POWER OF MODIFTINQ THE PHYSICAL CHARACTERS. 297 



parched soil — for the less prolonged duration of the elevated tempera- 

 ture derived from the action of the sun's rays. It is also to be remem- 

 bered, that vegetable soils at least absorb the sun's heat more rapidly 

 tlian the lighter coloured sandy soils, and thus the plants which grow in 

 the former, which is sooner heated, may in reality be exposed to the 

 highest influence of the sun's warmth — for at least as long a period as 

 those which are planted in the latter. 



The only power we possess over these relations of soils to heat, ap- 

 pears to be, that by top-dressing with charcoal, with soot, or with dark- 

 coloured composts, we may render it more capable of rapidly absorbing 

 the sun's heat, and by admixture with sand, more capable of retaining 

 the heat which it has thus obtained. 



Sucn are the most important of the physical properties of soils. Over 

 some of them, the skilful farmer possesses a ready control. He can 

 drain his land, and thus render it cheaper to work and more easy to re- 

 duce to a fine powder. He can plough, subsoil, and otherwise work it 

 well,, and thus can make it more open and porous, more accessible both 

 to air and water. When it is light and peaty, he can lay heavy matter 

 over it — clay, and sand, and lime-stone rubble — and can thus increase 

 its density. He can darken its colour in some localities with peat com- 

 posts, and can thus make it more absorbent of heat and moisture, as well 

 as more retentive of the rain that falls. But here his power ends, and 

 how far any of the changes within his power can be 'prudently attempted 

 will depend upon the expense which, in any given locality, the operation 

 would involve. And even after he has done all which mere mechanical 

 skill can suggest, the soil may still disappoint his hopes, and refuse to 

 yield him remunerating crops of corn. 



" A soil," says Sprengel, " is often neither too heavy nor too light, 

 neither too wet nor too dry, neither too cold nor too warm, neither too 

 fine nor too coarse ; — lies neither too high nor too low, is situated in a 

 propitious climate, is found to consist of a well-proportioned mixture of 

 clayey and sandy particles, contains an average quantity of vegetable 

 matter, and has the benefit of a warm aspect and favouring slope." — 

 I'BodenJcunde, p. 203.] It has all the advantages, in short, which 

 physical condition and climate can give it, and yet it is unproductive. 

 And why ? Because, answers chemical analysis, it is destitute of cer- 

 tain mineral constituents which plants require for their daily food. The 

 physical properties, therefore, are only accessory to the chemical consti- 

 tution. They bring into favourable circumstances, and thus give free 

 scope to the operation, upon the seeds and roots of plants, of those che- 

 mical substances which Nature has kindly placed in most of our soils, or 

 by the lessons of daily experience is teaching the skilful labourer in her 

 fields to supply by art. 



And yet the study of the physical properties of soils is not without its 

 use, even in a theoretical point of view. It shows both the use of the 

 fundamental admixture of sand, clay, and vegetable matter, of which 

 our soils consist, and for what special end all the mechanical labours Oi 

 the husbandman are undertaken, and why they are so necessary. Plants 



13* 



