JOURNAL OF ACRKETffiAL RESEARCH 
DEPARTMENT OF AGRICULTURE 
Vol. VII Washington, D. C., December 4, 1916 No. 10 
NITRIFICATION IN SEMIARID SOILS—I . 1 
By W. P. Kelley, 2 
Professor of Agricultural Chemistry , Graduate School of Tropical Agriculture and Citrus 
Experiment Station , University of California 
HISTORICAL INTRODUCTION 
The distribution and amounts of nitrogen in the humus of the arid and 
semiarid soils of America and the activities of the microorganisms con¬ 
tained therein have been discussed at length by a number of writers. It 
has been especially emphasized that, as a rule, the humus and nitrogen 
are distributed more uniformly to a greater depth in the subsoil of the 
arid than of the humid regions. The soils are commonly very deep and 
are not sharply separated from the subsoils, but the actual percentage 
of nitrogen in the surface soil is frequently low and usually decreases in 
passing downward into the subsoil. The soils and subsoils in many parts 
of California, for example, contain less than 0.05 per cent of nitrogen 
and in many localities even less than 0.03 per cent. 
Hilgard (10), 3 Loughridge (26), Lipman (16), and others have directed 
attention to the great depth of root penetration in the semiarid region, 
and Loughridge has suggested that the apparent deficiency of nitrogen 
in the surface soils may be compensated for by the distribution of nitro¬ 
gen in the deep subsoils. He finds, for example, that the total nitrogen 
in the zone occupied by plant roots commonly compares favorably with 
that in humid regions. But, as is well known, the application of nitroge¬ 
nous fertilizers commonly results in marked stimulation to crops, and 
in many localities successful crop production depends upon the use of 
nitrogenous fertilizers. As much a£ 1,000 to 1,500 pounds of dried 
blood per acre has been applied annually to some of the Citrus groves 
of southern California, and in some cases as much as 2,000 pounds per 
acre has been applied. Corresponding amounts of other nitrogenous 
materials have been used. 
1 Paper 35, Citrus Experiment Station, College of Agriculture, University of California, Riverside, Cal. 
2 The writer acknowledges the valuable analytical assistance of Mr. A. B. Cummins. 
3 Reference is made by number to “ Literature cited,” p. 436-437. 
Journal of Agricultural Research, 
Dept, of Agriculture, Washington, D. C. 
gi 
(417) 
Vol. VII, No. 10 
Dec. 4, 1916 
Cal.— 7 
