Chemical rather than biological tests for soil fertility have been favoured due to their quickness although until recently they have correlated poorly with crop growth. The proposed Illinois soil nitrogen test may detect soils not requiring N fertilization. The test uses diffusion methods to overcome the apparent incomplete estimation of amino sugar N in neutralized soil hydrolysates using the steam distillation method.

The very high proportion of soil N occurring in amino sugar N in non responsive soils used by Mulvaney and Khan prompted a re-examination of the basis of amino sugar N determinations by a) alkali estimation of amino sugar N in situ using the diffusion method (Illinois soil N test) and b) alkali estimation of amino sugar N in neutralized soil acid hydrolysates using the steam distillation method.

Results show that both methods share common deficiencies that may lead to underestimation of amino sugar N e.g. inadequate temperature control.

Additionally not more than 1% of the N contained in polymeric amino sugars such as chitin, free or present in microbial cell walls and soil organic matter could be released as ammonia after 5 hours exposure to 2M NaOH at 50°C whereas 90% of the N present in glucosamine and galactosamine could be accounted for as ammonia under these conditions.

Acid hydrolysis completely solubilized chitin and steam distillation of neutralized hydrolysates with buffer accounted for >90% and 50% of glucosamine and galactosamine-N respectively as ammonia. Use of NaOH increased recovery of galactosamine-N to 80% during steam distillation.

It is suggested that the usefulness of the Illinois soil N test may be limited to local rather than universal use and that the search for chemical tests of soil fertility need to include a fuller understanding of the below ground system and its linkages above ground.