The Mehlich-III extraction is a soil testing procedure used to determine the amounts of micro and macro nutrients in a soil. A solution of NH4F, EDTA, NH4NO3, acetic acid, nitric acid, and DI water is used to extract nutrients from a soil sample. 20 ml of the extracting solution are added to 2 g of dried and ground soil and shaken. The slurry is filtered and the filtrate is analyzed using ICP-OES. Depending on the number of samples being processed, it will take approximately ten minutes per sample. One standard is analyzed for every twelve unknown samples.

Principle

The Mehlich-III extraction is a soil testing procedure used to determine the amounts of micro and macro nutrients in a soil. 

Equipment

  • Weigh boats
  • Balance
  • Volumetric flasks (100 ml)
  • Erlenmeyer flasks (1L)
  • Graduated cylinders (20 ml, 1L)
  • Adjustable volume pipettes (5 ml, <1 ml)
  • Stir plate/hot plate
  • Magnetic stir bar
  • Soil scoop
  • Shaker
  • Wooden funnel frame with plastic funnels
  • Lab tape
  • Filter paper
  • ICP vials and caps
  • Vial holder
  • Sharpie

 

Reagents

  • NH4F
  • Ethylenediaminetetraacetic acid (EDTA)
  • Deionized (DI) water
  • NH4NO3
  • Glacial acetic acid
  • HNO3 (max conc.)
  • Elemental standards (1000 ppm; Ca, Mg, P, K; B, Fe, Mn, …)

 

Solution prep

Mehlich-III stock solution

  • Weigh 13.89 g NH4F and 7.305 g EDTA in a weigh boat and add to a 100 ml volumetric flask
  • Dilute to 100 ml using DI water

Mehlich-III extractant

  • Weigh 20 g NH4NO3 in a weigh boat and add to a 1L Erlenmeyer flask
  • Using an adjustable pipette, add 4 ml of the Mehlich-III stock solution, 11.5 ml glacial acetic acid, and 0.82 ml HNO3 to the flask
  • Dilute to 1L using DI water
  • Add a magnetic stir bar to the flask
  • Stir and heat on low until all NH4NO3 is dissolved
  • Cover and store in the refrigerator; use quickly (within 10 days)

 

Extraction procedure

Sample prep

  • Weigh 2 g of dried and ground soil to be tested and put in a labelled centrifuge tube. Add 20 ml of Mehlich-III extractant and cover tightly. Repeat for 12 samples, even if more samples could be placed in the frame or shaker. The procedure is only reliable if all soils have been exposed to the extractant solution for the same amount of time.
  • Weigh 2 g of ground NAPT soil and put in a labelled Falcon tube. Add 20 ml of Mehlich-III extractant and cover tightly
  • Put the Falcon tubes in the shaker for five minutes

 

Filtration

  • Choose a wooden funnel frame with clean plastic funnel tubes and place on the lab bench
  • Place a folded filter paper in each funnel
  • Put a band of tape on the wooden frame and label with sample IDs
  • Remove the soils from the shaker after five minutes
  • Empty the tubes into the corresponding filter funnel
  • When the solution has filtered through, empty it into labelled ICP sample vials
  • If the samples will not be analyzed right away, cap them and store in the refrigerator
  • Analyze within 24 hours