Soil and Plant Analysis 9394490434, 9789394490437

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ISBN 939449043-4

9 789394 490437

SOIL AND PLANT ANALYSIS

NIPA GENX ELECTRONIC RESOURCES & SOLUTIONS P. LTD. New Delhi-110 034

About the Author Dr. Rahul Dev Behera holds Ph.D. degree in Soil Science & Agricultural Chemistry from the Odisha University of Agricultural Technology, Bhubaneswar, Odisha in 2021. He has currently working as SMS (Soil Sc. & Ag. Chem.), Krishi Vigyan Kendra, Bolangir, OUAT, Odisha. He has published 22 nos of research papers in International and National journals. He has published 4 number of books with 10 book chapters. He worked with different rural farmers in Malkangiri and Bolangir District of Odisha with transferred the different new technologies to the farmers. He has tested more than 2000 numbers of soil samples at soil testing laboratory of KVK and distributed more than 5000 number of Soil Health Card to the farmers. He has given different skill and new technology training to the farmers and have also asked them to adopt them, which they have increased the production. He was awareded Scientist Award in Agriculture Science by International Multidisciplinary Research Foundation, 2017, Fellowship Award in Environmental Science by Environment & Social Welfare Socity, 2017 and Dr. APJ Abdul Kalam Award with medal in Soil Science by Odisha Rajya Sanskruti Surakhya Mancha, 2017. He gives different information to the farmers through Radio talk and articles through newspapers as well as through Booklet and leaflets.

SOIL AND PLANT ANALYSIS

Rahul Behera, Ph.D SMS (Soil Science & Agricultural Chemistry) Krishi Vigyan Kendra, Bolangir Odisha University of Agricultural Technology Odisha-751 003

NIPA GENX ELECTRONIC RESOURCES & SOLUTIONS P. LTD. New Delhi-110 034

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eISBN: 978-93-94490-89-5

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Preface In Odisha most of the area (41.16%) is under cultivable land. Most of the people are dependant on farming. They use huge amount of fertilizers without applying balanced fertilizers. They do not test soils and apply unbalanced fertilizers which leads to less growth of crops and yields. Despite our best and diverse efforts for sustainable crop production, the imbalance use of major nutrients (N, P, K, Ca, Mg & S) and micronutrients (Fe, Mn, Cu, Zn & B) decreases the production of the crop. In agriculture eco-system, soil and plant are indispensable for food production. For conservation and judicious use of natural resources, analysis of soil and plant is of utmost importance. Therefore, for any application of inputs or management of crop to be sustainable, testing of soil/plant parts in respect of plant nutrients is pre-requisite. There is qualitative and quantitative analysis in respect of nutrient in mandatory to arrive at appropriate conclusion/recommendation. To provide the soil testing laboratory with sustainable technical literature on various aspects of soil testing, including testing methods and formulations of fertilizer recommendations etc, the Krishi Vigyan Kendra, Bolangir has decided to bring out this book. The book provides elaborate information on major soil types of India, their composition, plant nutrient and their function, typical deficiency symptoms of nutrients in plants, apart from procedure of sample collection and methods of analysis. This manual contains analytical concepts on nutrients covering principles, materials, equipments, methods. Calculations and interpretations. Moreover, clear and easily understandable words/sentences have been used in this manual for better clarity of the analytical methods used. This book can also be used as a tool for soil testing in research laboratories for advancing research and preparing Soil Health Card for the farmers. It is expected that this book will be useful for the technical staff of the soil testing laboratories in doing day-to-day analytical work and framing fertilizer use recommendations. Dr. Rahul Dev Behera SMS (Soil Sc. & Ag. Chem.) KVK, Bolangir

Contents Preface ........................................................................................................... v 1. Collection of Soil Sample .......................................................................... 1 2. Processing of the Soil Samples ................................................................. 5 3. Determination of Soil Texture by Feel Method ......................................... 7 4. Determination of Bulk Density, Particle Density and Pore Space in Soil .......... ........................................................................11 5. Determination of Water Stable Aggregates ............................................ 13 6. Soil Moisture Determination ................................................................... 15 7. Determination of the Field Capacity ....................................................... 17 8. Determination of the Infiltration Rate of Water by Double Ring Infiltrometer Method.......................................................... 19 9. Determination of the Maximum Water Holding Capacity of Soil ...................................................................................... 21 10. Determination of Soil Texture by Bouyoucos Hydrometer Method ............................................................................... 23 11. Analytical Chemistry-Basic Concepts and Calculations ......................... 27 12. Determination of Electrical Conductivity of Soil ..................................... 43 13. Determination of Organic Carbon in Soil by Walkley: Black Method .......................................................................... 49

Contents / viii

14. Determination of Lime Requirement by Woodruff Buffer Method ........................................................................................ 59 15. Determination of Neutralizing Value of Sample ..................................... 61 16. Determination of Available Nitrogen by Kjeldahl Method ...................... 63 17. Determination of Available Phosphorous in Soil by Bray’s-1 Method .................................................................................... 65 18. Determination of Available Phosphorous in Soil by Olsen’s Method ...................................................................................... 69 19. Determination of Available Potassium in Soil (Jackson,1973) ................ 71 20. Determination of Exchangeable Calcium in Soil ..................................... 73 21. Determination of Exchangeable Magnesium in Soil ............................... 75 22. Determination of Available Sulphur in Soil by Turbidimetric Method ............................................................................. 77 23. Determination of Available Sulphur in Soil by Seed Solution Method ............................................................................. 79 24. Determination of Available Fe, Mn, Cu & Zn in Soil .............................. 81 25. Determination of Exchange Acidity in Soil ............................................. 85 26. Preparation of Indicators ........................................................................ 87 27. Determination of Nitrogen in Plant Sample ............................................ 89 28. Determination of Phosphorous in Plant Sample ..................................... 91 29. Determination of Potassium in Plant Sample ......................................... 93 30. Determination of Sulphur in Plant Sample .............................................. 95 31. Determination of Calcium in Plant Sample ............................................. 97 32. Determination of Magnesium in Plant Sample ....................................... 99

1 Collection of Soil Sample Aim of the Experiment: To collect the soil sample from the given plot.

Principles Soil testing identifies various constraints in soil that limit crop production. Fertilizer recommendation and soil management practices to be adopted are based on the soil test report. Soil testing starts with collection of soil samples from the field and ends with interpretation of soil test results leading to necessary recommendations for increasing crop production. To obtain correct soil test values, the soil sample should be a proper representative of the soil of the particular area. Since soil is heterogeneous in nature, specific techniques are followed to collect the soil sample. A soil sample is the representative of the soil of a particular area. It represents the average physical, chemical, biological properties and nutrient status of the soil of that area. Soil samples are usually collected from 0 to 15 cm depth since the roots of most of the field crops derive nutrients and moisture from this depth. The collected soil samples are analysed in the laboratory in order to identify various constraints in the soil like soil acidity, alkalinity, salinity etc. and to follow appropriate management practices to increase the crop production as well as to prescribe fertilizer recommendation for higher crop production. Soil heterogeneity is more in the vertical dimension than in the horizontal dimension. To average out the horizontal variation, samples are drawn from a number of spots well distributed throughout the field excluding some unusual areas. Soil samples drawn from different spots are mixed thoroughly to prepare the composite sample. The entire field is thoroughly surveyed before collection of soil sample and is divided into sampling units on the basis of Physiography, Texture, Structure, Colour and Previous cropping history Physiography includes vegetation, topography and the presence of other natural features like hills, rivers etc. If the soil is uniform in all these characteristics it is treated as one sampling unit. For variation in single characteristic in any part of

2

Soil and Plant Analysis

the field, it is treated as another sampling unit. Maximum area under one sampling unit should not exceed 2 hectare or 5 acre. One composite sample is collected for each sampling unit. If the soil is dry and hard, samples are collected by phowrah.

