THE SOIL AND THE PLANT
Apparatus required.
_Dry powdered soil, sand, clay, leaf mould, seeds. Six funnels, disks,
stands and glass jars [3]. Six glass tubes about 1/2 in. diameter and
18 in. long [2]. Muslin, string, three beakers. Six lamp chimneys
standing in tin lids [3]. Pot experiments (p. xiii), growing plant.
Two test tubes fitted with split corks (Fig. 35)._
If you have ever tried to grow a plant in a pot you must have
discovered that it needs much attention if it is to be kept alive. It
wants water or it withers; it must be kept warm enough or it is killed
by cold; it has to be fed or it gets yellow and starved; also it needs
fresh air and light. These five things are necessary for the plant:
Water,
Warmth,
Food,
Fresh air,
Light.
We may add a sixth: there must be no harmful substance present in the
soil.
Wild plants growing in their native haunts get no attention and yet
their wants are supplied. We will try and find out how this is done.
{65}
[Illustration: Fig. 29. Loam and sand both retain water, but loam
better than sand]
Water comes from the rain, but the rain does not fall every day. How
do the plants manage to get water on dry days? A simple experiment
will show you one way. Put about four tablespoonsful of dry soil on to
the funnel shown in Fig. 29 and then pour on two tablespoonsful of
water. Measure what runs through. You will find it very little; most
of the water sticks to the soil. Even after several days the soil was
still rather moist. Soil has the power of keeping a certain amount of
water in reserve for the plant, it only allows a small part of the rain
to run through. Do the experiment also with sand, powdered clay, and
leaf mould. Some water always remains behind, but less in the case of
sand than in the others. In one {66} experiment 30 cubic centimetres
of water were poured on to 50 grains of soil but only 10 cubic
centimetres passed through, but when an equal amount was poured on to
50 grains of sand no less than 20 cubic centimetres passed through.
Very sandy soils, therefore, possess less power of storing water than
do soils with more clay or mould in them, such as loams, clays or black
soils.
[Illustration: Fig. 30. Water can rise upwards in soil. It can, in
fact, travel in any direction, from wet to dry places]
Further, water has a wonderful power of passing from wet places to dry
places in the soil. Tie a piece of muslin over the end of a tube and
fill with dry soil, tapping it down as much as you can, then stand the
tube in water as in Fig. 30. Fill another with sand {68} and place in
water. Notice that the water at once begins to rise in both tubes and
will go on for a long time, always passing from the wet to the dry
places. It rises higher in the soil than it does in the sand. Enough
water may pass up the tube in this way to supply the needs of a growing
plant. Fill a glass lamp chimney with dry soil, packing it down
tightly, put into water and then sow with wheat. The plants grow very
well. A longer tube may be made from two chimneys fastened together by
means of a tin collar stuck on with Canada balsam or sealing wax (Fig.
31). Our plants grew well in this also, but on a sandier soil, where
the water could not rise so high, it might happen that they would not.
[Illustration: Fig. 31. Wheat growing in soils supplied from below
with water. All the water the plant gets has to travel upwards]
Thus we shall expect great differences in the moisture of various
soils. In some districts there is much more rain than in others, and
therefore the soils get a larger supply of water. Sandy soils allow
water to run through while a loam holds it like a sponge, in a loam
also the water readily moves from wet to dry places. Further, water
runs down hills and collects in low-lying hollows or valleys; here,
therefore, the soil is moister than it is somewhat higher up. What
will be the effect of these moisture differences on plants?
You must find out in two ways. Visit a soil that you know is dry--a
sandy, gravelly or chalky soil in a high situation--and look carefully
at the plants there, then go to some moister, lower ground and see what
the plants show. You cannot be quite certain, however, that anything
you see is simply due to water supply, because there may be other
differences in the soil as well. So you must try the second method,
and that is to find out by experiments what is the effect of varying
{69} quantities of water on the plant growth. Both methods must be
used, but it may be more convenient to start the experiments first, and
while they are going on to collect observations in your rambles.
[Illustration: Fig. 32. Mustard growing in soils supplied with varying
quantities of water. 16 very little water, 3 a nice amount of water,
15 too much water]
Fill four glazed pots with dry soil: keep one dry; one only just moist;
the third is to be very moist and should be watered more frequently
than the second; and the fourth is to be kept flooded with water, any
way out being stopped up. Sow wheat or mustard in all four and keep
out of the rain. The result of one experiment with mustard is shown in
Fig. 32. Where no water was supplied there was no growth and the seeds
remained unaltered. Where only little water was supplied (Pot 16) the
plants made some growth, but not very much: the leaves were small and
showed no great vigour; {70} where sufficient water was given (Pot 3)
the plants grew very well and had thick stems and large leaves; where
too much water was given (Pot 15) the plants were very sickly and small.
