May 14, 1917 Soil Nitrogen and Nutrition of Citrus Plants 203 
soil S. From 14 to 28 days there was an increase in nitrates in all soils 
except B and C, which continued to show no increase in nitrates when 
held at 28°; when held in the field at a temperature of 17 0 to 20°, these 
two soils showed gains of 3.56 and 3.15 mgm., respectively. After 56 
days a somewhat smaller amount of nitric nitrogen was found in soils B 
and C than in the control samples, when held at 28°, but the increase in the 
other soils varies from 24.78 mgm. in soil T to 56.14 in soil O. When held 
at a temperature of 17 0 to 20° for 56 days, soils B and C showed an in¬ 
crease of 3.04 and 1.26 mgm., respectively. Soil T also showed a much 
higher nitrifying power when held at the lower temperature, but in the 
other soils the influence of temperature within the range of the experi¬ 
ment does not seem to have been an important factor. 
From the data presented in Tables I to X it is obvious that the nitrify¬ 
ing power of a soil as determined in the laboratory by the addition of the 
usual amount of dried blood may be very different from that exhibited 
by the soil under the field conditions, and the results secured must there¬ 
fore be interpreted with great care. The data presented in Table X show 
that, on the addition of 1 per cent of dried blood, soils B and C failed to 
give any increase in nitric nitrogen, even after 56 days' incubation; but, 
on the addition of 0.1 per cent of dried blood, these two soils nitrified as 
rapidly as the other soils. 
It is observed that both of the virgin soils failed to nitrify 1 per cent 
of dried blood, while many of the soils cultivated for some years nitrified 
this amount of dried blood very rapidly. The power of southern Cali¬ 
fornia soils to nitrify 1 per cent of dried blood se£ms to be rather closely 
correlated with the character of the organic content. Those soils which 
have received additions of organic materials frequently nitrify 1 per 
cent of dried blood when the adjacent virgin soil or cultivated soils which 
have received no organic matter fail to give any increase in nitrates. 
Even those plots which have received only dried blood or bone meal 
frequently nitrify 1 per cent of dried blood, while the virgin soil, or soil 
from plots which have received no nitrifiable matter, fails to give any 
increase in nitrates. 
In semiarid soil the growth of native vegetation is frequently very 
limited, owing to the meager rainfall, and the organic content of many 
virgin lands is consequently very low. It is well known that the proper 
physical, chemical, and biological characters of a soil are largely de¬ 
pendent upon the presence of organic matter. It therefore seems reason¬ 
able to suppose that the process of nitrification, which is so closely associ¬ 
ated with the decomposition of organic matter, should become weakened 
and fail to function properly under abnormal conditions, such as are 
obviously produced by the large accumulation of ammonia which invari¬ 
ably follows the addition of 1 per cent of dried blood. 
The results presented above indicate that the use of leguminous crops 
may be of great value not only in maintaining active organic matter in 
the soil but also in maintaining the nitrogen supply. In Tables I to IX 
