CULTIVATION AND TILLAGE
Apparatus required.
_Plot experiments, hoeing and mulching. Thermometer. Soil sampler
(Fig. 42, p. 88). This tool consists of a steel tube 2 in. in diameter
and 9 in. long, with a slit cut along its length and all the edges
sharpened. The tube is fixed on to a vertical steel rod, bent at the
end to a ring 2 in. in diameter, through which a stout wooden handle
passes. It is readily made by a blacksmith._
Farmers and gardeners throughout the spring, summer and autumn, are
busy ploughing or digging, hoeing or in other ways cultivating the
soil. Unless all this is well done the soil fails to produce much; the
sluggard's garden has always been a by-word and a reproach. In trying
to understand why they do it we must remember that plant roots need
water, warmth and air; if the soil is too compact or if there is too
much water the plant suffers, as we have seen.
[Illustration: Fig. 40. After harvest the farmer breaks up his land
with a plough and then leaves it alone until seed time]
One great object of cultivation is, therefore, to prevent the soil
being too compact and too wet. After the harvest the farmer breaks up
his ground with a plough and then leaves it alone till seed time (Fig.
40). A gardener does the same thing with a fork in his kitchen
garden--he cannot very well elsewhere, or the plant roots might {83}
become too cold. If there is frost during the winter both farmer and
gardener are pleased because they say the frost "mellows" the ground;
you can see what they mean if you walk on a frosty morning over a
ploughed field. The large clods of earth are no longer sticky, they
already show signs of breaking up, and if they are not frozen too hard
can easily be shattered by a kick. The change has been brought about
in exactly the same way as the bursting of water-pipes by frost. When
water freezes it expands with enormous force and bursts open anything
that confines it; water freezing in the pores of the soil forces the
little fragments apart. This action is so important that further
illustrations should be looked for. A piece of wet chalk left out on a
frosty night often crumbles to pieces. It is dangerous {84} to climb
cliffs in the early spring because pieces of rock that have been split
off during the winter frosts by the expanding water may easily give
way. Frost plays havoc with walls built of flints and with old bricks
that are beginning to wear. If there are several frosts, with falls of
rain or snow and thaws coining in between, the soil is moved about a
good deal by the freezing and melting water. Bulbs and cuttings are
sometimes forced out of the ground, whilst grass and young wheat may be
so loosened that they have to be rolled in again as soon as the weather
permits. When the ground has been dug in autumn and left in a very
rough state all this loosening work of the frost is very much helped,
because so much of the soil may become frozen. If in spring you dig a
piece of land that has already been dug in autumn, and then try digging
a piece that has not, you will find the first much easier work than the
second in all but very sandy soils.
A little before the seeds are sown, the soil has to be dug or
cultivated again so that it may become more level and broken into
smaller pieces. The farmer then harrows and the gardener rakes it, and
it becomes still finer. Very great care is bestowed on the preparation
of the seed bed, and it will take you longer to learn this than any
other part of outdoor gardening. The soil has to be made fine and dry,
and no pains must be spared in getting it so.
When at last the soil is fine enough the seed is put in. But it is not
enough simply to let the seed tumble into the ground. It has to be
pressed in gently with a spade or a roller, not too hard or the soil
becomes too sticky. Fig. 41 shows this operation being carried out on
the farm. Then the soil should be left alone.
[Illustration: Fig. 41. Rolling in mangold seeds on the farm]
{86} If you watch an allotment holder who grows onions really well
working away at his seed-bed you will see what a beautifully fine tilth
he gets. If you try to do the same you will probably fail; his seeds
will be up before yours and will grow into healthier plants. Only
after long practice will you succeed, and then you will have mastered
one of the great mysteries of gardening.
As soon as the plants are up they have to be hoed, and the more often
this is done the better. Hoeing has several useful effects on the
soil; during summer time some experiments may be made to find out what
these are. A piece of ground is wanted that has got no crop on it.
