For determination of physical and chemical properties of soil, sampling is one of the major steps. Meticulous attention should be paid for collecting an adequate amount of soil needed for analysis.
Soil samples are collected from selected sites (at different depths), after removing the surface debris, in polythene bags and then suitably labelled for future recording.
Physical Characteristics of Soil:
Soil temperature is measured by using soil thermometer. The bulb is buried in the soil to the desired depth and the temperature can be read on the dial. For determining the soil colour, collected soil sample is spread over a piece of cardboard uniformly and then match the colour with Munsellcolour chart containing standard colour chips arranged systematically on cards carried in a loose notebook.
Soil texture can be determined by mechanical analysis. A rapid method (Cenco-Wilde hydrometer method) is described:
Rapid Determination of Soil Texture (By Cenco-Wilde Hydrometer Method):
The soil is passed through a 2 mm mesh sieve onto a piece of paper. A sample of about 40 g of sieved soil is then taken with a measuring spoon. The sample is then placed in a 125 ml flask with a wide neck and a dispersing agent (0.5 g of sodium oxalate) is added.
Finally water is added up to the 100 ml mark. A rubber stopper is inserted and the flask is shaken vigorously. After 40 seconds a tube of 14 cm height and 2.5 cm diameter is filled with some of the suspension to the 60 ml mark. A Cenco-Wilde hydrometer is floated immediately in the suspension, and the reading is taken as soon as possible.
The maximum amount of fine soil material which can be read on the scale is 45 percent. If a content higher than 45% is to be determined, only 30 ml of the suspension is transferred to the test tube and diluted to the 60 ml mark with water. The test tube is vigorously shaken and placed on a level surface.
Then the hydrometer is floated on it. The reading multiplied by two gives the % of fine soil material. [Before analysis the hydrometer should be placed in the test tube with a solution of the dispersing agent to check whether or not the solution level and mark of the hydrometer correspond.]
Specific gravity of the soil is usually determined by Pycnometer. The water-holding capacity is also determined by a simple process. The sample of soil, collected by means of metal core cylinders, are placed in a flat pan. Water is poured into the pan until it nearly reaches the upper edge of the cylinders.
The samples are left until they attain complete saturation, which is indicated by the shiny film of free water on the surface. The cylinders are placed in a double layer of ordinary smooth 200 pound weight desk blotter and allowed to drain for exactly 24 hrs. The sample is weighed, dried in an oven at 110°C, and weighed again.
The content of moisture is reported on a volume basis:
Weight of drained soil sample — 157g
Weight of new dry soil — 122g
Content of water — (157-122) = 35g
Field capacity — 35% by volume
The soil moisture content is usually measured by simple gravimetric method. A soil sample is weighed in an aluminium container, placed in an oven, and dried to constant weight at 105°C. Then the sample is weighed again, and the content of moisture is expressed as a percentage of the oven dry weight.
Chemical Characteristics of Soil:
The technique of chemical analysis of soils varies considerably depending on the purpose of investigation, availability of equipment, skill of the analyst and the value of crops.
Normally the pH of soil water is determined by a glass electrode (using pH meter). Usually soil is mixed with distilled water (1: 2.5) and then the suspension is used for the determination of pH directly by electrode (after comparing with standard pH solution). The total acidity and alkalinity of soil can be determined by acid-alkali titration.
Electrical conductivity of the soil is determined from a suspension (soil: water = 1: 5) by using conductivity bridge. Cation exchange capacity of the soil is determined by a very simple method. A 20 g sample of air dry soil is placed in a 500 ml conical flask with 400 ml N Ammonium acetate. The flask is shaken on a mechanical shaker for 30 mins.
The suspension is filtered using a funnel under suction and Whatman No. 2 filter paper. The flask is then ringed with ammonium acetate and the last rinsing is followed immediately by 300 ml of ethyl alcohol which is run through the soil core in small portions. The soil sample is then removed from the filter and placed in 500 ml shaking flask with 400 ml acidified 10% KC1.
The suspension is shaken for 30 minutes. The filtering process is repeated. The leachate is made up to 500 ml with distilled water and placed in 800 ml Kjeldahl flask with 1 teaspoonful of magnesium oxide and a small piece of paraffin. The leachate is distilled, the distillate is caught in 4% boric acid and titrated with N/14 sulphuric acid using methyl red-bromocresol green indicator.
The exchange capacity is calculated as:
mEq per 100 g = ml acid and X normality of acid X ( 100 / Wt. of sample (g))
From ammonium acetate leachate as mentioned above the content of available Ca++ and Mg++ are titrimetrically determined.
