant and soil for nutrients concentration will be more useful and economic for applying fertilizers either as basal or foliar and would be the long term strategy to cope up with nutritional problems.

For rapid tissue test to assess the nutrient status, different parts of plant should be taken as indicator tissue and some of the representative crops are furnished below:

Introduction

It is necessary to ascertain the quality of irrigation water so as to gauge the possible effects of this water on the soil. The irrigation water often results in water logging, salinity and alkalinity problems. It is therefore very important to know its quality. Following are the basic criteria or characteristics on which irrigation water quality is judged.

1. Presence of total soluble salts in irrigation water.
2. Proportion of carbonates and bicarbonates in relation to calcium and magnesium in irrigation water.
3. Proportion of sodium in relation to other cations in irrigation water.
4. Concentration of specific ions in irrigation water.

Several parameters are employed to judge the quality of irrigation waters No parameter in complete in judging the quality of irrigation water. Further the quality of water should be judged by considering the crop and soil factors. Any judgment without considering the above factors may not be useful. Researchers devised many parameters by following above said criteria. Some of the commonly adopted parameters used to judge the quality of irrigation water are:

Introduction

In classification of irrigation water, it is assumed that the water will be used for average conditions with respect to soil texture, infiltration rate, drainage, quantity of water used, climate and salt tolerance of the crop. Large deviations from the average for one or more of these variables may make it unsafe to use what, under average conditions, would be a good water; or may make it safe to use what, under average conditions, would be a water of doubtful quality. The relationship to average conditions must be kept in mind in connection with the use of any general method for the classification of irrigation water.

However, the quality of irrigation water is evaluated, mainly by the quantity of dissolved salts, proportion of anions, monovalent and divalent cations, sodium adsorption ratio, residual sodium carbonate, potential salinity, specific ion toxicity, proportion of magnesium, calcium in water etc.

Soil pollution is any physical or chemical change in soil that adversely affects the health of plants and other organisms living in and on it.

Soil pollution is important not only in its own right but because so many soil pollutants tend to move into surface water, groundwater, or air.

The chief soil pollutants are salts, petroleum products, heavy metals, and agricultural chemicals. Point source releases of industrial solvents (both LNAPLs and DNAPLs) and feedstock chemicals (e.g., for making plastics) often occur as the result of LUSTs (leaking underground storage tanks).

Salinization, a common problem in irrigated arid and semiarid regions, makes soil unfit for growing most crops. It is extremely difficult to remove excess salts from salinized soils.

A variety of techniques are used for soil remediation, which is cleaning up contaminated soil.

1. These include physical techniques such as
   1. Soil vapor extraction (SVE), moving air through the soil.
   2. Thermal conduction heating, heating the soil before SVE.
2. Bioremediation methods include
   1. Landfarming, fertilizing, liming, and repeatedly plowing contaminated soil to stimulate bacterial decomposition of high molecular weight hydrocarbons like those in diesel oil.
   2. Bioventing, pumping air at low rates through soil to provided O₂.
   3. Biosparging, pumping air into aquifers to provided O₂.
   4. Phytoremediation, particularly useful for removing heavy metals.

