JOURNAL OF Afflmnm RESEARCH 
Voiy. IX Washington, D. C., May 14,1917 No. 7 
RELATION OE THE TRANSFORMATION AND DISTRIBU¬ 
TION OF SOIL NITROGEN TO THE NUTRITION OF 
CITRUS PLANTS 1 
By I. G. McBeth, 
Physiologist , Soil-Bacteriology and Plant-Nutrition Investigations , Bureau of Plant 
Industry , United States Department of Agriculture 
INTRODUCTION 
The total nitrogen content of Citrus lands is frequently low (i) 3 , and 
it is well recognized that the quantity of available nitrogen formed 
through the natural processes of nitrification soon becomes inadequate 
for the needs of Citrus plants unless an effort is made to maintain the 
nitrogen supply by the addition of commercial fertilizers, cover crops, 
manure, etc. It has been suggested (7) that the scarcity of organic 
nitrogen in semiarid soils is offset, to some extent at least, by the great 
depth to which nitrification takes place in these soils; but, while it is 
true that nitrification may take place at greater depth in semiarid soils 
than in humid soils, there seems to be little doubt that the bulk of the 
nitrates formed in semiarid soils are formed in the surface layers. Few 
soils are plowed deeper than 9 inches; consequently the cover crops, 
fertilizers, manure, or other nitrogenous material added are confined to 
this layer, and the nitrification process naturally is most vigorous in the 
soil to which nitrogenous organic matter is added from time to time. 
In Citrus groves the common practice is to cultivate frequently to a 
depth of about 6 inches. Comparatively few feeding roots can there¬ 
fore exist in the upper 6 inches of soil. If the rainfall in southern Cali¬ 
fornia was sufficient and was evenly distributed throughout the year, 
the nitrates formed or carried above the feeding roots by capillarity 
would be moved downward and brought within reach of the roots at 
frequent intervals. But the rainfall between April and December is 
usually too scanty to cause a movement of nitrates from the surface 
layers of soil; and, as the furrow system of irrigation, which is used most 
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. 
2 Reference is made by number to “ Literature cited,” p. 251-252. 
(183)' 
Journal of Agricultural Research, 
Washington, D. C. 
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Vol. IX, No. 7 
May 14,1917 
Key No.G—no 
