250 
Journal of Agricultural Research 
Vol. IX, No. 7 
It is well known that the addition of nitrogenous fertilizers to Citrus 
soils may be influential in bringing about important changes in the 
chemical composition of the soils, and it is possible that these changes, 
especially in the absence of organic matter, may be responsible, in some 
measure at least, for the apparent correlation between high nitrate con¬ 
tent and mottling. 
It must also be recognized that high nitrate content of the surface soil 
is frequently associated with unfavorable soil conditions. In a large 
percentage of badly mottled groves a rather impervious plowsole develops 
just below the cultivated zone. The plowsole is a serious obstacle to 
irrigation and frequently results in an inadequate soil moistening and a 
very uneven and unsatisfactory distribution of the nitrates and other 
soluble plant food. It would seem that the extremely variable supply 
of plant food and soil moisture may be an important factor in mottling. 
Indeed, Smith and Smith (io), in 1911, expressed the view that the most 
prevalent type of mottling is due to such conditions. 
SUMMARY 
(1) Semiarid soils frequently fail to nitrify dried blood when added in 
1 per cent quantities, but invariably nitrify blood when added in amounts 
not greater than are ordinarily applied under the field conditions. 
(2) The addition of dried blood to semiarid soils in 1 per cent quan¬ 
tities frequently caused large amounts of ammonia to accumulate in the 
soil. The addition of dried blood or other nitrogenous substances 
applied as fertilizers caused no marked increase in the ammonia content 
of the soils. 
(3) When 1 per cent of dried blood is added to semiarid soils, as much 
as 50 per cent of the nitrogen added may be lost during an incubation 
period of six weeks. As the soils frequently give off a strong ammoniacal 
odor, it is believed that this loss is due, in a large measure at least, to the 
volatilization of ammonia. 
(4) Ammonification or nitrification studies on semiarid soils in which 
1 per cent of dried blood is added are of questionable value and may lead 
to erroneous conclusions. 
(5) Green manures, especially the legume varieties, nitrify very rapidly. 
As much as 50 per cent of the nitrogen contained in green plant tissues 
may be converted into nitrates in 30 days. 
(6) Green manures furnish a valuable source of energy for the non- 
symbiotic nitrogen-fixing organisms. 
(7) The furrow system of irrigation frequently causes a very unsatis¬ 
factory distribution of the soil nitrates. In many Citrus groves more 
than two-thirds of the nitric nitrogen in the upper 4 feet of soil is found 
in the surface 6 inches, in which, because of the frequent cultivation, few 
feeding roots are found. 