Materials Required for Collection of Soil Samples 1. Phouragh / Trenchoe

2.

Khurupi

3. Screw auger

4.

Soil sampling tube

5. Bucket

6.

Tray / polythene sheet

7. Polythene bag

8.

Drawing sheet

9. Pencil

10.

Thread

Procedure Collected samples randomly from different spots well distributed throughout the field. Draw imaginary ‘Z’ shape (N shape) lines and collect samples from different spots located on these lines. Collect samples from minimum 16 to 20 spots for one acre area of land. Remove the grasses, weeds, concretions from the selected spot to collect the sample. While removing the grasses, weeds etc care should be taken for least disturbance of the soil surface. Prepare a ‘V’ shape hole to a depth of 15 cm by phaworh. Remove a thin uniform slice of 1.5" thickness parallel to one side of the hole and collect the same soil in a bucket or tray. Reduce the size of the soil sample collected (large quantity) from a number of spots to 0.5 to 1 kg of soil which is required for laboratory analysis by quartering process. Keep the collected soil samples on a polythene sheet that should be free of contamination with any chemical. Remove the grasses, weeds, concretions or any other foreign materials, if noticed from the soil. Mix the soil thoroughly and prepare a round heap. Divide the whole sample into four equal quarters and remove diagonally two opposite quarters. Mix properly the soil sample of other two opposite quarters. Repeat this process till about 0.5 to 1 kg of soil is left. This is the composite soil sample which is kept in a polythene or cotton bag. Write the following information on a piece of drawing sheet by pencil or a ball pen. Keep the information sheet inside the bag containing composite soil sample. 1. Name of the cultivator 2. Location a) Village / Town

Collection of Soil Sample

3

b) Block c) District 3. Type of land a) Low/Medium/High land b) Virgin or cultivated land 4. Depth of collection 5. Date of collection 6. Previous crops grown and management practices followed if any 7. Signature of the collector If the soil is moist or wet, put the polythene bag containing the composite soil sample into a second polythene bag. Then insert the label containing the required information into the second polythene bag instead of the first one so that the label does not become wet and get damaged. Use screw auger to collect the soil sample when the soil is moist. Insert the auger into the desired depth (0-15 cm) by rotating it clockwise and then pulled out straight without any rotation from the soil. Collect the soil adhered to the spiral end of the auger. It is convenient to use the screw auger since samples can be drawn from a large number of spots in short time with less labour. Because a small quantity of soil is collected from each spot by the screw auger, quartering process may not be needed. Put a piece of drawing sheet written with the information as mentioned above into the bag containing the collected soil sample. Collect the soil sample by soil sampling tube, if the soil is wet. Press the soil sampling tube into the desired depth of the soil and then pull out from the soil. Collect the soil present inside the tube.Tie the bag containing the composite soil sample by a thread or rubber band and send to the laboratory for necessary analysis.

Precaution 1. Don’t collect the soil sample near the bonds. Collect samples leaving sufficient distance from the bonds. 2. Don’t collect the soil sample from or near a manure pit, heap or swampy area in the field. Samples are to be collected leaving sufficient distance from these spots. 3. Samples should not be collected from the area under shade of the tree. It should be collected leaving sufficient distance from this area.

4

Soil and Plant Analysis

4. If crops like brinjal, tomato or potato etc. have been grown then collect the soil samples from the furrows leaving the ridges. Nutrient contents below the ridges are higher than of the actual soil, since fertilizers are placed below the ridges.

2 Processing of the Soil Samples Aim of the Experiment: The composite soil samples, collected, are air-dried and processed before analysis in the laboratory.

Materials Required 1. Polythene sheet/ paper sheet 2. Wooden hammer 3. 2 mm sieve 4. Polythene bottle of 500 g to 1 kg capacity

Air-drying of the soil Spread the soil sample thinly on a polythene or paper sheet. Remove the grasses, weeds, concretions or any other foreign materials if noticed. Dry the soil under shade till its moisture content is in equilibrium with the moisture content of the atmosphere. This is called air-drying of soil. It takes nearly 5 to 7 days for airdrying.

Grinding of the soil The soil sample is ground by a wooden hammer and is passed through a 2 mm sieve. The soil particles passing through the sieve are collected and preserved in the polythene bottle for laboratory analysis. The soil particles which pass through the 2 mm sieve are having the diameter of 2 mm or less and are called the fine earth fraction of soil. The particles retained on the sieve are having the diameter of > 2 mm and are called the coarse fragments. The fine earth fraction of the soil is collected leaving the coarse fragments.

Precautions 1. The soil is to be dried under shade but not under sun because of the possibility of the loss of organic matter which may bring a change in the physical and chemical properties of the soil.

6

Soil and Plant Analysis

2. For grinding the soil, use of iron hammer is avoided because of the possibility of contamination of soil with iron, which interferes in various chemical analysis like organic carbon content, available Fe content in soil etc.

Calculation of the Mesh Diameter in Sieves In stating the size of the sieve openings, one widespread custom is to specify the number of meshes per linear inch without referring to the actual size of the openings. For example a 2 mm sieve is specified as 8 mesh sieve. A 8 mesh sieve contains 8 meshes or openings per linear inch. In each linear inch 63% i.e. 0.63¢¢ is occupied by the diameter of the meshes and the rest 37% is occupied by the metallic wires encircling the meshes or openings. Therefore, the sum of all the meshes in one linear inch = 0.63” or 16 mm.  The diameter of each mesh 

16 mm no.of meshes per linear inch

Diameter of each mesh in a 8 mesh sieve

16  2 mm 8

Diameter of each mesh in a 60 mesh sieve 

16  0.27 mm 60

N.B.: Thickness of the metallic wires encircling the openings or meshes decreases proportionately with increase in the number of meshes per linear inch so that the value 0.37” occupied by the wires encircling the meshes in one linear inch remains almost constant.

3 Determination of Soil Texture by Feel Method Aim of the experiment : To determine the soil texture by feel method

Principle Soil texture is the relative proportion of sand, silt and clay in soil by weight. As per the classification of the inorganic soil particle by the International Society of Soil Science, diameters of the sand, silt and clay particles vary from 2 to 0.02 mm, 0.02 to 0.002 mm and less than 0.002 mm, respectively. There are 3 broad textural groups i.e sand, loam and clay and each broad textural group is subdivided into different textural classes depending upon the proportion of sand, silt and clay. Broad textural group

Basic soil textural class

Texture

Sand or sandy soil (contains 70% sand and d” 15% clay by weight

Sand Loamy sand

Coarse textured soil

Loam or loamy soil

Sandy loam

Moderately coarse textured soil

Loam, Silt loam, Silt

Medium textured soil

Sandy clay loam, Silty clay loam, Clay loam

Moderately fine textured soil

Sandy clay Silty clay Clay

Fine textured soil

Clay or clayey soil (contains e”35% clay by weight)

Soil texture is the first identifying character of soil. It provides many useful information on the soil properties like its water holding capacity, nutrient retention capacity, stickiness, plasticity etc. Soil texture indicates the suitability of the soil for growing a crop. Although the accurate textural class of the soil can be determined by Bouyoucos hydrometer method but it is time consuming and may not serve the purpose of the farmer who wants an immediate information on his soil before growing a crop. Feel method, in this case, is extremely useful to determine the soil texture immediately on the spot and to provide information on

8

Soil and Plant Analysis

his soil before growing the crop. This method is more an arts rather than science since the accuracy of the determination of the textural class by feel method depends on the experience of the person analysing. The students are, therefore, advised to practice a number of samples to make themselves well acquainted with this method.

