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Soil Chemical Analyses

ASL AVAILABLE SOIL ANALYSIS

  1. pH Measurement
  2. Electrical Conductivity (EC)
  3. Total Kjeldahl Nitrogen  
  4. Available Phosphorus_Olsen P Method
  5. Available Phosphorus_Bray P Method
  6. Available Potassium
  7. Organic Carbon (Walkley and Black Method)
  8. Active Iron and Manganese Determination
  9. Available Zinc and Copper
  10. Total Exchangeable Bases (TEB)
  11. Cation Exchange Capacity
  12. Exchange Aluminum and Acidity (KCl method)
  13. Particle Size Analysis
    • Pipet Method
    • Hydrometer Method
  14. Available Boron (Hot Water Method)


ABSTRACT OF ANALYTICAL METHODS FOR SOIL SAMPLES

 

Sample Preparation

Soil samples received at field moisture are air dried for 3-4 days at 35 to 40 C. These are then ground to pass a 2 mm stainless steel sieve to produce "fine earth" for the determination of moisture, pH, electrical conductivity, exchangeable bases, CEC, available P and particle size. Powdered sample (60 mesh) is used for the analysis of Kjeldahl N, Org C and total elements.

 

Moisture  Determination

Ten g of fine earth is dried at 105oC for at least 12-24 hrs, cooled and weighed. The loss in weight expressed as % represents moisture content.

 

pH and Electrical Conductivity (1:1 soil to water ratio)

Twenty g of fine earth is placed in a container and 20 ml of distilled water added. The suspension is shaken for 30 min and allowed to settle. Electrical conductivity and pH of the solution are then measured.

Note: It is important to measure conductivity before taking the pH reading, as the potassium chloride from the reference electrode can increase the EC.

 

Kjeldahl Nitrogen

One hundred mg of air-dried, finely ground (60 mesh) sample is digested with 1 g of K2SO4 and Se (100:2 ratio) and 2 mL of conc. H2SO4 in a digestion tube at 390oC for 2-1/2 hr. The digest is cooled and made up to 20 mL. After shaking the residue is allowed to settle. Nitrogen is determined colorimetrically as indo-phenol blue on a Technicon AutoAnalyzer at 625 nm.

 

Organic Carbon

Using the N analysis figure and assuming a C/N ratio of 10, enough soil sample (estimated to contain 10-15 mg C, but not exceeding 2 g soil), is allowed to react with 10 mL of N K2Cr2O7 and 20 mL conc. H2SO4 in a 125 mL Erlenmeyer flask. After cooling, 80 mL of 0.4N BaCl2 solution is added to the mixture, the contents mixed by swirling and the solids allowed to settle. Organic C is estimated from the intensity of the green color (measured at 625 nm) which is due to chromic sulfate that resulted from the reduction of the dichromate ions. Solutions of sucrose containing 5, 10, 15, 20, 25, and 30 mg C/2 mL are used for calibration.

 

Exchangeable Bases

Five g of soil mixed with enough quartz sand to facilitate percolation are leached with 10 portions of 20 ml of neutral normal NH4OAc into a 250 mL volumetric flask. The leachate is made up to 250 mL and analyzed for Na, K, Ca and Mg by AAS. The leached soil is saved for the determination of Cation Exchange Capacity.

 

Cation Exchange Capacity (CEC)

The leached soil from the exchangeable bases extraction is washed with 4 portions of 25 mL of 80% industrial ethanol. The washings are discarded. The adsorbed NH4+ ions are replaced by leaching with 4 portions of 20 mL each of N KCl (pH 2.5). The leachate is made up to 100 mL and analyzed for NH4+-N as indophenol blue.

 

 

Available Phosphorus

Olsen Method

Five g soil is shaken for 30 min with 100 mL of extracting solution (0.5 M NaHCO3 at pH 8.2) and polyacrylamide as precipitating agent. Phosphorus is determined in the filtrate colorimetrically as reduced phosphomolybdate at 625 nm using ascorbic acid as the reducing agent.

Bray 2 Method

2.5 g of soil is shaken for 1 min with 50 mL of extracting solution containing 0.03N NH4F and 0.1N HCl). Phosphorus is determined colorimetrically as in Olsen Method above.

 

 

Particle Size Analysis (Texture  Analysis)

Pipette  Method

Twelve g of air-dried, 2 mm soil sample is pretreated first with 20 ml hydrogen peroxide and dried at 80oC. The soil is further treated with 20 ml hydrogen peroxide and then dried at 100oC. The soil is then cooled and weighed. The mineral soil is shaken overnight with 100 mL water and 10 mL of solution containing sodium metaphosphate and Na2CO3. The mixture is quantitatively transferred to a 250 mL graduated cylinder and enough water added to make up the total volume to 200 mL and shaken. Twenty-five mL of the suspension is pipetted from a depth of 6 cm, evaporated to dryness and the weight taken. This weight representing 1/8 of the clay fraction, is multiplied by 8 to get the corrected weight. The rest of the slurry is washed through a 50 sieve and the fraction retained on the sieve quantitatively recovered, dried and weighed. This represents the sand fraction. Percent clay and % sand are calculated on the basis of the weight of the mineral soil. The silt content (%) is determined by difference.

Hydrometer Method

Forty g of air-dried, 2-mm soil sample in a 500 ml plastic bottle is mixed with 100 mL Calgon solution and 100 ml deionized water and shaken for 1 hr using a reciprocating shaker. The soil suspension is transferred quantitatively to a graduated cylinder and make up to 1-L with deionized water (include a blank consisting of 100 mL Calgon solution and 900 mL H2O). The mixture is allowed to stand overnight. Using a plunger, the suspension is stirred vigorously for 1 minute. After plunging, take hydrometer reading for RSILT+CLAY exactly 40 seconds after the completion of stirring. Add a couple of drops of amyl alcohol if the surface of the suspension is covered with foam. Repeat the process determining hydrometer reading on a blank solution and record as RSC. Remove the hydrometer carefully and rinse with deionized water. Let the cylinder stand undisturbed. After 6 hours, determine the temperature of the suspension at about 5 cm depth. Using the temperature correction values in Table 1.1 determine the settling time for the clay fraction. Based on time after initiation of settling, reinsert the hydrometer carefully and record as RCLAY. Repeat the process determining hydrometer reading on a blank solution and record as RC.

 

Determine oven dry soil moisture on a 20 0.1 g sample of soil.

 

Table 1. The influence of suspension temperature on the hydrometer determination of

soil clay (<2 um) based on a particle density of 2.65 g cm-3 and a solution density of

0.5 g L-1. (Source: Gee and Bauder (1986))

 

 

Temperature                 C Settling time for clay (hours & minutes)

18                                                           8:09

19                                                           7:57

20                                                           7:45

21                                                           7:35

22                                                           7:24

23                                                           7:13

24                                                           7:03

25                                                           6:53

26                                                           6:44

27                                                           6:35

28                                                           6:27

 

For every 1C above 20C, a 0.36 graduation is added to the hydrometer reading, and for every 1C below 20C, a 0.36 graduation is subtracted.

 

The correct hydrometer readings are obtained by correcting for temperature and

subtracting the blank reading.

 

Sand % = (oven dry soil weight) - ( RSILT+CLAY - RSC ) x 100

(oven dry soil weight)

 

Clay % = ( RCLAY - RC) x 100/(oven dry soil weight)           Silt % = 100 - (Sand % + Clay) 

 

  • -page updated 2011Jan07






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