Introduction

Many of the globe’s major food crops viz., wheat, rice and vegetables have reached “yield stagnation” or “yields plateau”. This can be attributed to the land degradation and soil diminishing capacity to supply nutrients at the required rate due to over mining of essential nutrients. Further, imbalance use of fertilizers and lesser use of organic manures over the years have badly affected the soil health. To bring back life to the soil and relieve it from nutrient stress; fertilizers and manures/bio-fertilizers have to be applied in a scientific manner. At the same time, their indiscriminate use can result either in their under utilization and toxicity or leaching to the lower horizons, leading to underground water and create pollution problems. The soil is also the main source of sixteen mineral elements identifies in view of their importance for plant growth. These are N, P, K, S, Ca, Mg, Na, Fe, Mn, Zn, Cu, Mo and Cl (Tisdale et al., 1985). Nitrogen, potassium and phosphorus have been designated as major nutrient elements in order of their removal by the plant; sulphur, calcium and magnesium as secondary nutrients while zinc, copper, iron, manganese and molybdenum are called micro nutrients as these are taken by plant in traces. 
Nutrients need of vegetables depends upon the yield response to applied nutrients from fertilizers, agro-climatic conditions, variety and nutrient availability from the soil pool. The nutrient status of the soil further depends upon the quantity of total and available nutrients present in the soil pool. The soil tests helps in selecting the proper manuring schedule for various crops in a sequence by knowing about the nutrient build up or depletion in the soil profile. The adequacy of soil testing methods to correctly predict the fertilizer needs varies with the soil type, climate/soil moisture and crop species. The rainfall also influences the efficiency of soil testing methods, especially nitrogen and potassium when it becomes imperative to have tissue analysis for modify the fertilizer recommendations. Plant tests also help to identify the nutrient deficiency in standing crop and in the event of deficiency of a particular nutrient helps in taking corrective measures (Sud 
et al., 2008).

Estimation of organic carbon in soil (Walkley and Black, 1934)

Principle: Soil organic carbon is oxidized to CO₂ by heat of dilution obtained when a known amount of soil is treated with excess of standard K₂Cr₂O₇ in the presence of concentrated H₂SO₄. The excess of K₂Cr₂O₇ not reduced by the organic matter is back titrated with standard ferrous ammonium sulphate in the presence of phosphoric acid or sodium fluoride using diphenyl indicator. At the point colour changes from violet to blue or bright green.

Apparatus: 500 ml conical flasks, Pipettes of 10 and 20 ml, Burette, Balance, Measuring cylinder, Asbestos sheet.

For plant analysis to be meaningful, collection of particular plant part(tissue) at the right stage of growth is very important. A research worker in soil fertility is generally interested in finding out the nutrient uptake by crop at different stages. The stage at which nutrient uptake is to be studied will vary with different crops. One may also be interested in collecting a plant sample when some abnormality is observed in the plants and a nutrient deficiency or toxicity is suspected. In such cases no time should be lost in collecting the sample otherwise secondary reactions that often take place in the mineral content of plants will conceal the real cause of deficiency or toxicity. In such cases only the affected plants are to be sampled as compared to general sampling technique described below.

For research projects each plot receiving differential fertilizer or some other treatment is to be sampled separately. For general advisory purposes one sample would be good enough for the entire field (not exceeding 2 hectares). One should, however, take care that the sample comes from a field or a part there of having a uniform stand. If a field is patchy and differences in stand and plant growth are obvious, each such area should be sampled separately.

Irrigation water always contains some soluble salts irrespective of its source. The suitability of waters for a specific purpose depends on the types and amounts of dissolved salts. Some of the dissolved salts or other constituents may be useful for crops. However, the quality or suitability of waters for irrigation purposes is assessed in terms of the presence of undesirable constituents, and only in limited situations is irrigation water assessed as a source of plant nutrients. Some of the dissolved ions, such as NO₃^- are useful for crops.

Collection of water samples

A representative water sample (500 ml) is collected in a glass or polyethylene bottle, which should be properly washed/rinsed with the same water that is being sampled. Floating debris or any other contaminant should be avoided while collecting the sample. After proper labelling (e.g. source of water, date of collection, and type of analysis required), the sample should be sent to the laboratory without undue delay.
Some of the anions such as SO₄ and NO₃ may be quite low in irrigation waters. Hence, the large volume of the sample has to be first concentrated by evaporating it to about 100 ml in order to obtain their detectable amounts.