The weights were:--
Green weight After drying
Plants with too much water 3.9 0.5
" " too little water 5.3 0.9
" " a nice quantity of water 17.7 2.6
Fig. 33 shows two pots of wheat, one kept only just sufficiently moist
for growth, the other kept very moist but not too wet. You can see
what a difference there is; in the drier pot the leaves are rather
narrow and the plants are small, in the moister pot the leaves are wide
and the plants big. But there was also another difference that the
photograph does not bring out very well--the plants in the rather dry
soil were, as you can see, in full ear, ripe and yellow, while those in
the very moist soil were still green and growing. We see then
(1) that on moist soils there is greater growth than on dry soils, but
the plants do not ripen so quickly;
(2) in very wet soils mustard--and many other plants also--will not
grow.
Water is not itself harmful. It is easy to grow many plants in water
containing the proper food, but _air must be blown through the water at
frequent intervals_. In the water-logged soil of Pot 15 the trouble
arose not from too much water but from too little air. Air is wanted
because plants are living and {71} breathing in every part, in the
roots as well as in the leaves.
[Illustration: Fig. 33. This wheat growing on very moist soil was
still green and growing vigorously, whilst this wheat growing on rather
dry soil was yellow and ripe]
Now turn to what you have seen in your walks. You would probably
notice that on the drier, sandy or gravel ground there was nothing like
as great a growth of grass or of other plants as on the moister soil.
This is so much like what we found in the pot experiments that we shall
not be wrong in supposing that the difference in water supply largely
accounted for the difference in growth. But you may also have noticed
something else. Plants in the drier soil have generally {72} narrow
leaves and the grasses are rolled up and fine, whilst those on the damp
soil, including the grasses, have usually broad leaves. Thus in the
dry sandy soil you may find broom, spurrey, sheep's fescue, pine trees,
all with narrow leaves; whilst on the moister soil you may find
burdock, primroses, cocksfoot and other broad-leaved plants. Figs. 34
_a_ and _b_ show some plants we found on a dry, gravelly patch on
Harpenden common, and on a moist loam in the river valley below.
[Illustration: Fig. 34 _a_. Plants collected on dry sandy soil.
Broom, sheep's fescue, crested dogstail and gorse, all with narrow
leaves]
{73}
Before we can account for this observation, we must ascertain a little
more closely what becomes of the water the plant takes up. It
certainly does not all stay in the plant, and the only way out seems to
be through the leaves. Put a test tube on the leaf of a growing plant
and fix a split cork round the stem: leave in sunlight for a few hours
and notice that water begins to collect in the test tube (Fig. 35).
The experiment shows that water passes out of the plant through the
leaves.
[Illustration: Fig. 34 _b_. Plants collected on moist loam. All have
wide leaves]
This experiment was first made by Stephen Hales, and described by him
thus in 1727: "Having by many {74} evident proofs in the foregoing
experiments seen the great quantities of liquor that were imbibed and
perspired by trees, I was desirous to try if I could get any of this
perspiring matter; and in order to do it, I took several glass chymical
retorts, _b a p_ [Fig. 36] and put the boughs of several sorts of
trees, as they were growing with their leaves on, into the retorts,
stopping up the mouth _p_ of the retorts with bladder. By this means I
got several ounces of the perspiring matter of vines, figtrees"--and
other trees, which "matter" Hales found to be almost pure water. The
test tube experiment should now be made with a narrow-leaved grass like
sheep's fescue and with a wide-leaved grass like cocksfoot. You will
find that wide-leaved plants pass out more water than those with narrow
leaves, and hence wide-leaved plants occur in damp situations or on
damp soils like loams and clays, while narrow-leaved plants can grow on
dry, sandy soils.