The flame photometer represents one of the most efficient modern tools of chemical analysis of Ca++, Mg++ and K++.
Total nitrogen, total phosphorus, sulphate, organic carbon etc. are also determined mostly by titration followed after appropriate digestion of soil. Most of the trace elements present in soil like Zn, Cu, Fe, Mn, Co, Ni, Al etc. determined by atomic absorption spectroscopic method.
For rapid determination of various physical and chemical characters of soil, some portable kits are available, which contain various reagents for qualitative tests.
The details of different soil parameter analysis is described:
(a) Soil pH:
The pH of soil is measured directly by pH electrode after making soil suspension (1 part soil and 5 parts water i.e., 20 gm soil is suspended in 100 ml distilled water).
(b) Conductivity:
Conductivity is a measure of the current carrying capacity, thus it gives a clear idea of soluble salts present in the soil. By preparing soil suspension (1 : 5 ratio) as stated above, the conductivity was directly measured by conductivity meter and data was expressed as μ Mohs.
(c) Chlorides:
The chloride content of the soil was directly measured by titrimetric method, involving direct titration of the soil solution (filtered) with AgNO3 using K2Cr2O7 as an indicator.
The detailed procedure is given:
Procedure:
20 gm soil is suspended in 100 ml distilled water and then stirred mechanically for about one hour at regular intervals. Then the suspension was filtered through Whatman No. 50 filter paper using Buchner funnel and vacuum pump. Then chloride content of 50 ml of such filtered suspension is determined by titration against 0.02 N AgNO3 using 2 ml of 5% K2Cr2O4 solution as indicator.
The end point of the titration will be indicated by pertinent red tinge of solution.
The chloride content was then calculated by the formula:
Chloride (%) × (ml × N) of AgNO3 × 35.5/ml soil solution × 2
To convert the value in mg/100 g, multiply the values in % with 1,000.
(d) Sulphate:
Sulphate was measured by gravimetric method:
Procedure:
1. Soil suspension (1: 5) was filtered after rapid shaking.
2. Then about 50 ml of solution was taken in a conical flask its pH was made up to 4.5 to 5 by addition of 50% HCl solution.
3. A few drops of methyl red indicator (0.1%) is added to the solution and, finally, excess amount of BaCl2 solution (100 gm. BaCl2 in 1 lit solution) is added and the mixture was boiled for some time (about 20 min).
4. Finally, warmed after cooling, the precipitate is filtered through pre-weighed Whatman filter paper 42. Then the precipitate was washed by warm water and then the ppt was oven-dried along with filter paper to obtain the weight of the ppt. of BaSO4.
The SO4 content was then calculated from the formula:
Sulphate (%) = mg BaSO4 × 411.5/ml soil sample × 2,000
To convert value in mg/100 gm. multiply the result in % with 1,000.
(e) Total Alkalinity:
By the titrimetric method total alkalinity of soil solution can be measured, using a strong acid (HCl or H2SO4 in presence of methyl orange or phenolphthalein indicators.
Procedure:
1. Soil solution was prepared as described earlier.
2. Then 100 ml of such solution is titrated by using 2 drops of phenolphthalein indicator against 0.1N HCl, till the disappearance of pink colour. This is phenolphthalein alkalinity (PA).
3. Now 2-3 drops of methyl orange is added to the same sample and the titration continued further, until the yellow colour changes to pink at end-point. This is total alkalinity (TA).
4. Finally, the total alkalinity is calculated by the formulae TA as
CaCO2 (mg/1) = (A × Normality) of HCl × 1,000 × 50/ml of sample
(A = ml of total HCl used with phenolphthalein and methyl orange)
(f) Nitrogen:
Nitrogen in the soil can be measured after digestion with conc. H2SO4 in presence of a catalyst. Then such digested samples is distilled by steam to liberate alkali which is ultimately absorbed by acid solution. From the residual acid assay, the amount of nitrogen of the soil samples can be measured effectively.
Procedure:
1. Ten grams of soil samples (powder) is transferred to Kjeldahl flask and 25 ml of distilled water, 20 gm. of catalyst (mixture of 20 gm. CuSO4, 3gmHgO, 1 gm. Selenium powder, 20 gm. Na2SO4), 35 ml of conc. H2SO4 is added.
2. Then the mixture is digested over flame at low heat for 30 min. Then continue the digestion with high heat for 3-5 hours.
