y 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