506 
NATURE 
[JuLty 16, 1914 
the Philippines on a scale of 1 : 7,500,000. The 
coast-line, rivers and lakes are in blue; the rail- 
ways and sea-routes, with distances, are in red; 
and town names are printed in black. The general 
effect is excellent, and the map should meet the 
needs of the class-room satisfactorily. 
LETTERS VO Tae EDITOR: 
[The Editor does not hold himself responsible for 
opinions expressed by his correspondents. Neither 
can he undertake to return, or to correspond with 
the writers of, rejected manuscripts intended for 
this or any other part of Nature. No notice is 
taken of anonymous communications. | 
Forests and Floods. 
SOME time ago the question of the effect of forests 
in checking floods was discussed in the pages of 
Nature. The subject was lately recalled to my recol- 
fection while watching gardening operations in the 
vegetable borders. I was then much struck by the 
different conditions, after rainfall, of newly dug 
ground and ground that had lain undisturbed for a 
year. The gardener was proceeding to put in plants 
in the newly dug part, but found it much too wet to 
be worked in. It was suggested he might continue 
the digging of the rest of the border and leave the 
planting until later. On breaking up the undisturbed 
ground, it was found to be fairly dry and in quite 
good condition for digging. These conditions 
seemed to indicate that newly disturbed ground holds 
a much greater proportion of the rainfall than does 
consolidated ground in which the soil particles are 
more closely packed together. In the latter the water 
seems to pass much more freely through it than in 
the former, possibly due to there being a continuous 
water film from the surface to the water table. It 
not only passes more quickly to the lower level, but 
much more of it passes, while the disturbed ground 
retains a much greater proportion of it to the benefit 
of the vegetation. 
Experiments might be made to get further informa- 
tion on this point by means of proper water-tight 
tanks filled with earth, and comparative readings 
taken of the drainage water in tanks in which the 
soil had become consolidated by rainfall and those 
in which the soil had been recently disturbed. As 
this would take a long time to accomplish, it has 
not been done, but perhaps some others may be in- 
duced to make the tests, as the knowledge of the 
subject may be useful in agricultural operations. It 
does not directly bear on dry farming, as that is a 
question mostly of surface soil mulch produced by 
stirring the surface soil and so breaking the water film 
connecting it with the subsoil. But it would seem 
to indicate that before dry farming can be started, 
the surface soil, to a depth sufficient to hold the 
rainfall, should first be thoroughly pulverised to 
prevent the rainfall passing downwards and beyond 
the range of the roots. 
Though satisfactory tests have not been made yet 
I had an .opportunity of making some experiments 
on somewhat similar lines. There were three pots 
full of soil lying out in the garden. These had pre- 
viously been in use in some experiments with plants. 
The soil in all of them was alike, having been taken 
from the kitchen garden. These pots and soil had 
been lying out for more than a year; and as the soil in 
them was thoroughly consolidated, the question was 
put to them. First, the pots were all weighed; as 
the weather had been fine for some time, the soil was 
NO. 2333, VOL. 93| 
pretty dry. The pot having the medium weight was 
then selected, the soil emptied out, broken up, and 
returned to the pot. Water was now poured slowly 
over the soil in all the pots in 4-0z. doses at a time. 
The first thing noticed was that the water entered the 
soil of the undisturbed pots more slowly than 
the other, and, secondly, that the water came more 
quickly through the soil in these pots than through the 
other. Water was added to the pots until they ceased 
to take up any more. After draining, they were 
weighed again, and the result is given in the table :-— 
Weights in Pounds and Ounces of the Three Pots. 
No. 1, No. 2. No. 3. 
Consolidated Pulverised Consolidatcd 
soil soil soil 
Ib. oz Ib. oz lb. oz. 
Dry 7 (95 FiglO Tas 
Wet au hea, Joe LO 9 64 9 I 
Water held by soil I o% Liz Pia 
It may be further mentioned that it was thought 
that some of the soil in the consolidated pots might 
not have got thoroughly wetted, owing to the water 
running quickly through them; the three pots were 
therefore afterwards put in a vessel of water to soak ; 
they were then drained and weighed, but the result 
showed but little change, showing that all the pots 
had got as much water as the soil would hold. An 
examination of the above table shows that the dis- 
turbed soil holds a much greater amount of water 
than the consolidated soil. No definite conclusion can 
be drawn from these figures as to the relative retain- 
ing powers of the soil in the two conditions, as no 
two soils are likely to be equally affected. The only 
thing to be noted is that the pulverised soil has a 
much greater power of holding water than the con- 
solidated. 
It may be asked: what has all this to do with forests 
and floods? If we are correct in supposing that soil 
by becoming consolidated and the particles close 
packed, by the action of the rainfall, causes it first 
to resist the entrance of heavy rainfalls, and secondly, 
after it has entered the soil, to facilitate its passage 
through it to depths beyond the range of being of use 
to vegetation. If this be so, then anything that 
breaks up the close packing of the grains and stirs 
the soil will tend to enable the water to enter the soil, 
and will also tend to enable it to retain it. Now the 
roots of trees in forests, by their constant growth and 
expansion, stir the soil and prevent it getting consoli- 
dated. The soil under trees will therefore always be 
in the best condition for absorbing and retaining the 
rainfall. And the surplus is only slowly parted with 
to feed the drainage, whereas on bare soil, or soil 
on which the vegetation is poor, tends to reject the 
rainfall, causing the water to run off the surface, and 
what enters is quickly passed downwards to swell the 
drainage water. From the above it would appear 
that bare and poorly cultivated land will tend to cause 
floods by speedily getting quit of its rainfall, while 
| forest land will retain and only slowly part with it. 
The decaying vegetation on the surface under trees 
has also a beneficial effect, as it absorbs water and 
acts as a mulch, preventing drying. 
It is well known that rains in early summer, unless 
when torrential, give rise to small amounts of flboding 
compared with winter rains of the same amount. 
There are a number of reasons for this which our 
space does not admit of treating, but it is probably 
in part due to the stirring action on the soil of the roots 
of grasses and other plants, as that is the season 
when root action is most active. 
Ardenlea, Fallxirlx. 
June 29. 
Joun AITKEN. 
