476 
Journal of Agricultural Research 
: Vol. XIV, No. z-z 
DISCUSSION OF RESULTS 
distribution of available moisture in surface layers 
After prolonged dry weather following good rains the soil without 
plant cover through the first few inches shows a rapid rise in moistness 
from the surface downward, while where there is a full stand of plants, 
as in the grassfields, the rise is slight and a low ratio extends beyond 
the twelfth inch. Where a moderate rain has fallen after the latter 
condition has once been established, there will be a high and compara¬ 
tively uniform degree of moistness through several inches and then a 
sharp fall, but where very heavy rains have fallen there will be almost 
uniformly high ratios. These conditions are illustrated by figure 5, 
showing graphically part of the data from Tables IX, X, and XII. 
relative importance of successive soil levels as sources of 
MINERAL NUTRIENTS 
It is customary in making chemical studies of the soil to distinguish 
sharply between the 6-to-9-inch portion reached by tillage implements, 
called the "soil” or "surface soil,” and that below, referred to as the 
"subsurface,” or sometimes as the "subsoil.” As it has been assumed 
that nutrients are secured through the roots mainly from the former, 
this has received the chief attention. As a source of nitrogen the sur¬ 
face layer will be much the more important because of the generally 
much higher content of nitrogen and the more aerobic conditions found 
in this. 
On the soils of at least the Missouri River territory, such as represented 
in the present study, it is evident that a similar assumption for the 
mineral nutrients is not justifiable. The decline in moistness within the 
surface foot of the grassfields is quite uniform (Table IX), the withdrawal 
of water appearing as rapid in the lowest 3-inch section as in the second; 
and there is nothing in the data to suggest that this uniformity does not 
extend to a considerable distance below the twelfth inch. Almost the 
same remarks apply to the cornfields after the plants have made their 
main growth of stalk. 
However, the similar readiness with which water is given up to the 
roots from the two levels specified does not necessarily indicate that they 
are equally important as sources of mineral nutrients to the crops. 
Aside from the fact that with annuals these are largely absorbed during 
the early stages of growth there is the important consideration that on 
nonirrigated lands the uppermost of the two sections will be in a moist 
condition a much greater proportion of the time, owing to many of the 
summer showers not being sufficiently heavy to cause any increase in 
moistness beyond the first few inches, as is well illustrated in the above 
tables. Hence, as a source of mineral nutrients the first 6 inches will be 