The basic philosophy of nutrient management should be to apply fertilizers at rates to ensure high fertilizer use efficiency so that the amount of unutilized fertilizers is reduced to environmentally acceptable levels. Leaching of nitrate into public drinking water supplies has become a national problem. There is no realistic alternative but to use increasing quantities of inorganic fertilizers for maintaining food supplies in order to feed the increasing population. Used properly, fertilizers can, in fact, lead to better environment because greater vegetative growth and higher crop yields leave large quantities of residues in the soil, thus reducing erosion; absorb more carbon dioxide, a major source of green house effect and lessen the need to bring marginal and forest land into cultivation. The declining or stagnating yield trend in India has been attributed to multiple nutrient deficiencies and imbalance of nutrients. Therefore, the most widely recognized alternative is integrated nutrient management (Trehan et al., 2008).

Soil test based fertilizers is the maintenance of soil fertility and plant nutrient supply to an optimum level for sustaining the desired productivity through optimization of the benefits from fertilizers, organic manures, green manures, bio-fertilizers, non-conventional sources and crop residues. Soil test based fertilizers  aims at maximization of the use efficiency and minimization of the avoidable losses of nutrients from all the sources such that triple objective of maximization of crop yields, sustenance of soil, water and air quality and improvement of socio-economic conditions of farming community is accomplished.

Water quality standards (Classification)

Some countries have established national standards and criteria for evaluation and classification of water quality. The classification of irrigation waters has been made according to one or more of the following criteria.

1.    Salinity or total concentration of soluble salts (EC)
2.    Sodicity hazards (SAR)
3.    Alkalinity hazards (RSC)
4.    Combined evaluation on the basis of EC and SAR by USSL diagram
5.    Combined evaluation on the basis of EC and Adj. SAR
6.    Combined evaluation on the basis of EC, SAR and RSC

The essential plant nutrients, or essential elements, are listed in the Table 41. It enumerates 16 chemical elements of which the essentiality for plant growth and reproduction has been established according to the criteria formulated by D.I. Arnon and P.R. Stout, 1939. This listing has existed since 1954 with chlorine as the latest addition (cited by Hopkins 1999).

These plant nutrients are classified into various groups:

1.    Major elements and trace elements : Of the 16 listed, 9 are major elements or macronutrients and 7 are trace elements or micronutrients. Major elements are those which are needed by plants for their growth and reproduction in relatively large amounts while trace elements are needed in smaller amounts compared to the former. The major elements are further sub classified into structural elements (carbon, oxygen and hydrogen), primary macronutrients (nitrogen, phosphorus and potassium), and secondary macronutrients (calcium, sulfur, and magnesium). Micronutrients are Zn, Cu, Mn, Fe, Mo4 and boron (Table 41).

Cation exchange: The ‘soil cations’ essential for plant growth include ammonium, calcium, magnesium, and potassium. There are three additional ‘soil cations,’ which are not essential plant elements but affect soil pH. The additional ‘soil cations’ include sodium, aluminum and hydrogen.

The major distinguishing characteristic of cations is their positive charge. Just like a magnet, a positive charge is strongly attracted to a negative charge. When soil particles have a negative charge, the particles attract and retain cations. These soils are said the have a cation exchange capacity. Although most soils are negatively charged and attract cations.

The ‘soil cations’ are further divided into two categories. Ammonium, calcium, magnesium, potassium, and sodium are known as the ‘base cations,’ while aluminum and hydrogen are known ‘acid cations.’ Unlike the other base cations, sodium is not an essential element for all plants. Soils that contain high levels of sodium can develop salinity and sodicity problems.

The words ‘base’ and ‘acid’ refer to the particular cation’s influence on soil pH. As you might suspect, a soil with a lot of acid cations held by soil particles will have a low pH. In contrast, a highly alkaline soil predominately consists of base cations.

Cations in the soil compete with one another for a spot on the cation exchange capacity (Table 42 and Fig 18). However, some cations are attracted and held more strongly than other cations. In decreasing holding strength, the order with which cations are held by the soil particles follows: aluminum, hydrogen, calcium, potassium and nitrate, and sodium.