May r4,1917 
Soil Nitrogen and Nutrition of Citrus Plants 
239 
or nitrification. The soils are very poor in both organic matter and 
total nitrogen, and the quantity of nitric nitrogen found in the control 
plots shows that the nitrification of the organic matter takes place very 
slowly. Furthermore, if the nitrogen-fixing or nitrifying bacteria were 
responsible for the production of the niter spots in these soils, it would 
seem that the niter spots in the soils receiving applications of active 
organic matter would be higher in nitrates than the soils to which no 
organic matter was applied. It is seen that this is not the case. 
Inasmuch as the higher ground surrounding the experimental field is 
dry land and the water table is far below the zone which would make 
it possible for the water to move to the surface by capillary action, it 
would seem that the nitrate content of the soils can not be influenced 
by deposits of nitrates occurring in the country rock. 
The formation of niter spots or streaks in Citrus soils is so definitely 
correlated with the fertilization and furrow system of irrigation that it 
would seem that there can be no doubt of the accuracy in the interpreta¬ 
tion of the forces responsible for their formation in these soils. 
DISTRIBUTION OF NITRATES IN SOILS IRRIGATED BY AN OVERHEAD 
SYSTEM OF IRRIGATION 
During the season of 1915 a number of samples were drawn from a 
grove at Covina which is irrigated by an overhead system. 
A portion of the grove received a mulch of bean straw, while another 
part remained unmulched. On July 14, as shown in Table XXVI, the 
highest nitrate content of the unmulched soil was at a depth of 6 to 18 
inches, while the mulched soil showed the highest nitrate content at a 
depth of 18 to 30 inches. The mulch applied to this soil had presumably 
prevented rapid evaporation from the soil, and the water had therefore 
penetrated the soil to a greater depth than in the unmulched soil, thus 
carrying the nitrates somewhat deeper. Another set of samples was 
taken from this grove on August 2. In the meantime the grove had 
been given a light irrigation amounting to approximately one-half inch 
of water. In the overhead system of irrigation used in this grove the 
irrigation pipes are placed over every third row of trees. The middle 
farthest away from the pipes usually received less water than the middles 
on either side of the row above which the pipes are placed. The sam¬ 
ples taken on July 14 were from the dry middles, and the nitric nitrogen 
in the unmulched soil, as stated above, had apparently not been carried 
below a depth of 18 inches. The samples taken from the wet middle 
on August 2 showed the highest nitrate content at a depth of 18 to 30 
inches. Since the quantity of water added between the samplings was 
too small to effect the distribution of the nitrates, it would seem that 
the heavier irrigation given the sgil near the irrigation pipe had caused 
a downward movement of the nitrates below 18 inches, while the lighter 