Set out three strips each six feet wide and six feet long, leave one
entirely alone, hoe the second once a week, and the third three times a
week; put labels on so that no mistake can arise. The surface of the
untouched plot becomes very compact and glazed in appearance; the other
soils look nice and crumbly. Take the temperature of the soils by
placing a thermometer into it at various depths--half inch, three
inches, and six inches--also take the temperature of the air; enter up
the results as in the table, which shows what happened at Harpenden.
Air
Date temperature Soil temperature
Hoed Hoed
Untouched once three times
weekly weekly
1910
June 20th 30 1/2 inch 35 31.5 31.5
3 inches 30.5 29.8 28.8
6 inches 27 26.5 24
June 27th 18 1/2 inch 17.5 17 17
3 inches 16.7 16.3 16.2
6 inches 15.8 15.5 15.6
{87}
The thermometer readings are in degrees centigrade.
Remarks. June 20th: Hot sunny day, there had been no rain since June
11th.
June 27th: Cold, cloudy day, several cold, wet days during the past
week.
On the cold day there was very little difference between the plots, but
on the hot day the hoed plots were cooler than the others. Now only
the top inch is touched by the hoe, and so it appears that the layer
thus loosened shields the rest of the soil from the sun's heat. If
this is the case we ought to find that any other loose material would
act in just the same way. We must, therefore, set out a fourth plot
alongside the others, cover it with straw or cut grass (a cover like
this is called a mulch), and take the temperature there. Some of the
results were as follows:--
Air
Date temperature Soil temperature
Hoed plot Mulched plot
1910
Sept. 24th 15 1/2 inch 17.5 12.25
3 inches 12.5 11.75
6 inches 12.25 11.5
Oct. 5th 17 1/2 inch 17 15.5
3 inches 16.7 15
6 inches 15.5 14.5
Remarks. Sept. 24th: Warm day after a rather cold spell. Oct. 5th:
After a long spell of dry, warm weather.
The untouched plot had become smothered in weeds and could no longer be
used for this experiment. The mulched soil is, however, cooler even
than the hoed soil, and our expectation that mulching would keep the
soil cool has turned out to be correct.
{88}
[Illustration: Fig. 42. Soil sampler. (See p. 82 for description)]
It may be expected that the hotter soil--the unhoed plot--will also be
drier than the others, and this can be found out by a simple
experiment. Take a sample by making a hole six indies deep with
straight and not with sloping sides: this is best done by driving a
tube two inches wide into the soil (Fig. 42): if you have not got such
a tool you may use a trowel, but you will have to be very quick and
very careful. Weigh the soil--or a part of it if you have got a great
deal--then set it to dry in a warm place for three or four days. Weigh
again when it is dry: the difference gives the loss of water: find what
it would be in a hundred parts. Our results were:--
{89}
Date Percentage of water in the
Untouched soil Soil hoed once Soil hoed three
weekly times weekly
1910
June 4th 21.1 19.5 17.9
June 20th 14.7 16.0 16.0
June 27th 19.3 18.4 20.5
Remarks. June 4th; The weather is still cold and the summer has not
yet begun.
June 20th: Hot day following on some hot, dry weather.
June 27th: Rain had recently fallen.
When hoeing is done in the early part of the summer it dries the soil,
and the more frequent the hoeing the drier the soil (see June 4th
results). But later on, when the hot weather begins, the hoed soil
loses much less moisture than the untouched plot; the latter lost 6.4
per cent. in 16 days in the top six inches, whilst the soil hoed once
weekly lost 3.1 per cent., and the one hoed three times weekly lost
only 1.4; the two hoed soils are now equal, and are both moister than
the untouched soil. When more rain comes they get just as wet as the
others: hoeing does not prevent water from sinking in, but it does
prevent water from getting lost.
Our experiment has, therefore, shown us that hoeing makes a loose layer
of soil which shields the rest of the soil from the sun's heat, and
prevents it getting too hot or too dry. A hoed soil is cooler and
moister, and therefore better suited for the growth of plant roots than
an unhoed soil.
{90}
The mulch of straw or dried grass was found to have the same effect in
conserving the water as the loose layer of soil obtained by hoeing.
