We would like to determine the total organic nitrogen in soil samples and are looking for an easy and fast way to do it in the lab, preferably using very little soil material (
Hi Guillaume,
You need to perform two different steps of analyses to determine the ‘Organic N content’ in soil. One analysis to determine the ‘Total N’ content and the other analysis to determine ‘Inorganic N’ (NH4-N and NO3-N) content in the soil. ‘Organic N’ content in the sample can be then determined by the difference (Organic N = Total N - Inorganic N).
Total N can be determined following Duma’s combustion with the help of a ‘CN elemental analyzer’. This method can be performed for a sample size less than 1.0 g soil. It required air dried, finely ground soil sample encapsulated in a ‘tin foil’. It is a ‘dry combustion method’ and fairly simple.
To determine inorganic N (exchangeable NH4-N and NO3-N) you need to follow a wet extraction method using a standard soil extractant (most preferably 2M KCl), followed by the analysis of the solution using a ‘flow-injection type Auto-Analyzer’. For this analysis, you need ‘field moist’ soil sieved through 2-4 mm sieve and require at least 5.0 g moist soil.
If your soil has high levels of clay, then there is a possibility that you may need to perform a third analysis to determine the ‘Clay-Fixed NH4-N’.
Standard protocols for these analysis can be available in any good text book on ‘Methods of Soil Analysis’ and can also be found in the following web site:
http://ral.cfans.umn.edu/soil-analysis-and-methods/
Hope these hints are useful.
-SJ
There are three fundamental approaches to measuring total N. The first is based on the original Dumas technique, involving oxidation of the sample in the presence of copper oxide to produce N2 gas, the volume of which is measured. In the second approach, the ‘wet’ Kjeldahl digestion method, organic and mineral N are reduced to NH4 + in hot, concentrated sulfuric acid in the presence of a catalyst. The third approach is based on near infrared spectroscopy and is relatively rapid and inexpensive, but requires careful calibration.
Details of all these approaches are available in the website: http://aciar.gov.au/files/node/10169/MN136%20Part%202.pdf
I'm not sure how high/low tech you want to go. I suggest that unless you have thousands of samples you will not want to invest the resources needed to first chemically analyse a few hundred samples (at least) to reference quality and use the data to to calibrate an IR technique. In any event you say that you do not have sufficient sample for this approach!
Although you should strictly extract the nitrate and ammonia, e.g. using KCl, and measure them colorimetrically, if you don't need these data the values are typically negligible relative to org N, i.e. you could simply do a semi-microKjeldahl N digestion followed either by distillation of colorimetry and ignore their contribution. I suggest fine grinding to make sure that you can get a representative sample. Any plant material will cause positive bias.
If you have suitable digestions conditions, notably a digestion block to run at about 200 degrees C and add sufficient K2SO4 to raise the BP of the H2SO4 above 200 degrees, i.e. so the block T controls the digestion T, and use a Se catalyst, digestion will be quantitative in about 60 mins. This can be stringently tested before you begin with your samples by testing the recovery of N from pure TRIS base. I suggest that you do recoveries from soil samples too.
You can see
http://ucce.ucdavis.edu/universal/printedprogpageshow.cfm?pagenum=0&progkey=1360&county=5120
this book (Methods of soil analysis. Part2: Chemical and Microbiological Properties.)
(Page A.L., R.H. Miller and D.R. Keeney (1982) )
www.researchgate.net/publication/256086506_Correlation_between_contents_in_organic_matter_and_organic_Nitrogen_in_greenhouse_Soils
Article Correlation between contents in organic matter and organic N...
I found a method that applies ammonium-selective electrode to the analysis of TON in soil.
full text query required:
Article Determination of total organic nitrogen in soils, plant mate...
You have to do it in the lab, no indigenous method is there in my knowledge.
I can send the procedure and requirements if you want.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Climate Science
- Climate Change