May 14, 1917 
Soil Nitrogen and Nutrition of Citrus Plants 
205 
water which is distributed in the furrows quickly passes under the forces 
of gravity qnd capillarity. The downward movement of the water is 
frequently interrupted by a rather impervious plowsole; under such con¬ 
ditions the capillary forces cause a rapid lateral movement of the water, 
and in the course of a few hours the moisture may spread to all of the 
intervening space between the furrows. The action of the capillary 
forces is of paramount importance in securing an even distribution of 
water; but these forces, operating under the conditions mentioned above, 
while giving an even distribution of moisture, may have the reverse effect 
upon the soluble salts in the soil, especially the highly soluble salts, such 
as the nitrates, which possess a relatively high order of diffusibility. 
The investigations in the distribution of nitrates in furrow-irrigated 
soils was commenced in July, 1913, and during the latter part of the 
season several hundred samples of soil were collected from furrow-irrigated 
Citrus groves in Riverside County. The analyses indicated an extremely 
uneven distribution of nitrates in these soils. Surface scrapings collected 
from brown spots, which generally occur in furrow-irrigated soils imme¬ 
diately after irrigation, frequently showed a nitrate content amounting 
to more than 0.5 per cent of nitrogen, while samples taken a few inches 
immediately beneath the brown spots were invariably low in nitrates and 
not uncommonly contained as little as 1 part per million. The very large 
amount of nitric nitrogen in the brown spots led to a study of the vertical 
distribution of the nitrates in 6-inch sections. The analyses indicated 
that in many groves as much as 75 per cent of the nitrate in the upper 3 
feet was confined to the surface 6 inches of soil during the summer months. 
Because of the frequent cultivation of Citrus groves, very few feeding 
roots were found in the upper 6 inches of soil. It therefore appears that 
the large store of available nitrogen found in the surface layers can be of 
little value in the nutrition of Citrus plants until carried down within 
reach of the feeding roots. The investigation of the vertical distribu¬ 
tion of nitric nitrogen was continued during the winter months, and it was 
observed that the nitrates began to move downward as soon as the winter 
rains were sufficient to penetrate the soil to a greater depth than 6 inches. 
At the end of the rainy season the nitrates in the surface layers were ex¬ 
tremely low. After the beginning of the rainy season there was no evi¬ 
dence of the brown spots, which had been so characteristic after every 
irrigation. However, as soon as the irrigation of the new season began, 
the brown coloration was again in evidence, though not so abundant as 
during the latter part of the previous season. Analyses of scrapings from 
the brown spots again showed them to be typical niter spots. During the 
spring and summer months extensive investigations were carried out on 
the nitrifying and nitrogen-fixing power of these soils. No correlation 
could be established between the high nitric content of the surface soil 
and the activity of the nitrifying or nitrogen-fixing organisms, but the 
field observations and preliminary laboratory studies seemed to show 
that there was a close correlation between the irrigation and the occur- 