Some results were:--
Percentage of moisture in
Date Hoed soil Mulched soil
1910
Sept. 24th 19.6 20.7
[Illustration: Fig. 43. Cultivation and mulching reduce the loss of
water from soils]
These results are so important that some indoor experiments should be
made to furnish more proof. Fix up three inverted bell jars with corks
and bent tubes as shown in Fig. 43, fill all with dry soil well pressed
down, then add water carefully till it appears in the glass tubes.
Next day mark with stamp paper the level of liquid in each tube and
then leave one jar {91} untouched, carefully cultivate with a penknife
every two or three days the top quarter of an inch of the second, and
cover the third with a layer of grass. After a week notice again the
levels of the liquid and mark with paper; you find that the water has
fallen most in the untouched jar, showing that more has been lost from
this than from the jars covered with a mulch either of soil or of dry
grass.
A slate or flat stone acts like a mulch; if you leave one on the soil
for a few days in hot weather and then lift it up on a hot day you will
see that the soil underneath is quite moist; you may also find several
slugs or other animals that have gone there for the sake of the
coolness and the moisture. Plants and trees also keep off the sun's
heat and so make the soil cold and moist. Grass land is in summer and
autumn, and even in early winter, cooler near the surface than bare
land. At Harpenden we found:--
Soil temperature
Date Grass land Bare land
1910
Sept. 24th 1/2 inch 13 17.5
3 inches 12.5 12.5
6 inches 12.5 12.25
Oct. 5th 1/2 inch 17.5 17
3 inches 15 16.7
6 inches 14.5 15.5
Even if the ground is not covered a certain amount of protection is
still possible. Trees are often planted round ponds to prevent
evaporation of the water. The wind helps to dry the soil very much,
and a hedge {92} that shields from the wind not only protects the crop
but also keeps the soil moist: a road with high hedges at each side
remains wet for a long time after more exposed parts have dried. The
effect on the temperature can be well seen on a day when a N.E. wind is
blowing. Fix up on a piece of the experimental ground a little hedge
made of small pea-stakes or brushwood, and take the soil temperature at
one inch depth, both on the windward and on the leeward side. Two
results were:--
Temperature at 1 inch depth--sheltered side 15.5
" " " " windward side 14
We have already seen that on the hot day, June 20th, the top half-inch
of soil was hotter than the air: the mercury in the thermometer rose
directly it was put into the soil. There is nothing very unusual about
this; if you touch a piece of iron lying on the soil you find it hotter
than the air. Lower down the soil had the same temperature as the air,
and still lower it was cooler[1]. The sun's heat travels so slowly
into the soil in summer that months pass before it gets far down, but
then, as it takes so long to get in, it also takes a long time to get
out, and it takes still longer to get either in or out if there is a
mulch or if grass is growing.
During the early winter you may notice that the first fall of snow soon
melts on the arable land but remains longer on the grass; towards the
end of the winter, however, the reverse happens and the snow melts
first on the grass. There is no difficulty in explaining this. The
arable land is, as we have seen, warmer in autumn and early winter than
grass land, {93} and so it melts the snow more rapidly. But during
winter the grass land loses its heat more slowly, and therefore it is
warmer at the end of the winter than the arable land, hence the snow
melts more quickly.
In Chap. V. it was pointed out that dark coloured soils rich in humus
are greatly favoured by gardeners and farmers. The value of humus can
easily be shown: take a sample of soil from a garden that has for a
long time been well manured and another from a field close by--next to
it if you can--and find the amounts of moisture present. Two soils at
Rothamsted gave the following results:--
Date April 6th May 6th July 6th Oct. 28th
Moisture in dark soil
rich in humus 20.0 18.0 20.7 23.3
Moisture in lighter
soil poor in humus 13.1 11.9 12.0 17.5
Humus, therefore, keeps the water in the soil and saves it from being
lost.
Another beneficial effect of hoeing is to keep down weeds. Weeds
overcrowd the plant, shut out light, take food and water, and occupy
space. Few plants can compete against weeds, some fail very badly in
the struggle. Sow two rows of maize two yards apart; keep one well
hoed for a yard on each side and leave the other alone to struggle with
the weeds that will grow. Fig. 44 shows the result of this experiment
at St George's School. At Rothamsted a piece of wheat was left
unharvested in 1882, and the plot has not been touched since; the wheat
was allowed to shed its seed {94} and to grow up without any attention.
