FIXATION OF AMMONIA IN SOILS 1 
By I. G. McBeth, 
Physiologist , Soil-Bacteriology and Plant-Nutrition Investigations , Bureau of Plant 
Industry , United States Department of Agriculture 
INTRODUCTION 
It is well known that the capacity of a soil to serve as a reservoir for 
plant food depends in a large measure upon its power to retain water- 
soluble substances, such as potash, phosphoric acid, and ammonia, 
against the leaching action of rains. 
There are undoubtedly many forces which operate in the retention of 
soluble salts in soils, and it is quite likely that these forces are very 
different in different soils. The extremely complicated nature of the 
phenomena has led to many apparently conflicting observations; and it 
would seem that an entirely satisfactory explanation of the fixation 
processes can not be secured until more refined methods have been 
developed which will make possible a more exact knowledge of the 
physical and chemical forces involved. 
In studying the nitrifying power of semiarid soils at different depths, 
it was observed that, when ammonium sulphate was added to soils 
drawn from considerable depths, little or no increase in nitrates 
resulted during the first two weeks’ incubation; but, notwithstanding 
the lack of nitrification, the ammonia added could not be recovered 
from the soil by any of the ordinary methods for determining ammonia 
in soils. In the early work only small quantities of ammonium sulphate 
were added, and it was thought possible that the ammonia might have 
been assimilated by microorganisms. Larger quantities of ammonium 
sulphate were then added. Little or no increase in nitrates was secured, 
but only a small percentage of the ammonia added could be recovered 
from the soils drawn from a considerable depth. The results obtained 
with the surface soil were quite different, about 95 per cent of the nitrogen 
added being recovered as ammonia or nitrates. This observation seemed 
to indicate a strong ammonia-fixing power for the deep soil layers, a 
condition which, if true, would lead to complications in studying the 
ammonifying and nitrifying power of the soils at different depths. 
The work reported in this paper is limited to a study of the fixation of 
ammonia by soils, and is an outgrowth of the observations stated above. 
1 The work discussed in this paper was carried out in cooperation with the University of California 
Citrus Experiment Station and Graduate School of Tropical Agriculture at Riverside. The writer wishes 
to express his indebtedness to Director H. J. Webber and members of his staff for many courtesies and 
facilities extended during the course of the work. 
(14 1 ) 
Journal of Agricultural Research, 
Washington, D. C. 
hv 
Vol. IX, No. 5 
Apr. 30, 1917 
Key No. G —108 
