May i, 1925 Soil Mulch in Absorption and Retention of Moisture 
829 
near the surface exposed to evapora¬ 
tion. This factor might have a bear¬ 
ing on increasing the inhibitory effect 
of the mulch on absorption during the 
colder seasons of the year, but if so it 
has been less important on the Adams 
Station than the other factors outlined 
above, as evidenced by the fact that 
the inhibitory effect has been apparent 
before any freezing weather occurred. 
That the inhibitory effect of the 
mulch on absorption tends to be pro¬ 
portional to the depth of the mulch 
was brought out by discussion of the 
data in Table II. The deeper mulch 
of the dry fall plowing was a more ef¬ 
fective inhibiting agent than the more 
shallow mulch of the disking. Any 
condition which tends to retain the 
largest percentage of current showers 
near the surface undoubtedly would 
aggravate the inhibitory effect of the 
mulch on absorption, and increased 
depth in the mulch has such an effect. 
Under normal field conditions a stirred 
soil tends naturally to settle as it is 
wet, and a deeper mulch, requiring a 
larger amount of moisture to entirely 
wet through it, will naturally remain 
an effective inhibiting agent for a 
longer time than would one more shal¬ 
low. The volume and frequency of in¬ 
dividual rains naturally have a very 
important bearing on this factor. 
In no instance has the writer been 
able to find a record of the mulch fail¬ 
ing to conserve moisture when it is 
clearly shown that it was confined to 
that single function. The data pre¬ 
sented in this study for the summer 
periods are, therefore, in complete 
agreement with the findings of others 
so far as this one phase is concerned. 
In general, the theoretical operation 
of the soil mulch in preventing the loss 
of soil moisture through evaporation 
might seem too well known to require an 
extended discussion; yet, because the 
issue has been confused in some cases, 
a certain elaboration is desirable. It 
has generally been considered as estab¬ 
lished beyond controversy that soil 
moisture moves within the soil in re¬ 
sponse to the forces of capillarity. In 
accordance with this concept, as evap¬ 
oration removes moisture from the sur¬ 
face soil, other moisture moves up 
from more moist underlying soil, and 
being in turn removed by evaporation, 
a more or less continuous movement 
from below to the surface is estab¬ 
lished. The stirring, incident to cre¬ 
ating the soil mulch, decreases the num¬ 
ber of points of capillary contact, and, 
by exposing additional surface to evap¬ 
orating agencies, increases evapora¬ 
tion in the immediate surface soil to 
a rate beyond the ability of capillary 
movement to replace losses. As a re¬ 
sult, the surface soil becomes dry, and 
being already loosened by the stirring, 
the mulch effect is created. Bucking¬ 
ham ( 7 ), Alway and Clark (1), and 
others have shown that such a dry 
soil retards capillary movement, hence, 
when present as a loose surface layer, 
necessarily would check evaporation. 
In introducing the review of litera¬ 
ture in their discussion of the soil 
mulch, Call and Sewell (9) make the 
statement that “ capillary movement 
without the presence of a water table 
(with a dry subsoil) has not been 
demonstrated.” From the data pre¬ 
sented in Table IV, it is very evident 
that moisture withdrawal, and hence 
movement, occurred in both the 
mulched and unmulched soils. This 
movement was both upward and 
downward, but despite the fact that 
downward movement, aided by grav¬ 
ity, tends to be greater than that in 
an upward direction, as noted by 
Harris and Turpin (17), this latter 
was slight in the unmulched soil, and, 
varying with season, there occurred a 
withdrawal of moisture extending into 
the third foot. How much deeper 
this upward movement might or 
might not have extended with greater 
moisture content and deeper penetra¬ 
tion is, of course, a matter of con¬ 
jecture, yet it was extensive enough in 
any case to demonstrate that upward 
movement occurs without a water 
table. That this was a capillary 
movement seems the logical conclusion, 
since Buckingham ( 7 ) and Bouyoucos 
(5) both have shown that internal 
evaporation and diffusion have very 
little effect on moisture movement 
within the soil. 
It is well known that the rapid 
evaporation of arid regions often 
greatly exceeds the ability of any soil 
to conduct moisture to the surface by 
capillarity, resulting in a naturally dry 
surface condition, this dry surface act¬ 
ing in preventing further loss of mois¬ 
ture as a “ natural mulch” ( 7 ). Many 
have felt that such a natural mulch 
might make unnecessary the arti¬ 
ficially created one, and that benefits 
have been ascribed to the latter which 
may, in reality, be just as effectively 
secured by natural means and without 
unnecessary labor. The data pre¬ 
sented, however, would not indicate 
the “natural mulch” to be equal to the 
artificially created one on the Adams 
Station, and, eliminating concurrent 
precipitation and its effects on soil 
moisture content in the mulched and 
unmulched soil, it seems very doubtful 