[Illustration: Fig. 35. Plants give out water through their leaves]
Another thing you will notice is that fields lying at the side of a
river and liable to be flooded, and fields {75} high up in wet hill
districts, are covered with grass. In a clay country there is also a
great deal of grass land and not much ploughed land; if you live where
there is much clay you can easily discover the reason. Clay becomes
very wet and sticky when rain falls, and very hard in dry weather: it
is, therefore, difficult to cultivate. Farmers cannot afford to spend
too much money on cultivation, and so they prefer grass, because once
it is established it goes on indefinitely and does not want ploughing
up and re-sowing. And besides, farmers have learned by experience that
grass can tolerate more water and less warmth than most other English
crops. There is much more grass land in those parts of England where
the rainfall is high and the temperature rather {76} low--e.g. the
northern parts of England--than in the eastern counties where the
rainfall is low.
[Illustration: Fig. 36. Stephen Hales's Experiment (from _Vegetable
Staticks_, Vol. I. 1727)]
The difference in water supply, therefore, leads us to expect the
following differences between sandy soils and clays or loams:--
On sandy soils (the water content being small) the wild plants and
trees usually have small leaves. Cultivated plants do not give very
heavy crops, but they ripen early.
On clay soils (the water content being good) wild plants and trees
usually have larger leaves. Cultivated plants give good crops, but
they ripen rather late. If the water content is too good or the clay
is too sticky the land is generally put into grass.
[Illustration: Fig. 37. Hill slope near Harpenden. Woodland at the
top, arable land lower down. In the valley there is grass land but
this is hidden by the cottages]
Plants require to be sufficiently warm. Some like tropical heat and
can only be grown in hot houses; others can withstand a certain amount
of cold and will grow up on the mountains. Our common cultivated crops
come in between and will not grow in too cold or exposed a situation;
thus you find very little cultivated land 800 ft. above sea level, and
not usually much above 500 ft. At this height it is left as grass
land, and higher up as woodland, moor, or waste land. Grass requires
less warmth and can therefore grow at greater heights than many other
crops. If you start at the top of a hill in Derbyshire, and walk down,
you will see that the top is moorland, lower down comes grass land,
still lower you may find arable land, and if the valley is damp you
will find more grass at the bottom. Figs. 37 and 38 show typical views
of the hill slopes further south: they are taken near Harpenden. The
top of the hill in each case is over 400 ft. above sea level, and has
never been thought worth cultivating, but has always been left as {78}
wood because it is too exposed for farm crops. On the lower slopes the
arable fields are seen, while at the bottom bordering the river is
rough grass land, shown in Fig. 39. The top is too cold and windy, and
the bottom too wet, to be worth cultivating.
[Illustration: Fig. 38. View further along the valley, woodland and
arable above rough grass land near the river]
As the plant root is alive it wants air. The effect of keeping air out
can be seen by sowing some barley or onion seeds in the ground and then
pouring a lot of water on and plastering the soil down with a spade.
Sow another row in nicely crumbled soil, not too wet, press the seeds
well in, but do not plaster the soil. This second lot will generally
do much better than the first. If the ground round a plant is
frequently trodden so that it becomes very hard the plant makes much
less growth than if the soil were kept nice and loose. A good gardener
takes very great pains in preparing his ground before he sows his
seeds, and he is careful that no one should walk on his beds lest his
plants should suffer.
SUMMARY. We may now collect together the various things we have learnt
in this chapter. Plants require water, air, warmth, food, and light,
and they will not grow if harmful substances are present. The
rain-water that falls remains for some time in the soil, and does not
at once run away or dry off: water can also move from wet to dry places
in the soil. Therefore the plant does not need rain every day, but can
draw on the stock in the soil during dry weather. A sandy soil is
usually drier than a loam or a clay, especially if it lies rather high:
plants growing on a sandy soil make less growth and have narrower and
smaller leaves than those on a moister soil.
Situations more than five or six hundred feet above sea level are, in
England, as a rule, too bleak and {80} exposed for the ordinary
cultivated crops. Such land is, therefore, either grass land,
moorland, downland or woodland.
The roots of plants are living and require air. The soil must not be
trodden too hard round them or air cannot get in, nor can it if too
much water is present.
Grass can put up with more water and less warmth than most cultivated
crops.
[Illustration: Fig. 39. Rough grass pasture near the river, above that
is arable land and still higher is woodland]
Instances of these facts may be found in going down any hill 500 ft. or
more in height: the top is usually wood or waste, being too cold for
crops, below this may come grass land, lower still arable land. It is
both warmer and moister in the valley (since water runs down hill), and
so we can account for the proverbial fertility of valleys. But just
near the river, if there is one, the ground may be too wet for crops,
and therefore grass is grown. Clay land that is rather too wet to
plough is usually left in grass.
{82}