Weeds flourished, but the wheat did not; the next year there was but
little wheat, and by 1886 only a few plants could be seen, so stunted
that one would hardly recognise them. The ground still remains
untouched, and is now the dense thicket seen in Fig. 45. Most of our
land would become like this if it were neglected for a few years.
[Illustration: Fig. 44 _a_. The hoed plot, no weeds. Maize cannot
compete successfully against weeds]
{95}
Farmers occasionally leave their ground without a crop for a whole year
and cultivate it as often as they can to kill the weeds. This practice
is called "fallowing," and is very ancient; it is much less common now
that crops like mangolds and swedes are grown, which can, if necessary,
be hoed all the summer.
[Illustration: Fig. 44 _b_. Untouched plot, many weeds]
We have already seen (p. 69) that ordinary cultivated plants will not
live in a water-logged soil. {96} Wherever there is an excess of water
it must be removed before satisfactory results can be obtained. Fig.
46 shows a field of wheat in May where the crop is all but killed and
only certain weeds survive on a patch of undrained land that lay wet
all the winter. Draining land is difficult and somewhat expensive;
trenches are first cut to a proper depth, and drain pipes are laid on
the bottom, taking care that there is a gentle slope all the way to the
ditch. The rain soaks into the soil and gets into the pipes, for they
are not joined together like gas or water pipes, but left with little
spaces in between; it then runs out into the ditch. Usually only clay
soils need drainage, but occasionally sandy soils do also (see pp. 30,
106). A great deal of drainage was carried out in England between 1840
and 1860, and it led to a marked improvement in agriculture and in
country life generally. There is, however, a great deal that wants
doing now.
[Illustration: Fig. 45. A plot of wheat left untouched since 1882 at
Rothamsted has now become a dense thicket]
The addition of chalk or lime to soil was found in Chap. III. to
improve it very much by making it less sticky and less impervious to
air and water. Chalk or lime does more than this. It puts out of
action certain injurious substances or acids that may be formed, and
thus makes the conditions more favourable for plants and for the useful
micro-organisms; farmers and gardeners express this by saying that it
"sweetens the soil." A United States proverb runs: "A lime country is
a rich country." Very many soils in England are improved by adding
lime or chalk. There are considerable areas in the south-eastern and
eastern counties where the soil is very chalky; here you find a
wonderfully rich assortment of flowers and shrubs. Where there is too
much chalk the soil is not fertile, because it lets water {98} through
too easily, as was shown on p. 26: but for this very reason it is
admirable for residential purposes.
There are some exceptions to the rule that plants need lime. Some
plants will not tolerate it at all; such are rhododendrons, azaleas,
foxgloves, spurrey, and broom; wherever you see these growing you may
be sure that lime is absent.
Lime really differs from chalk, but changes into it so quickly in the
soil that the action of both is almost, though not quite, the same.
[Illustration: Fig. 46. A wheat field in May. The large patch in the
centre where the crop is doing badly lay under water for much of the
winter because of the bad drainage]
SUMMARY. The various things we have learnt in this Chapter are:--
Autumn and winter cultivation are needed to loosen the soil so that
rain can soak in and not lie about in pools, and also to facilitate
working in spring.
The soil has to be broken down very finely and made rather dry for a
seed bed. The seed has to be rolled in and then left entirely alone.
As soon as the little plants are up the soil must be hoed, and the more
often this is done the better. Hoeing keeps the soil cool and moist in
hot weather, the loose layer acting like a mulch of straw. Anything
else that shields the soil from the sun or the wind has the same action
but is not so effective as the mulch. Further, hoeing keeps down
weeds, which successfully compete against almost any cultivated plants.
Humus also prevents the loss of moisture from soils.
Drainage may be necessary to remove excess of water.
Liming or chalking the soil is beneficial, not only because of the
improvements mentioned in Chap. III., but also because certain
injurious substances are thereby removed. There are, however, some
plants that will not tolerate lime.
[1] At great depths below the surface the temperature rises again from
quite another cause.
{100}