CARBON AND NITROGEN CYCLES IN THE SOIL 211 



markedly from denitrification proper ; it apparently goes on 

 when sugar is added to the soil (151^). But such assimilation 

 does not necessarily involve any loss of nitrogen, for as the 

 organisms die they are probably decomposed with formation 

 of ammonia and nitrates once again. 



Doryland (86) has shown that this assimilation of am- 

 monia and nitrates depends on the amount of energy material 

 available, and may become considerable in favourable circum- 

 stances (pp. 1 86, 266). 



The Sulphur Cycle. 



Sulphur enters into the composition of several plant con- 

 stituents, and it appears to undergo a series of changes in the 

 soil whereby it is converted into sulphate, in which form it is 

 readily taken up once more by plants. The conditions of this 

 change have been investigated by Brown and his co-workers 

 (60 b and c) who call the process "sulfofication". 1 



Effect of Bacteria on Soil Phosphates. 



It is frequently suggested that the phosphates in the soil 

 are made more soluble through the activity of bacteria, but 

 there is no sufficient proof. British agriculturists in the 

 eighteenth century recognised that bones acted better as 

 manure after fermentation than before, but the older chemists 

 attributed the action to the decomposition of the organic 

 matter and consequent greater accessibility of the phosphate. 

 More recent bacteriological investigations by Stoklasa 2 and by 

 Sackett, Patten, and Brown, 3 however, show that certain bac- 

 teria have the power of dissolving both bone and mineral 

 phosphates in culture solution, but the mechanism of the pro- 

 cess is not clear. It is uncertain whether this action goes on 

 in the soil : direct analysis has failed to demonstrate it and 

 the evidence from field experiments is conflicting (p. 191). 



1 See also Kappen and Quensell, Landw. Versuchs-Stat., 1915, 86, and 

 J. W. Ames and T. E. Richmond, Soil Sci., 1918, 5, 311. 



2 Stoklasa, Duchacek, and Pitra, Centr. Bakt. Par., 1900, 6, 526 and 554. 



3 Sackett, Patten, and Brown, Michigan Special Bull., 43, 1908. 



14* 



