TOTAL NITROGEN
SA05
Last Revised:
June 2008
1. PURPOSE
This exercise determines the total N (excluding nitrate) in a soil sample by the Kjeldahl method.
2. REAGENTS & EQUIPMENT
2.1 Air-dried and ground soil
2.2 Kjeldahl digestion and distillation apparatus
2.3 Boric acid solution with indicator
2.4 Concentrated sulfuric acid
2.5 40% NaOH
2.6 Catalyst mixture (potassium sulfate/copper sulfate)
2.7 Standardised 0.01 M HCl
3. PROCEDURE
The following procedure is based on the use of semi-automated digestion and distillation apparatus. If
you are using manual methods, you will need to make appropriate changes to the procedure.
3A. Digestion
3.1 Digest and distil a reagent blank with each batch of assays, and prepare all samples in duplicate
– that means you will need three Kjeldahl flasks if analysing one sample!
3.2 Weigh accurately about 1 g (see Note 4.1) of finely ground (<0.5 mm), air-dry soil and transfer
into a into a 100 mL digestion flask.
3.3 Add deionised water (2 mL) and, after swirling for a few minutes, add catalyst mixture (approx.
2 g), and conc. H2SO4 (10 mL by measuring cylinder) with caution.
WARNING: Handle concentrated H2SO4 with extreme care, and do not remove form the fume hood.
It is very corrosive.
3.4
Run the heating program set up for this digestion.
3B. Distillation & titration
3.5 Transfer the digestion vessel to the distillation apparatus.
3.6 Fill a conical beaker to the necessary level with the boric acid/indicator mixture and place on the
outlet stage. Ensure the outlet tube is covered.
3.7 Follow the operating instructions to carry out the distillation.
3.8 Titrate the distillate with standardised 0.01M HCl. The end point is indicated by a colour change
from pale green to red.
4. NOTES
4.1 A 1 g sample mass is suitable for %N levels of 0.1-0.4%. Adjust mass according if levels are
outside this range.
Soil Analysis Practical Manual
SA05
5. REPORT
Calculations
1.
Subtract any titre value obtained for the blank from the sample titres.
2.
Calculate the moles of HCl in the titration.
3.
Given a 1:1 ratio, determine the moles of NH3, and therefore, the moles of N in the sample.
4.
Calculate the mass of N.
5.
Calculate the %w/w N in the soil.
6.
Average the two sample values.
Questions
1.
Compare your %N to the value required by grain crops in Table 6.2.
2.
Explain why any nitrate or nitrite in the soil will not be detected by this method. Describe how
the method is modified to ensure total N is measured.
3.
What are the symptoms of nitrogen deficiency in plants?
4.
How is N added to depleted soils?
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