emains 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