Procedure Take some crushed soil (30-40 g) in a watch glass. Make a round heap. Add water slowly to make the soil moist. Then knead the soil thoroughly by the fingers. All the characteristics like feel by finger, ball formation, roll formation, ring formation and ribbon formation and stickiness are to be tested with this kneaded moist soil.

Feel by finger Rub the soil between the thumb and fore finger. Observe the feeling of the soil. The sand particles are gritty to feel. Silt particles are smooth or buttery to feel and clay particles are sticky to feel. If the soil is entirely gritty to feel, the texture may be sand or loamy sand. If the soil is sticky to feel it may be clay loam or clay. If the soil is gritty as well as sticky to feel it may be sandy clay loam or sandy clay. If it is very smooth and sticky it may be silty clay loam or silty clay. Sand and loamy sand can be distinguished from each other by ball formation. Similarly clay loam and clay ; sandy clay loam and sandy clay; silty clay loam and silty clay can be distinguished from each other by roll, ring and ribbon formations as well as stickiness.

Ball formation Form a ball with the kneaded moist soil. Taste the strength of the ball by pressing between the forefinger and thumb and observing the pressure needed to break the ball.

Roll formation Roll the ball between the palms of the hand and observe the length of the roll formed. Coarse textured soil Moderately coarse textured soil Medium textured soils Moderately fine textured soils Fine textured soils

: : : :

Roll formation does not occur Formation of short roll occurs Little tendency for ring formation Ring formation occurs but there are cracks at the outer surface of the ring : Ring formation occurs with smooth surface

Determination of Soil Texture by Feel Method

9

Ribbon formation Squeeze the ball between the fore finger and thumb to make a ribbon as long as possible until it breaks by its own weight. Fine textured soils

: Ring formation occurs with smooth surface

Coarse textured soils

: No ribbon forms

Moderately coarse textured and medium textured soils

: Ribbon forms but the length of the ribbon is < 2.5 cm

Moderately fine textured soils

: Ribbon forms with length 2.5 to < 5 cm

Fine textured soils

: Ribbon forms with length  5 cm

Stickiness To determine the stickiness little more water is added to the moist soil to make it slightly wet. Press the wet soil between the thumb and the fore finger. Throw the hand so that the soil mass outside the finger is removed. Now separate the fingers and observe the stickiness of the soil. Non sticky : Soil mass does not adhere to fore finger or thumb. If adheres, it comes out (i.e. removed from the fingers) cleanly by slightly throwing the fingers. The coarse textured soils are non sticky. Slightly sticky : Soil mass adheres to the fingers but comes out by moderately throwing the fingers. The soil shows little stretching while separating both the fingers. Moderately coarse textured and medium textured soils are slightly sticky. Sticky : Soil mass adheres to both fore finger and thumb and tends to stretch when both the fingers are separated. Soil mass does not come out by throwing of the fingers. Moderately fine textured soils are sticky. Very sticky : Soil mass adheres strongly to both fore finger and thumb and is decidedly stretched when both the fingers are separated. Soil mass does not come out even with violent throwing of the fingers. Fine textured soils are very sticky.

10

Soil and Plant Analysis

Properties of soil textural classes Textural class

Fill by finger

Ball formation

Roll formation

Ring Ribbon Stickiness formation formation

Sand

Very gritty No

No

No

No

Nil

Loamy sand - do -

Ball forms, but No breaks with little pressure

No

No

Nil

Sandy loam

Gritty

Ball forms, but Short rolls resists more pressure than loamy sand

No

4 dS m-1. Specific soil management practices are required for reclamation of saline and saline-alkali soils to obtain higher yield potential. Conductance 

Ampere Volt

The unit of conductance is mhos. The current strength is expressed in ampere. One ampere is the flow of 1 coulomb of charge per second. The charge of one gramequivalent of a cation or an anion is 96,500 coulombs. Therefore 1 coulomb 1

of charge is carried by 96,500 1.04 10 geq of cations or anions. If the conductance of a solution is 1 mhos that means 1.04 ×10-5 gramequivalent of cations and anions move per second towards the respective electrodes under an applied potential difference of one volt. Therefore, if the conductance of a solution is 1 mmhos, then 1.04×10-5 milliequivalent of cations and anions move per second towards the respective electrodes under the applied potential difference of one 5

44

Soil and Plant Analysis

volt. Since 1 geq of a cation or anion carries 6.023 ×1023 no. of positive or negative charges (Avogadro’s number), therefore one milliequivalent of the cation or anion carries 6.023×1020 number of positive or negative charges. This shows when the conductance of a solution is 1 mmhos, then (1.04×10-5) × 6.023 ×1020 = 6.264 ×1015 no. of positive charges and the same number of negative charges move per second towards the cathodes and anodes, respectively. Conductance of a solution is measured by a conductivity meter which consists of a conductivity cell that is dipped into the solution whose conductance is to be measured. The conductivity cell contains two platinum electrodes of rectangular size spaced at a specific distance. When the electrodes are connected to an external source of electromotive force, current flows through the solution or in other words the solution conducts electricity. Conductance of the solution is directly proportional to the cross sectional area of the electrodes and is inversely proportional to the distance between the two electrodes. As the cross sectional area of the electrode increases, it bears more numbers of charges when connected to the external EMF and attract more no. of the oppositely charged ions towards itself. Therefore more of the ions move towards their respective electrodes contributing more of the currents. As the distance between the two electrodes increases the electrostatic force of attraction of the electrodes to the oppositely charged ions decreases. Therefore, less numbers of the ions move towards their respective electrodes contributing less of the current.

