Dec. 4, 1916 
Nitrification in Semiarid Soils 
419 
ditions prevail in the humus-poor soils of our coast valleys and of other valleys in the 
state, in which either the soils have always been deficient in organic matter or have 
become depleted in that respect through excessive oxidation under favorable cli¬ 
matic conditions assisted by constant summer cultivation. 
In connection with the above and other investigations, Lipman has 
found that ammonification is active in California soils generally and that 
in some cases greater amounts of ammonia than nitrate occur in soil 
samples freshly drawn from the field. This condition was especially 
noted in soils on which the plant diseases known as dieback and mottle- 
leaf of Citrus spp. occur. In these soils Lipman (20, 21) found nitrifi¬ 
cation to be inactive. He suggested a causal relationship between the 
active ammonification and inactive nitrification on the one hand and the 
abnormal plant growth on the other, the latter being attributed to. 
enforced ammonia absorption occasioned by the inability of the soil to 
transform ammonium nitrogen into nitrates. 
As contrasted with the above hypothesis, Kellerman and Wright (11) 
have pointed out that the physiological disease known as mottle-leaf of 
Citrus trees may be caused by excessive accumulations of nitrates in the 
soil; but the source of the nitrate, whether being actively formed at the 
present time or otherwise, is not clear from their publication. 
Beckwith, Vass, and Robinson (3) have studied the effects of lime on 
the ammonification of dried blood and peptone, and the nitrification of 
dried blood and ammonium sulphate in six soils from Oregon. In every 
case they found that active ammonification took place, but in two of the 
soils less nitrate was found after four weeks’ incubation where either 
dried blood or ammonium sulphate had been added than in the portions 
to which no nitrogenous material was added. In one case the further 
addition of lime failed to induce the nitrification of these materials. An 
increase in the nitrate content in the check portions was found in every 
case, indicating the presence of the nitrifying organisms in the soil. 
From the foregoing partial review of the literature on this subject it 
is apparent that radical differences of opinion are held with reference 
both to the formation and the movement of nitrates in the soils of the 
semiarid region. 
In view of the economic importance of nitrogen and the scientific 
interest attached to nitrification, the writer has for some time been 
engaged in a series of studies on this subject at the University of Cali¬ 
fornia Citrus Experiment Station, at Riverside, Cal. At this place a 
fertilizer experiment with Citrus trees has been maintained during the 
past nine years. The plots of this experiment and other semiarid soils 
near by have been used in these investigations and have made it possible 
to compare the data obtained in the laboratory studies with the effects 
produced in the field. 
The relative rates of nitrification in the field and laboratory, the 
effects of soil treatments including different fertilizers, manure, and cover 
