478 
AGRONOMY: C. B. LIPMAN 
hence their nitrogen-content, owing to the large supply of decaying 
organic matter, may compare very favorably v^ith that of an average 
soil of the humid region. In our truly arid soils, however, which re- 
ceive fifteen inches of rainfall per year or less, it is quite the usual 
thing to find the total nitrogen supply below 0.05% in the air-dry 
surface soil. Frequently, indeed, under the conditions of the San 
Joaquin Valley the percentage of total nitrogen in the surface soil may 
be no more than 0.01% or 0.02%. Therefore, even if all of this nitro- 
gen could be rendered available for assimilation by the plant, the soil 
could not be expected to produce profitable crops for more than a few 
years. Fortunately, the roots of plants can draw more or less freely 
on the nitrogen supply of the soil below the first foot in depth, and thus 
crops have been produced at times on soils manifestly deficient in nitro- 
gen. It must be remembered, however, that, even in arid soils in 
which we commonly find nitrification proceeding at the remarkable 
depth of six feet below the surface, nitrification and hence the available 
nitrogen supply decreases in intensity rapidly with increasing depth. 
As a result of a total supply of nitrogen which is too meager, coupled 
with the relatively small fraction thereof which is rendered available 
as is pointed out below, nitrogen starvation with its various manifesta- 
tions in different plants is one of the prominent problems of soil fertihty 
in CaHfornia, and particularly in case of nonleguminous perennial 
plants. To illustrate this, it may be mentioned that it has frequently 
been found impossible to carry young fruit trees through one season of 
growth in tile San Joaquin Valley on soils which are otherwise well 
supplied with plant food elements, for lack of a proper nitrogen supply. 
More frequent even than the total starvation of crop plants on our 
typically arid soils is the occurrence of plants which languish for several 
years because of an insufficient supply of available nitrogen. With 
my coworkers I have obtained experimental and observational data, 
to appear in detail later as above indicated, which point significantly 
to a probable causal relationship between the lack of usable nitrates 
in the root zone and many features of backwardness or disease in our 
crop plants. These data indicate, almost without exception in the soils 
studied, that the lack of available nitrogen referred to is to be accounted 
for in one or more of four ways: 1st, a lack of sufficient nitrogen in the 
soil in to to; 2d, a feeble nitrifying power of the soil; 3d, accumulation 
of nitrates in the dry surface crust of the soil in which they can not be 
used by the feeding roots; and 4th, denitrification of nitrates produced 
within or added to the soil. 
Our investigations point to the conclusion that the second cause. 