Ca

C

1 

 C

C = Conductance of the solution in mmhos

a 

Ck  k  C

a = Cross sectional area of the electrode (cm2)

a 

 /a a

= Distance between the two electrodes (cm) = Cell constant (cm-1)

k is the proportionality constant called specific conductance or electrical conductivity of the solution (mmhos cm-1). To evaluate k, k = C when a =1 cm2 and  = 1 cm. Therefore, specific conductance or electrical conductivity of a solution is its conductance when the cross sectional areas of the electrodes are 1 cm2 and the electrodes are spaced

Determination of Electrical Conductivity of Soil

45

at 1 cm apart. In other words, specific conductance is the conductance of each one cc of the solution. Conductance of a solution may vary from instrument to instrument because of the variation in ‘a’ and ‘  ’ of the electrodes. But specific conductance is constant at a particular temperature irrespective of the values of ‘a’ and ‘  ’ of the electrodes in the instrument. Conductance of a solution increases with increase in temperature due to increase in the ionic mobility. Ionic mobility is the distance travelled by an ion per second towards the respective electrode under unit potential gradient i.e. a potential gradient of one volt per cm. Potential gradient 

Potential differnence applied between the two electrodes(volt) Dist an ce between the two electrodes(cm)

The increase in ionic mobility with increase in temperature is possibly due to increase in the intermolecular distance among the water molecules with rise in temperature. The ions, therefore, get a greater free path for movement towards their respective electrodes and can cover a greater distance per second. Therefore more numbers of ions move towards their electrodes contributing more of currents. The reverse occurs with decrease in temperature. Therefore for uniform comparison, electrical conductivities of the solutions are expressed at 25ºC. Since above 25ºC, the EC value increases, therefore it is multiplied by a factor 1.0 to convert the value at 25ºC. As the temperature decreases below 25ºC, the value of this factor increases above 1.0.

Reagent 1. Standard 0.02 M KCl : Dissolve 1.491 g of Analar grade KCl (dried at 110ºC in oven) in water and dilute to one litre. This solution has a conductivity of 2.768 mmhos cm-1 at 25ºC

Apparatus required 1. Conductivity meter

2. Physical balance

3. 100 mL plastic beaker

4. Glass rod

5. Thermometer

6. Filter paper

46

Soil and Plant Analysis

Procedure Take 25 g of soil in a 100 mL plastic beaker. Add 50 mL of distilled water. Stir the contents by a glass rod for several times during a period of 2 hours and then allow to stand till the soil particles settle down. Decant or filter the supernatant to another 100 mL beaker. Dip the conductivity cell of the conductivity meter into the filtrate and measure the conductance. Note the temperature of the solution by a thermometer. Before measuring the conductance of the soil solution, determine the cell constant of the instrument by measuring the conductance of a 0.02 M KCl. N.B. : In order to save time conductance or EC of the soil solution can be measured following the pH measurement. Wait, after pH measurement, till the soil particles settle down and a clear supernatant is obtained above the settled soil particles. Dip the conductivity cell into the supernatant to measure the conductance.

Calculation Cell constant of the instrument (  /a)

Csp of 0.02 M KC1at 25o C  Cm of 0.02 M KC1at 25o C Csp = Specific conductance (mmhos cm-1) Cm = Measured conductance (mmhos) Csp of 0.02 M KCl at 25ºC = 2.768 mmhos cm-1 Cm of 0.02 M KCl at 25ºC = Cm of 0.02 M KCl at the measured temperature × Temp. factor Csp of the soil solution at the measured temperature = Cm of the soil solution ×  /a. Csp of the soil solution at 25ºC = Csp of the soil solution at the measured temperature × temperature factor. N.B. Electrical conductivity (EC) of the soil refers to the specific conductance of the soil solution. EC decreases as the soil : water extraction ratio increases and vice versa. Electrical conductivity of a soil at 1:1 soil : water extraction ratio is more than that at 1:2 soil : water extraction ratio. Therefore, while writing the EC of a soil the soil : water extraction ratio should be specified (in brackets by the side of EC). For example = EC (1:2) = 0.181 dS m-1 in Bhubaneswar soil.

Determination of Electrical Conductivity of Soil

47

Concentration of salts in the soil water extract or irrigation water (meq per litre) = 12.5 × L mmhos cm-1 Where L mmhos cm-1 is the specific conductance of the soil water extract at 25ºC. Conc. of salts in the soil solution (ppm) = 640 × L mmhos cm-1 % salt in soil = 0.064 × L mmhos cm-1 × water : soil of extraction Csp of the saturation extract of soil = Csp of the soil (1:2) × 200 % water in soil at saturation

Conversion of mmhos cm-1 to dS m-1 1 Siemen = 1 mhos Let the conductance = C mmhos = C milliSiemen 

C ds 100

Let the cross sectional area of the electrode = a cm2 = a × 10-4 m2 Let the distance between the 2 electrodes = cm   102 m 1

  10 2  Cell cons tan t cm 1   102 m 1 4 a a a 10  a

1 Specific conductance (mmhos cm-1) = C mmhos  cm  C

Specific conductance (dS m-1) 

 mmnos cm 1 a

C     ds   10 2 m 1   C  ds m 1 100 a a 

 Specific conductance in mmhos cm-1 = Specific conductance in dS m-1. Temperature factor (ft) for determining conductivity at 25ºC EC at 25°C = EC at measured temp. × ft Cº

ft

Cº

ft

15

-

1.247

31

-

0.89

16

-

1.218

32

-

0.873

17

-

1.189

33

-

0.858

18

-

1.163

34

-

0.843

48

Soil and Plant Analysis

19

-

1.136

35

-

0.829

20

-

1.112

36

-

0.815

21

-

1.087

37

-

0.801

22

-

1.064

38

-

0.788

23

-

1.043

39

-

0.775

24

-

1.02

40

-

0.763

25

-

1

26

-

0.979

27

-

0.96

28

-

0.943

29

-

0.925

30

-

0.907

13 Determination of Organic Carbon in Soil by Walkley: Black Method Aim : To determine the organic carbon by Walkley-Black method.

Principles In the detection of soil organic carbon a known weight of soil is heated with an excess volume of standard K2Cr2O7 in the presence of Con. H2SO4. The soil is slowly digested at the low temperature by the heat of dilution of H2SO4 and the organic carbon in the soil is thus oxidized to CO2. The highest temperature attained by the heat of dilution reaction produced with the addition of Con. H2SO4 is approximately 12oC which is sufficient to oxidize the active forms of the soil organic carbon but not the more inert forms of carbon that may be present. The excess of K2Cr2O7 not reduced by organic matter is titrated against a standard solution of Ferrous Ammonium Sulphate in the presence of Phosphoric acid and Diphenylamine as indicator. While the actual measurement of oxidisable Organic Carbon, the data are normally converted to percentage organic matter using a constant factor, assuming that organic matter contains 58% Organic Carbon. However, as this proportion is not in fact constant, we prefer to report as oxidisable Organic Carbon, or multiplied by 1.334 as Organic Carbon.

Reagents 1. Standard 1 N K2Cr2O7 : Dissolve 49.04 g of K2Cr2O7 in distilled water and dilute to 1 litre. 2. Ferrous ammonium sulphate solution (0.5 N) : Dissolve 196.1 g of Fe(NH4)2(SO4)2.6H2O in 800 mL of distilled water containing 20 mL conc. H2SO4 and dilute to 1 litre. 3. Diphenylamine indicator solution = Dissolve 0.5 g of the indicator in 20 mL water and 100 mL conc. H2SO4 4. Conc. H3PO4 (85%)

50

Soil and Plant Analysis

5. Solid NaF 6. Ferroin indicator : Dissolve 1.485 g of O-phenanthroline monohydrate and 0.695 g of ferrous sulphate heptahydrate in water and adjust the volume to 100 mL.

Apparatus required 1. 2. 3. 4. 5.

