Soil Testing, Critical Levels of Different Nutrients in Soil
Soil Sampling (मृदा नमूनाकरण):- For determination of physical and chemical properties of soil, sampling is one of the major steps. 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.
[मृदा के भौतिक और रासायनिक गुणों के निर्धारण के लिए, नमूनाकरण प्रमुख चरणों में से एक है। मृदा के नमूने विभिन्न गहराई पर चयनित स्थलों से, सतह के मलबे को हटाने के बाद, पॉलिथीन बैग में एकत्र किए जाते हैं और फिर भविष्य की रिकॉर्डिंग के लिए उपयुक्त रूप से लेबल किए जाते हैं।]
Soil Testing (मृदा परीक्षण):-
1. Soil pH (मृदा 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).
(मृदा निलंबन बनाने के लिए मृदा का 1 भाग और 5 भाग जल अर्थात 100 मिली आसुत जल में 20 ग्राम मृदा को निलंबित किया जाता है। अब सीधे pH इलेक्ट्रोड द्वारा मृदा के pH को मापा जाता है।)
2. Conductivity (चालकता):- 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.
[चालकता विधुत वहन क्षमता का एक माप है, इस प्रकार यह मृदा में उपस्थित घुलनशील लवणों का एक स्पष्ट विचार देता है। ऊपर बताए अनुसार मृदा निलंबन (1: 5 अनुपात) तैयार करके, चालकता को सीधे चालकता मीटर द्वारा मापा जाता है और आंकड़ों को μ Mohs के रूप में अभिव्यक्त किया जाता है।]
3. Chlorides (क्लोराइड्स):- 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.
(मृदा की क्लोराइड मात्रा को सीधे अनुमापनीय विधि द्वारा मापा जाता था, जिसमें एक संकेतक के रूप में K2Cr2O7 का उपयोग करके AgNO3 के साथ फिल्टरित मृदा विलयन का सीधा अनुमापन किया जाता है।)
Procedure (विधि):- 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.
(20 ग्राम मृदा को 100 मिलीलीटर आसुत जल में निलंबित किया जाता है और फिर नियमित अंतराल पर लगभग एक घंटे के लिए यांत्रिक रूप से विलोडन किया जाता है। फिर बुचनर कीप और वैक्यूम पंप का उपयोग करके व्हाटमैन नंबर 50 फिल्टर पेपर के माध्यम से निलंबन को फ़िल्टर किया जाता है। फिर इस फ़िल्टर किए गए निलंबन के 50 मिलीलीटर को संकेतक के रूप में 5% K2Cr2O4 विलयन के 2 मिलीलीटर का उपयोग करके 0.02 N AgNO3 के प्रति अनुमापन करके क्लोराइड मात्रा निर्धारित की जाती है। अनुमापन का अंतिम बिंदु विलयन के प्रासंगिक लाल रंग द्वारा इंगित किया जाता है।)
The chloride content is then calculated by the formula (क्लोराइड सामग्री की गणना सूत्र द्वारा की जाती है):-
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.
4. Sulphate:- Sulphate was measured by gravimetric method.
Procedure:-
- Soil suspension (1: 5) was filtered after rapid shaking.
- 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.
- 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).
- 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.
5. 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:-
- Soil solution was prepared as described earlier.
- 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).
- 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).
- 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)
6. 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:-
- 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.
- Then the mixture is digested over flame at low heat for 30 min. Then continue the digestion with high heat for 3-5 hours.
- Finally the flask is cooled down and the contents transferred to the distillation flask gently after dilution.
- 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.
- 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.
- 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.
7. 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:-
- 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.
- 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).
- A blue colour will
appear, which is then used in spectrophotometer for measuring OD at 690 nm.
- 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.
8. Potassium (exchangeable):- The exchangeable
potassium is determined by leaching the ion in ammonium acetate solution and
then content measured by flame photometer.
Procedure:-
- 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.
- The suspension is then
filtered using What-man filter paper no. 50. Then the soil was washed twice
with same alcohol on filter paper.
- 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.
- Then the suspension was
filtered and the filtrate is then used for exchangeable potassium detection by
using flame photometer.
- 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
9. 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:-
- 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.
- Kept the flask for
reaction (30 min approx.).
- 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.
- 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.
- 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.)
- 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.
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