3. Finally the flask is cooled down and the contents transferred to the distillation flask gently after dilution.
4. Then 25 ml boric acid mixed indicator is taken in a 500 ml conical flask. (The mixture was prepared by 4 g boric acid in 100 ml warm dist. water, then in 100 ml alcohol, 0.5% bromoeresol green, 0.1% methyl red is added in 2: 1 ratio. Finally, 5 ml of dye mixture is added to 100 ml of boric acid so that the colour is pink by adjusting pH only). The distillation will continue till at least 150 ml of condensate is collected in the conical flask thus placed.
5. The mixed indicator in condensate turns blue due to the dissolution of ammonia. Finally the contents is titrated with 0.1 N HCl until the colour changes to light brown-pink.
6. One blank set is also triturated using boric acid mixed indicator only.
Finally, nitrogen content was calculated by the following formula:
Nitrogen (%) = a – b × N of HCl × 1.4/S
where a = ml of HCl used with sample
b = ml of HCl used with blank
S = weight of soil taken.
(g) Phosphorus (available):
Phosphorus in soil is generally determined as available phosphorus, which can be extracted from coil with 0.002 N H2SO4 (1 soil: 200 H2SO4). Then soluble phosphorus in the extract can be determined by conventional colorimetric technique.
Procedure:
1. One gram oven dry soil is taken in 500 ml conical flask and to which 200 ml of 0.002 N H2SO4 is added. The suspension is shaken for 1/2 hour. Then the suspension is filtered to get a clear solution.
2. 50 ml of clear solution is then taken in a conical flask to which 2 ml of ammonium molybdate solution (25 g of ammonium molybdate in 175 ml of dist. water is mixed with 400 ml H2SO4 solu. is which 280 ml conc. H2SO4 was poured) is added followed by 5 drops of SnCl2 solution (2.5 g of SnCl2 in 100 ml of glycerol by heating on a water bath).
3. A blue colour will appear, which is then used in spectrophotometer for measuring OD at 690 nm.
4. A standard curve should be prepared early with known concentration of phosphate by following the same method. By comparing with standard curve, the available phosphate content will be determined by using this formula
Phosphate (%) = mg P/l soil solution/50
To convert the value in mg/100 g, multiply the result in % with 1,000.
(h) Potassium (exchangeable):
The exchangeable potassium is determined by leaching the ion in ammonium acetate solution and then content measured by flame photometer.
Procedure:
1. 50 gm. of air-dried soil is taken in 500 ml beaker and add about 100 ml of 40% alcohol. Then stir the same for 15 minutes.
2. The suspension is then filtered using What-man filter paper no. 50. Then the soil was washed twice with same alcohol on filter paper.
3. Now scrape the soil and transfer to 200 ml beaker to which 100 ml ammonium acetate (57 ml of acetic acid in 800 ml dist. water and then NH4OH is added to make the volume 1 liter and pH was adjusted at 7.0). The suspension was stirred and kept overnight.
4. Then the suspension was filtered and the filtrate is then used for exchangeable potassium detection by using flame photometer.
5. A standard curve was at first prepared using 0 to 100 mg/1 potassium is the said instrument. Then by comparing the present data, the unknown concentration from the soil can be determined:
Phosphate (%) = mg K/l of soil extract × v /10,000 × S
where v = Total volume of soil extract prepared
S = Weight of soil taken
(i) Organic Matter:
The organic matter present in the soil is digested with excess of potassium dichromate and sulphuric acid and the residual unutilized dichromate is then titrated with ferrous ammonium sulphate.
Procedure:
1. 10 gm of oven-dry soil is taken in 500 ml conical flask and to which 10 ml of 1 N K2Cr2O7 solution (49.04 g of K2Cr2O7 in 1 liter water) and 20 ml conc. H2SO4 is added. Then the mixture is shaken.
2. Kept the flask for reaction (30 min approx.).
3. Then the mixture is diluted with dist. water up to 200 ml and then 10 ml phosphoric acid and 1 ml of diphenyl amine indicator (0.5 gms diphenyl amine in 100 ml H2SO4 and 20 ml water) is added.
4. The solution became dark blue, which is then titrated against 0.4 N ferrous ammonium sulphate [156.86 g Fe(NH4)2 (SO4)2 6H2O is dissolved in dist. water by adding 14 ml conc. H2SO4 and volume made up to 1 lit]. The end point of the titration is noted by change of colour of mixture from blue to brilliant green.
5. A blank set is run in the same manner without soil sample.
(Potassium dichromate solution content may be subjected to alteration depending on the carbon content of soil samples.)
6. Finally carbon content and organic matter are determined by the following formula:
Carbon (%) = 3.951/8 (1 – T/S)
organic matter (%) = % C × 1.724
where, g = weight of sample in gram
S = ml of ferrous solution with blank titration
T = ml of ferrous solution with sample titration.