Analytical balance 500 mL conical flask 10 mL volumetric pipette 100 mL and 500 mL measuring cylinders Burette

Procedure • • • • • • • •

Take 1 g soil in a 500 ml conical flask Add 10 ml potassium dichromate solution Add 20 ml H2SO4 Wait for 30 minutes Add 200 ml tap water Add 4 drop feroin indicator Then titrate against Ferrous Ammonium Sulphate (FAS) End point is brick red colour

Reactions Organic matter contains different organic compounds. Organic compounds are the compounds of C, H and O. Carbon is the skeleton of the organic compounds. When the concentrated H2SO4 is added to the K2Cr2O7 solution heat is evolved (heat of dilution) due to reaction of H+ with H2O (which is the solvent in the K2Cr2O7 solution) forming H3O+ ions. H++H2O  H3O+ + energy This energy is used in breaking the bonds in the organic compounds and carbon in the elemental form is released. This carbon is oxidized by Cr2O7= as follows. K2Cr2O7 + 4H2SO4  K2SO4 + Cr2(SO4)3 + 4H2O + 3O

(1)

1.5C + 3O ® 1.5CO2

(2)

K2Cr 2O7 + 1.5C + 4H2SO 4  K2SO 4 + Cr 2(SO4)3 + 1.5CO2 + 4H2O (3) (Adding equation 1 and 2)

Determination of Organic Carbon in Soil by Walkley

51

The unreacted K2Cr2O7 is then titrated against ferrous ammonium sulphate. K2Cr2O7 + 6 Fe (NH4)2 (SO4)2 .6H2O + 7H2SO4  (unreacted) K2SO4 +Cr2 (SO4)3 + 3 Fe2(SO4)3 + 6 (NH4)2SO4 + 43H2O

Ionic equation The ionic equation shows the species really involved in this redox reaction. Cr2O7 = + 8 H+  2 Cr +++ + 4H2O + 3O

(1)

1.5C + 3O  1.5CO2

(2)

Cr2O7 = + 8 H+ + 1.5C  2 Cr +++ + 1.5CO2 + 4H2O (Adding equation 1 and 2) Cr2O7 = + 6 Fe++ + 14H+  2 Cr +++ + 6Fe+++ + 7H2O (unreacted)

Action of the diphenylamine indicator Diphenylamine (DPA) is a redox indicator. The original compound is in reduced form and is colour less. The oxidized form of this compound is violet colour Diphenylamine is oxidized by K2Cr2O7 with internal structural changes and the new compound formed is called Diphenylbenzidine (violet). The reactions occur as follows. H N

H N diphenylbamine H N

H N

+ 2H+ + 2e

diphenylbenzidine H N

diphenylbenzidine (colorless) N

N

+ 2H+ + 2e

diphenylbenzidine violet (violet)

The redox reaction of DPA can be represented in the simple form as follows. DPA + DPA  DPB + 2 H+ + 2e¯

52

Soil and Plant Analysis

(col.less) DPB  DPB violet + 2H+ + 2e¯ (col.less) DPA = Diphenylamine DPB = Diphenylbenzidine. The 1st step of the reaction is nonreversible, whereas the 2nd step of the reaction is reversible. When DPA is added it is oxidized by the unreacted K2Cr2O7 to produce the violet colour. On titration with FAS, the unreacted K2Cr2O7 is first reduced. After all the unreacted K2Cr2O7 is reduced, the oxidized from of the indicator is then reduced by gaining electrons from FAS and becomes colour less. Green colour appears at this point because of the presence of Cr+++. NaF or H3PO4 is added to get a sharp end point. The FAS added during titration is oxidized to Fe+++ by K2Cr2O7. When all the K2Cr2O7 is reduced the next electron supplied by Fe++ during titration is captured by Fe+++ instead of the oxidized form of DPA because of the greater electron affinity of Fe+++ than of the oxidized species of DPA. This creates a difficulty in getting a sharp end point. Therefore NaF or H3PO4 is added to inactivate the Fe+++ ions by forming complex, with Fe+++. Fe+++ + 6 F   (FeF6)3- (Hexafluoro ferrate) 2Fe+++ + 3H3PO4  Fe2(H3PO4)36+ The Fe2(H3PO4)36+ complex is possibly formed by the overlapping of the 2s orbitals of oxygens in H3PO4 containing lone pair of electrons by the vacant d2sp3 hybrid orbitals of Fe+++. The standard reduction potential* of Cr 2 O  – Cr +++ ion system 7

0

( E Cr O Cr    ) is 1.2 volt at 25ºC 2 7

E 0Fe   Fe 

is 0.78 volt at 25ºC

Standard reduction potential of the oxidized and reduced form of the diphenylamine indicator E 0( OR ) is 0.75 volt at 25ºC. Standard reduction potential of Fe+++ - Fe++

Determination of Organic Carbon in Soil by Walkley

53

0

ion system after formation of complex with F¯ ( E FeF63 FeF64 ) is 0.5 volt at 25ºC. Since the standard reduction potential of the indicator is less than the standard reduction potential of the Cr2O7 ® Cr+++ ion system but greater than the standard reduction potential of Fe+++ ® Fe++ ion system after forming complex with , therefore, there is no difficulty in obtaining a sharp end point. Similarly, since the (after forming complex with H3PO4) is less than the E0 value of the oxidized and reduced form of the indicator, therefore, a sharp end point can be obtained. *Standard reduction potential (E0) of a system is its reduction potential with reference to the standard hydrogen electrode at a particular temperature when all the species in the system are in unit activity. Greater is the E0 value of a system, greater is the tendency of the oxidized species, present in it, to gain electron from a reducing agent.

Use of ferroin indicator Ferroin is a redox indicator. The compound is formed when Fe++ from a complex with 3 molecules of 1,10-phenanthroline (orthophenanthroline). Each nitrogen in the structure in ferroin has a lone pair of electrons in its ‘s’

N

N

N

N

N

N

N

N

1, 10-phenanthroline

Ferroin (Fe++ complex of 1, 10 phenanthroline)

orbital of the valency shell. The six vacant d2sp3 hybrid orbitals of Fe++ is overlapped by the 2s orbitals of six nitrogen (present in 3 molecules of 1,10phenanthroline) to form the complex. The original compound is in reduced from and is intense red in colour. The oxidized form (Ferric orthophenanthroline) is pale blue in colour. Fe(Ph)3

Fe(Ph)3

(Intensered)

(Pale blue)

Ph represents the compound orthophenanthroline

54

Soil and Plant Analysis

It is oxidized to ferric orthophenanthroline by losing one electron to K2Cr2O7 and changes to pale blue colour. But the pale blue colour is not noticed as it is suppressed by the orange colour of . When the unreacted is titrated against FAS, the is reduced to Cr+++ which exhibits green colour. With gradual reduction of the intensity of orange colour decreases and intensity of green colour increases due to increase in the concentration of Cr+++. The green colour of Cr+++ also suppresses the pale blue colour of the oxidized indicator. Appearance of green colour indicates that the titration is close to the end point. After all the unreacted K2Cr2O7 is reduced, the oxidized form of ferrion is reduced by the FAS added from the burrette. As soon as the reduction of the oxidized ferrion starts, a maroon colour suddenly appears indicating the arrival of the end point. The maroon colour is due to the combination of the intense red colour of the reduced form of ferroin with the green colour of Cr+++. Since the standard reduction potential of the oxidized and reduced form of the ferroin indicator is 1.1 volt at 25ºC which is less than the standard reduction potential of - Cr+++ ion system but greater than that of the Fe+++-Fe++ ion system, therefore a sharp end point can be obtained without addition of NaF or H3PO4. The efficiency of this method is 77% i.e. 77% of total organic carbon can be determined by this method. The other 23% of carbon are in the highly resistant compounds like cellulose, hemicellulose, pectin, lignin and waxes and cannot be determined by this method.

