830 
Journal o f Agricultural Research 
Vol. XXX, No. 9 
if the “natural mulch” is ever equal in 
efficiency to the artificial in the actual 
retention of moisture already in the 
soil. 
Since there is, therefore, a proved 
upward movement of moisture in the 
soil under conditions of rapid removal 
by evaporation, and since the data 
presented show that the soil mulch can 
and does check a certain part of this 
movement and loss, there can be little 
doubt that as a purely retentive agent, 
the soil mulch has a positive effect. 
The two effects of the soil mulch in 
absorption and retention have been 
shown and their operation has been de¬ 
scribed. The data for the period of 
summer rainfall indicate how these two 
effects may be modified by type of rain¬ 
fall and intensity of evaporation. In the 
particular June period considered in this 
study there were two individual rains, 
each greater than 1 inch in amount, 
and both of which penetrated well 
below the mulch. Following these 
rains evaporation was rather active. 
When volume of rainfall is great 
enough to penetrate the mulch and to 
establish connection with the under¬ 
lying soil, the mulch no longer acts in 
absorption in the same manner as with 
lighter precipitation. If conditions fa¬ 
vor rather intense evaporation, tillage 
given immediately after such rainfall, 
by renewing the mulch, may retain a 
certain part of the moisture which has 
penetrated below the mulched area. 
When the mulch is not reestablished 
after the rain, under such conditions 
there is greater loss of moisture from 
the unmulched soil. In conserving 
natural precipitation, the most im¬ 
portant consideration is to reduce 
evaporation in the surface area to a 
point that moisture may have an op¬ 
portunity to move downward into the 
lower soil. If the rate of evaporation 
is not extreme, and moisture falling on 
the surface is not removed, either as 
run-off or by evaporation, before it 
has an opportunity to penetrate to 
lower levels, there is nothing to be 
gained by creating a mulch, either 
before or after a period of rainfall. 
Light showers not sufficient to pene¬ 
trate the mulch have little effect one 
way or the other on total moisture 
content, for either in a mulched or an 
unmulched soil such a limited amount 
of moisture near the surface is quickly 
removed if evaporation be active. 
The value of the soil mulch in conserv¬ 
ing moisture in a region of summer 
rainfall, therefore, is entirely dependent 
on the volume of individual showers 
and on the intensity of succeeding 
evaporation. Practically this may re¬ 
sult in little effect from the mulch one 
way or the other, as shown, for instance, 
by the data reported by Call and 
Sewell ( 9 ), by Barker (J), by Young 
{26), and others. 
In the foregoing discussion there is 
no intent to convey the idea that the 
balance of the inhibitory effect of the 
mulch on absorption and its positive 
effect in retention is the sole determiner 
of the final result in tillage practice. 
Soil type, temperature, relative hu¬ 
midity, the volume, character and 
distribution of precipitation, etc., may 
give other factors prominence so that 
the presence or absence of the mulch 
is not always of equal importance. 
In districts of heavy snowfall and of 
comparatively high winds the pro¬ 
tective effect of a heavy stubble in 
holding snow or in checking the evap¬ 
orative influence of the wind may 
contribute very largely, for instance, 
to final soil moisture content; and in 
such a case all of the benefits can not 
be ascribed to the absence of a mulch 
that might hinder absorption. If pre¬ 
cipitation is of such volume or char¬ 
acter that the mulch functions for only 
a short time, there will be little effect, 
either one way or the other, from 
mulch-forming tillage. Whether evap¬ 
oration is or is not active naturally has 
a decided influence on the result. 
These and other contributing factors 
must all be taken into account in 
regulating practice; yet, allowing to 
each factor its relative importance, 
the effect of the mulch as a mulch 
should still be given full consideration. 
CONCLUSIONS 
(1) The soil mulch has an inhibitory 
effect on moisture absorption, under 
conditions where individual rains are 
not of sufficient volume to fully pene¬ 
trate the mulch. 
(2) The mulch inhibits absorption 
by increasing the amount of current 
evaporation in the newly fallen mois¬ 
ture. The volume weight of the 
stirred mulch being less than that of 
an equal depth of unstirred soil, the 
moisture content of the mulched soil 
immediately after a rain is higher on a 
percentage basis. When conditions 
favor evaporation, the result is a greater 
total loss from the mulched soil. The 
final moisture content is due to a 
cumulative effect following several 
rains. 
(3) The soil mulch prevents the loss 
of moisture already in the soil by 
checking evaporation. 
(4) The practical use of the soil 
mulch in moisture conservation is 
