JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 
April 12, 1883. ] 
291 
12 th 
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Royal Society at 4.30 P.M. 
13th 
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Quekett Club at 8 P.M. 
14th 
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Royal Botanic Society at 3.45 P.M. 
15th 
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3RD Sunday after EASTER. Ghent International Exhi- 
16th 
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[bition (8 days). 
17th 
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Society of Arts at 8 P.M. 
EAETH TEMPERATURES. 
I^HEN treating of bottom beat it is usual to 
begin by quoting figures which prove that 
during the growing season the heat of 
the soil is higher than the air. As this 
fact is now perfectly well known, it is 
not necessary to take up space proving 
what is regarded as a truism. We will, 
therefore, at once plunge into the subject 
of bottom heat, and the effect of soil-warmth in 
promoting the growth of plants. 
In hot countries the heat of the soil frequently 
becomes so high as to be absolutely destructive. Under 
such circumstances cultivators have to keep down 
earth heat by irrigation, and on a small scale by mulch¬ 
ing, in order to rescue the land from utter sterility. 
Unless during exceptionally hot seasons and on thin 
soils with a hard or rocky substratum underneath, a 
too high earth temperature is seldom experienced by 
British gardeners, unless in the cultivation of some few 
alpines or plants from northern climes. Generally 
speaking, it is want of heat that we in our northern 
latitude suffer from, and especially so in those dis¬ 
tricts where clouds prevail and the sun seldom reaches 
through to warm the cold earth below. 
Increase of bottom heat is equal to increase of air 
heat; but while glass houses or sheltering walls and 
plantations have to be erected at considerable cost in 
order to secure a higher air temperature, higher earth 
temperature may be secured by applying the facts 
which science has discovered, and which cost us little. 
It is well known that dark surfaces absorb heat, while 
white surfaces reflect it. Acting on this, we in 1882 
darkened the surface of half of the space wherein our 
earliest batch of Potatoes grew, and also part of our 
Onion beds. The Potatoes in our darkened section 
were fully a week earlier than those grown on the un¬ 
darkened part. The soil was a brown loam. On 
ordinary garden soil, deeply darkened because much 
humus is present, such artificial darkening would not, 
of course, have an equal effect. In the case of the 
Onions the crop was very superior, not so much in 
weight as in ripeness and quality, to the portion grown 
on the undarkened part. But other means were taken to 
keep up the earth heat, which will presently be men¬ 
tioned. Had the season been at all an average one 
the results would have doubtless been even more 
marked. As it was there was remarkably little sun¬ 
shine after June. 
The last time that Tomatoes were grown out of 
doors here was in 1878. The climate is against their 
doing well. In that year all were grown on a border 
raised 1 foot by means of bricks—experiments the year 
before having shown the advantage of raised soil. 
Half of this border was darkened by impalpable char¬ 
coal dust, and half left au naturel. ■ Charcoal dust 
(moulders’ black used in ironfounding) was used in¬ 
stead of soot to guard against error; the use of soot, 
though securing the absorption of sun heat, having 
proved too stimulating and causing a luxuriance that 
was likely to be attributed to a wrong cause. Even 
common coal dust we have found effective. The plants 
grown in the darkened soil ripened fruit eleven days 
earlier than those under natural circumstances, and 
produced nearly double the crop, for the others failed 
to swell all they set—the early autumn frosts stopped 
them. 
The year before, on the same place, half were planted 
on the level ground, half on soil raised 1 foot. The 
year 1877 was extremely wet and sunless in this 
locality, but even the little sun we had warmed the 
raised border so that the plants benefited considerably, 
although no ripe fruit was got from either batch. The 
extra warmth, however, caused a healthier growth, 
and the plants produced a few green fruits, those on 
the flat remaining stunted and fruitless. 
On cold clays advanced gardeners have long practised 
planting fruit trees on raised mounds, experience 
having proved that better ripened wood and increased 
quantities of better fruit are thereby secured. The reason 
is that such mounds acquire a higher temperature than 
the soil on the flat. We think this fact has not been 
recognised in all its bearings, or raised borders would 
be more frequently seen. In these days, when root- 
cultivation is well understood and generally practised, 
it would be well worth while planting wall trees at 
least on borders raised out of the cold bottom to where 
the sun would early supply the earth with much-needed 
heat. Especially in the case of trees on dwarfing 
stocks or as cordons could this be done, as such require 
borders of no great dimensions. 
Barren cankered trees have been cured of their 
disease, and their barrenness replaced by fertility, by 
simply having their roots lifted out of the cold under 
soil and placed near the warmer surface. Dr. Lindley, 
quoting from the Memoirs of the Caledonian Horti¬ 
cultural Society, in his “ Theory of Horticulture,” 
mentions a case of this kind. Mr. Reid of Balcarras 
cured cankered trees by taking their roots out of 
soil which at 3 feet deep was during the summer 
months 44°, at 18 inches 50°, at 9 inches 57°, and 
at 6 inches 61°. In a raised border where the soil was 
dark we, ten years ago, found a thermometer indicate 
67 Q 6 inches from the surface ; near by, on the level, it 
was only 59°. The border, it should be added, had a 
slope of 25° or thereby. Such figures show that it is 
throwing away natural heat to make vinery or other 
borders on the ground level, flat, and 8 feet deep. 
Rather would we recommend raised borders with a 
slope to the sun, shallow, and roots close to the sur¬ 
face. Such would cost much less, for excavation would 
be saved, and less soil necessary. Indeed, as we shall 
show further on, it would enable us to keep cooler 
atmospheres, and that means less coal, less labour, and 
fewer insects. 
Earth heat may be considerably increased by cutting 
off the cold under soil from the warmer upper. In our 
district it is labour lost in ordinary seasons to put 
Vegetable Marrows out as is done with success in the 
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No 146 .—Vol. VI., Third Series. 
No. 1802.—VOL. LXIX., OLD SERIES. 