Interference of C1 in Organic Carbon Determination Samples collected from saline soils are rich in . Chloride interference creates a positive error in estimation of organic carbon i.e. higher amount of organic carbon is estimated than the actual amount present in soil. Cr2+ 6 + 14 H+  2 Cr+++ + 3 Cl2 + 7 H2O •

This interference of can be checked by the use of Ag2SO4 in the digestion mixture. Ag+ inactivates by precipitating it as AgCl and thereby preventing from oxidation. Ag2SO4 + 2 C1  2 AgCl + (Precipitate)

•

The % organic carbon determined with interference of C1 can be corrected by determining the % C1 content in the soil. The % C1 in soil is converted %C1

geq of in 100g soil to equivalent carbon % as follows % C1 in soil = 35.5

Determination of Organic Carbon in Soil by Walkley

55

(since eq. wt. of = 35.5 g)

%C1 = geq of carbon in 100 g soil 35.5 %C1 × 3 g of carbon in 100 g soil = 35.5 ( 1 geq of C = 3 g)

 %C1    3 % carbon in soil  35.5  This much % of organic carbon is determined in excess due to C1 interference than the actual % of organic carbon present in soil. Therefore, the actual % organic carbon in soil = % organic carbon determined with C1 interference  •

%C1 in soil 3 35.5

Alternatively the may be leached out on an asbestos filter. The asbestos and sample are then taken together for determination.

Calculation

 

% organic carbon in soil  10  1 

S 100   0.0039  B w

Where S = Sample titration value of FAS (mL) B = Blank titration value of FAS. (mL) w =Weight of soil taken (g) 0.0039 is the factor % organic matter = % organic carbon 

100 58

The formula has been derived as follows 10 mL of 1 N K2Cr2O7 = 10 meq of K2Cr2O7 In the experiment 10 meq of K2Cr2O7 is added Normality of FAS 

10 B

56

Soil and Plant Analysis

(The blank titration is carried out to determine the normality of FAS. Normality of FAS is calculated by the formula. V1N1 = V2N2 where V1 = volume of FAS = B mL, N1 = Normality of FAS, V2 = Vol. of K2Cr2O7 added = 10 mL, N2 = Normality of K2Cr2O7 = 1). Normality of FAS decreases with time since the Fe++ is oxidized coming in contact with atmospheric oxygen. FAS consumed in the sample titration 

 meq of unreacted K2Cr2O7 

10 S meq B

10 S meq B

10S    S  K2Cr2O7 reacted  10  B   10 1  B  meq     1 mole of K2Cr2O7 oxidizes 1.5 g atom of C (See eqn. 3 at page 52)

 6 g equivalent of K2Cr2O7 oxidizes 18 g of C  1 geq of K2Cr2O7 oxidizes 3 g of C  1 meq of K2Cr2O7 oxidizes .003 g of C  S  S 10 1   meq of K Cr O oxidizes 10 1    0.003 g C 2 2 7  B  B  

Total amount of carbon present in soil  10  1 

S 100   0.003 B 77

 S  10 1    0.0039 g (since efficiency of this method is 77%).  B Since this amount of carbon is present in w g of soil

 

% organic carbon in soil  10 1 

S 100   0.0039  B w

Back titration in estimation of organic carbon Back titration is carried out to correct the error due to excess end point in either blank or sample titrations. Excess end point in blank titration creates a positive error i.e. higher % organic carbon is estimated than the actual amount present in soil, whereas the excess end point in sample titration creates a negative error. I.

Back titration for blank : Back titration is carried out with 1 N K2Cr2O7 taken in a burette till the original violet colour just comes back.

Determination of Organic Carbon in Soil by Walkley

57

Let the blank titration value of FAS due to excess end point = B mL Let the volume of K2Cr2O7 added to bring back the original violet colour = x mL Vol. of K2Cr2O7 taken in the beginning of the experiment = 10 mL AS  (10+x) mL K2Cr2O7  B mL of FAS

B AS  10 mL of K2Cr2O7  10  10  mL of FAS The corrected blank titration value of FAS (Bc)  10 

B 10  mL of FAS

It is to be noted that since 10 mL of 1 N K2Cr2O7  Bc mL of FAS I mL of I N K2Cr2O7 

Bc 10 mL of FAS

II. Back titration for the sample. Like back titration for the blank, the back titration for sample is carried out with 1 N K2Cr2O7 taken in a burette till the original violet colour just comes back. Let y mL of K2Cr2O7 was added to bring back the original violet colour. y mL of K2Cr2O7



Bc 10 mL of FAS (since 1 mL of K2Cr2O7 a” Bc/10 mL of

FAS)

 Corrected sample titration value of FAS  S 



% organic carbon 10 1 



Bc y ml  Sc mL 10

Bc  100   .0039  Sc  w

If the blank titration is conducted with correct end point then Bc = B Similarly, if the sample titration is conducted with correct end point then Sc= S

Critical range 0.75

– High

14 Determination of Lime Requirement by Woodruff Buffer Method Aim : To determine the lime requirement by woodruff buffer method of given soil Sample

Principles The lime requirement (LR) of an acid soil is the amount of lime needed to neutralize the acidity from an initial condition to some target soil pH, like 7.0. Different methods are used to calculate a LR for specific soils. Lime requirement tests will always recommend adding enough lime to neutralize all the acidity associated with soil pH, much of the salt-replaceable acidity, and some of the residual acidity. Other methods are used to estimate the amount of lime to neutralize potential acidity. Buffer methods use a fast-reacting, weak liming agent that is added to a soil sample. The mixture is allowed to equilibrate, and the pH is measured. The LR is read directly from a table based on the pH of the soil after the buffer has been added: the lower the pH of the mixture, the higher the LR. Buffer techniques are the most commonly used LR test in the US, and different buffers are used in different areas based on the dominant soil minerals of the area.

Reagents Required •

Woodruff buffer solution : Dissolve 10 g calcium acetate, 12 g paranitrophenol, 10 g salisylic acid and 4 g calcium hydroxide in about 900 ml distilled water adjust pH to 7.0 ± 0.05 with dilute sodium hydroxide or acetic acid dilute to 1 ltr.

•

Calcium chloride solution : Take 14.7 g Cacl2 2H2O and add distilled water volume make up to 100 ml.

60

Soil and Plant Analysis

Procedure •

Take 10 g soil in a plastic beaker

•

Add 10 ml distilled water

•

Add 2 drops Calcium Chloride solution

•

Then take pH (it is the pH of salt)

•

Then add 10 ml woodruff buffer solution

•

Wait for 30 min.

