M.—AGRICULTURE 251 
divergence between the figures obtained and yet, applying the different 
standards of the two methods, it is claimed that there is a good agreement 
in both cases between the results of field trials and the recommendations 
of the methods. It should be pointed out that the method of comparison 
is purely qualitative, i.e. if a deficiency of potassium has been indicated 
by the test and there is a response to a dressing of potassium salts in the 
field, this is taken as a case of agreement. 
One general difficulty which applies to both methods is that they can 
be carried out only at institutes specially equipped for the purpose. The 
Mitscherlich method requires a whole season to carry out the test, while 
the Neubauer requires much supervision and extreme accuracy in the 
analytical work. Attempts have therefore been made to devise simpler 
biochemical methods suitable for the ordinary laboratory use ; of these, 
two are of special interest. 
The Azotobacter Method—The Azotobacter chroococcum is a well- 
known soil micro-organism which has the power of fixing soil nitrogen, 
and its use as a method for testing soils was first worked out in Denmark. 
The organism is very sensitive to acid conditions, and this was used as the 
basis of a method for measuring the ‘ lime requirements ’ of a soil. This 
method has been superseded by the more convenient and more accurate 
physico-chemical methods now employed. In addition to requiring a 
non-acid medium, the rate of growth of the organism depends also on the 
supply of phosphates, and later the method was adopted for the estimation 
of the available phosphates in the soil. The defect of the method is that 
there is no accurate means of estimating the development of the bacteria 
and that the results are therefore only qualitative in character. 
The Aspergillus Method—The principle of this method is the same as 
that described above, but it has the advantage of being more quantitative 
in character, as it is possible to collect the fungus and weigh it. It was 
found that under standard conditions the growth of Aspergillus niger is 
proportional to the amounts of available potash and phosphates in the soil. 
The mould is grown in a suspension of the soil in a culture solution 
containing all the constituents necessary for growth except the one being 
tested for. The mixture is inoculated with the organism and incubated 
for four to six days. During that time, the mycelium develops and covers 
the surface of the liquid like a felt. It is then removed, washed, dried 
and weighed. The estimations are carried out in triplicate or quadrupli- 
cate, and the results of a large number of experiments have shown that the 
standard error is of the order 4 per cent. The particular strain of 
organism employed and the form in which the nitrogen is supplied are 
important factors in the success of the determinations. 
The method has been worked out by Prof. Niklas and his colleagues 
at the Agricultural Research Station at Weihenstephen, near Munich, 
and has also been subjected to a critical examination by Dr. A. M. Smith, 
Edinburgh, who has tested the method with a variety of Scottish soils, 
as well as investigating the effect of different sources of nitrogen on the 
process. 
The results obtained by this method have been compared with those 
obtained by Neubauer’s method and have shown on the whole very good 