•

Again take pH

Calculation LR = (pH of buffer – pH of soil buffer suspension) x 10000

Example LR = 7 – 6.41 x 10000 = 0.59 x 10000 = 5900 kg Caco3/ha

15 Determination of Neutralizing Value of Sample Aim : To determine the neutralizing value of the given soil sample

Principle The term neutralization is used for a reaction between an acid and a base or alkali. Historically, this reaction was represented as acid + base (alkali)  salt + water For example HCl + NaOH  NaCl + H2O The statement is still valid as long as it is understood that in an aqueous solution the substances involved are subject to dissociation, which changes the ionization state of the substances. The arrow sign, ’!, is used because the reaction is complete, that is, neutralization is a quantitative reaction. A more general definition is based on Brønsted–Lowry acid–base theory. AH + B  A + BH

Reagents • • •

NaoH 0.25 N (Standard) HCL 0.5N (Standard) Phenolphalin indicator

Procedure • • •

Place 0.5-1g sample in 250 ml conical flask Add 50 ml 0.5 n HCL and boil it for 5 minutes Coal and titrate against standard alkali 0.25 N NaoH using 2 drops of phenolphalin indicator to pink colour end point.

16 Determination of Available Nitrogen in Soil by Kjeldahl Method Aim : To determine the available nitrogen in the given soil sample.

Principles The Kjeldahl method is mainly divided into three main steps. This method needs to be carried out in proper sequence. The sequences are as follows

1. Digestion In this process, a certain substance or sample is heated in the presence of concentrated sulphuric acid. The acid breaks down the organic substance by the process of oxidation and reduced nitrogen is liberated in the form of ammonium sulfate. Catalysts like copper, mercury, selenium, or ions of copper or mercury are also used in the process of digestion. The sample is said to be fully decomposed when we obtain a clear and colorless solution. (Note: Potassium sulfate is added to increase the boiling point of the medium).

2. Distillation In this process, a very small quantity of sodium hydroxide is added to convert the ammonium salt to ammonia. The distilled vapors are then trapped in a special trapping solution of HCl (hydrochloric acid) and water.

3. Titration The third step is the quantification of Ammonia. The amount of ammonia or the amount of nitrogen present in the sample is determined by the process of titration.

Reagents • •

0.32 % Kmno4 : Dissolve 3.2 g Kmno4 in 1 ltr distilled water 2.5 % NaoH : Dissolve 25 g NaoH in 1 ltr distilled water

64

• •

Soil and Plant Analysis

Parafin liquid : Commercial reagent Mixed indicator : Take 0.066 g Methyl red + 0.099 g Bromocresol green and volume make up 100 ml by ethanol

Procedure • • • • • • • • • • •

Take 20 g soil in a 800 ml Kjeldahl flask Add some distilled water Add 100 ml Kmno4 (Potassium Permanganate solution) Add 100 ml 2.5 % sodium hydroxide solution Add some about 2 ml paraffin liquid Add some glass breeds Add some distilled water and apply heat Take 20 ml 2 % boric acid in 100 ml conical flask Add 4 drops Mix-Indicator (Pink colour) After distillation pink colour changes to green Then titrate against 0.03 N H2SO4

Standardisation of H2SO4 Take 10 ml 0.1 N Sodium carbonate in a 100 ml conical flask. Add 4-5 drop mix indicator. In burrete take H2SO4. Titrate it which gives red/brick colour & point.

Calculation (Suppose strength = 0.035, X = titrate value) 1 ltr 1 N H2SO4 reacts with 14 g N 100 ml 0.035 H2SO4 reacts with 14 x 0.035 g N 1 ml 0.035 H2SO4 reacts with 14/100 x 0.035 g N x X 20 g soil contain = 14/100 x 0.035/20 g N x X 1000 g (1 Kg) soil contain = 14/100 x 0.035/20 x 1000 x X g N 2 x 106 Kg soil contain = 14/100 x 0.035/20 x 1000 x 2 x 106 x X g N = 14/100 x 0.035/20 x 1000/1000 x 2 x 106 x X g N kg/ha N = 0.014 x 0.035/20 x 2 x 106 x X kg/ha N

Critical range 500 =

Low Medium High

17 Determination of Available Phosphorous in Soil by Bray’s-1 Method Aim : To determine the available ‘P’ in soil by Bray’s-1 P method

Principles In Bray’s method a mixture of ammonium fluoride and hydrochloric acid (NH4F + HCl) act as the extracting agent. It works in two ways, firstly, the hydrogen ions from HCl have a solubilising effect, especially on Ca-P, which makes phosphate get extracted. Secondly, the fluoride ions (from ammonium fluoride) complex with the Al3+ and Fe3+ ions, thereby releasing the Al and Fe bound phosphorous into the extract. The liberated phosphorous (present as orthophosphates) in solution is treated with ammonium molybdate under acidic conditions. It forms a complex called ammonium phosphomolybdate. The complex is then treated with a reducing agent like, stannous chloride or ascorbic acid to obtain intense blue colour compound called molybdenum blue. The intensity of colour of this complex is proportional to the concentration of phosphate and can be read with the help of a photoelectric colorimeter at a wavelength of 660 nm or 880 nm depending on the reducing agent used. A number of standard solutions containing known concentration of the phosphorus are prepared. The colour is developed under conditions used for the soil extract and their absorbance is measured at a suitable wavelength. A plot of absorbance as a function of concentration is drawn and the concentration of the phosphorus in the soil extract is found out using this standard curve. This method is suitable for acidic soils and does not give good results with clay soils and calcareous soils. These soils have high degree of base saturation and neutralise the acid in the extractant, thereby reducing its solubilising effect.

Reagents •

Ammonium Floride : 1 N dissolve 37 g Ammonium Floride in distilled water and dilute the solution to 1 ltr. Store this in a polythene bottle.

66

Soil and Plant Analysis

•

0.5 N HCL : Dilute 20.2 ml of Conc HCL to a volume of 500 ml with distilled water

•

Extracting solution : Add 15 ml of Ammonium floride and 25 ml of 0.5 N HCL to 460 ml of distilled water.

•

Stanous Chloride stock solution : Dissolve 10 g of reagent grade stannous chloride with conc Hydrochloric acid (keep it in a glass stopered black glass)

•

Ammonium paramolybdate solution : Dissolve 15 g Ammo. Paramolybdate in 350 ml distilled water. Add 350 ml 10 N HCL to the flask slowly with stiring. Cool it in room temperature. To obtain a volume of 1 ltr add distilled water.

•

100 ppm ‘P’ solution : Add 0.4393 potassium hydrogen ortho-phosphate in 1 ltr distilled water

•

2 ppm ‘P’ solution : Take 5 ml 100 ppm ‘P’ solution in a 250 ml volume flask. Then volume make up to 250 ml.

•

Working stannous chloride solution : Take 1 ml stannous chloride stock solution in 33 ml distilled water

Standard preparation SL no

Std ‘P’ (ppm)

ml of 2 ppm ‘P’

ml of H2 O

ml of Amm. Chloride

ml of Sncl 2

Total volume

1

0

0

7

2

1

10

2

0.2

1

6

2

1

10

3

0.4

2

5

2

1

10

4

0.6

3

4

2

1

10

5

0.8

4

3

2

1

10

6

1.0

5

2

2

1

10

Procedure •

Take 3 g soil in a 100 ml conical flask

•

Add 21 ml of extracting solution (15 ml Ammonium Floride + 25 ml 0.5 N HCL + volume make up to 500 ml)

•

Shake it for exact 1 minute

•

Filter it in to 50 ml conical flask through ordinary filter paper

•

Take 1 ml aliquot in a test tube (glass)

•

Add 2 ml ammonium Chloromolybdate

Determination of Available Phosphorous in Soil by Bray’s-1 Method

67

•

Add 6 ml distilled water

•

Add 1 ml Sncl2 extracting solution

•

Then blue colour developed and take the reading by spectrophotometer in 660 wave length

Calculation Weight of soil = 3 g Bray’s-1 P solution taken = 21 ml Aliquot taken = 1 ml Total volume = 10 ml ‘P’ = 21/3 x 10/1 = 70 x 2 = 140 x ppm ‘P’

Critical range 40

= High

18 Determination of Available Phosphorous in Soil by Olsen’s Method Aim : To determine the available ‘P’ in soil by Olsen’s method

Principles Olsen P were found to bear a quadratic relationship, with Olsen’s extractant underestimating the content in phytoavailable P of soils with high Olsen P contents relatively to soils with low contents. The ‘‘change point’’ at which phytoavailable P began to increase rapidly per unit change in Olsen P was 53 mg Olsen P kg–1 soil. For the acid soils, a significant quadratic relationship was found between the amount of P desorbed to water and Olsen P at the three soil:solution ratios studied. However, these relationships became less significant when only the soils with an Olsen P value of less than 50 mg kg–1 were considered. For the acid soils, the change point at which P input to water began to increase rapidly per unit change in Olsen P was 20, 61 and 57 mg kg–1 at the 1:100, 1:1000 and 1:10000 ratio, respectively. At comparable Olsen P values, the calcareous soils released more phosphate to water than the acid soils. On the basis of our results, we suggest the following environmental threshold values for Olsen P in acid soils: 20 mg kg–1 for P desorption scenarios where the soil:solution ratio is high (e.g. drainage water) and 50 mg kg–1 for desorption scenarios where the soil:solution ratio is low (e.g., runoff, water in reservoirs). Both values are higher than the agronomic threshold above which plants are well supplied with P.

Reagents •

Sodium bicarbonate solution : Dissolve 42 g sodium bicarbonate in distilled water. Adjust the pH to 8.5 by adding small quantities 10 % NaoH and dilute HCL by adding and volume make up to 1 ltr.

•

Solution-A : Dissolve 12 g Ammonium molybdate in 250 ml distilled water. In 100 ml distilled water add 0.291 antimony potassium tartarate separately. Both these solutions are added to 1000 ml.

70

• •

Soil and Plant Analysis

5 N H2SO4 : Take some water in a 1 ltr flask and add 140 ml conc H2SO4 then volume make up to 1 ltr. Paranitrophenol Indicator : Dissolve 250 mg (0.25 g) of 1-nitrophenol in 100 ml distilled water.

Standard preparation •

•

Primary phosphate standard : AR grade potassium hydrogen orthophosphate (KH2PO4) is dried in air over at 600C for 1 hour and after cooling (desicator). Exactly 0.4393 is dissolved in a half litre of distilled water. 25 ml of 7N H2SO4 id added to make up the volume to 1 ltr by distilled water. Secondary standard (2 ppm) : Dilute 20 ml of 100 ppm solution to one ltr with distilled water.

Procedure • • • • • • • • • • • • •

Take 1 g soil in a 100 ml conical flask Add 1 pinch charcoal Add 20 ml sodium bicarbonate solution Shake it for 30 minutes Filter it in to 50 ml conical flask Take 5 ml aliquot in a 25 ml volumetric flask Add some water Add 1 drop para nitrophenol (yellow colour will appear) Then add 5 N H2SO4 drop wise. So that yellow colour will disappeared becomes colourless Add 4 ml L-ascorbic acid solution Volume makeup up to 25 ml Wait some times (nearly 30 minutes) Blue colour will developed. Then take reading by spectrophotometer by 882 nm wave length

Critical range 22 = >35 =

Very low Low Medium High Very high

19 Determination of Available Potassium in Soil (Jackson,1973) Aim : To determine the available potassium in soil with the help of flame photometer

Principles Potassium is extracted from the soil with the help of suitable extractant CH3COONH4 by shacking followed by filtration or centrifugation & is determined in the extract using flame photometer. The analysis photometer is based on the measurement of the intensity of characteristic lime emission given by the element to be determined . When a solution of a salts is sprayed into a flame the salt gets separated into its component atoms because of the high temperature. The energy provide by flame excites the atoms to higher energy levels (the electrons of atom go to high energy level). When the electrons return back to the ground or unexcited stage. They emit radiation of characteristic wave length cline emission spectrum. The intensity of these radiations is proportional to the concentration of particular element in solution which is measured through a photo cell in the flame photometer.

Reagents •

1N Neutral Ammonium Acetate : Dissolve 77.08 g Ammo. Acetate in 1 ltr distilled water.

•

Standard solution : Dissolve 1.907 g AR grade KCL in distilled water and make the volume one litre. This solution contains 1000mg K/L i.e 1000 ppm K.

72

Soil and Plant Analysis

• Working solution of K Standard

1000 ppm k

Volume make up

5 ppm

5 ml

100 ml

10 ppm

10 ml

100 ml

20 ppm

20 ml

100 ml

40 ppm

40 ml

100 ml

50 ppm

50 ml

100 ml

Procedure •

Take 5 g soil in a 100 ml conical flask.

•

Add 25 ml conical flask.

•

Shake it for 5 minutes in a mechanical shaker.

•

Then filter it.

•

Take reading by flame photometer.

Calculation Soil taken : 5 g, Ammo. Acetate taken : 25 ml Df = 25/5 = 5 x 2 = 10 x ppm k

Critical range 280

=

High

Precautions •

pH of the ammonium acetate solution should be adjusted at 7.0

•

In flame photometer gas and air pressure should not fluctuate to get steady reading.

•

Few ml of filtrate in the beginning should be rejected.

•

Use the proper filter of K and do not take the reading without filter.

20 Determination of Exchangeable Calcium in Soil Aim : To determine the exchangeable Ca in soil.

Reagents •

Hydroxyl amine : Take 5g Hydroxyl amine and volume make up to 100ml.

•

Potassium Ferrocyanide : Take 4g Potassium Ferrocyanide and volume make up to 100 ml.

•

10 % NaoH : Dissolve 10g NaoH in 100ml distilled water.

•

TEA : Commercial reagent.

•

Calcon : 0.2g in 25ml methanol.

•

EDTA solution (0.01 N) : Dissolve 1.8613 g disodium salt of EDTA (mol. Wt. 372.254 g) in distilled water and dilute to 1 lt. The strength of this solution changes with time if stored in glass container but not if stored in polythene container.

•

0.01 N Ca : Dissolve 0.5004 g AR CaCO3 dried at 1500C in 5 ml of approximately 6 N HCL and dilute the solution to 1 lt.

Procedure • • • • • • • • •

Take 5 g soil in a 100 ml conical flask. Add 50 ml ammonium acetate solution. (Take 77.08 g vol make up 1 lt) Then filter it by using a ordinary filter paper. Take 5 ml aliquot in a 100 ml conical flask. Add 10 drop Triethanolamine. Add 10 drop Potassium Ferrocyanide. Add 2.5 ml 10 % NaoH solution. Add 10 drop calcon reagent. Then take the titration by standard EDTA.

74

Soil and Plant Analysis

Calculation V x N x 50/V x 100/W

Critical level In soil

=