Dl{. SCHLEIDEN'S THEORY OF AGRICULTURE. ]51 



unci with few exceptions can modify it in till its peculiarities. 

 Each circumstance must, nevertheless, be carefully distinguished 

 in the inquiiy and its especial influence noted. The matter may 

 be considered under tAvo heads, the physical and chemical pro- 

 perties of the ground. 



A. a. The quantity and state of aggregation first require no- 

 tice. Other points being equal, the most favourable circum- 

 stance in soil is its depth. Some of the virgin forests of Ame- 

 rica, without any manure and with scarcely any labour, will 

 yield abundant crops of tobacco for centuries. The mean depth 

 of soil, how^ever, in our fields is about 6 inches, but the only 

 limit to improvement in this respect is a substratum of hani 

 stone or un tractable water. Some plants require a much deeper 

 soil than others. The white mountain clover, for instance, and 

 sainfoin succeed best in a shallow soil,lucern and common clover 

 in deep layers. Plants with spreading roots never grow so thick 

 as tliose whose roots run deep, and tliese, when compelled to ex- 

 pand their roots in wet ground, require more lOom, consequently 

 seed may be sown thicker in deep than in shallow soils. The 

 deeper the soil is, the more independent of course it is of the 

 subsoil. In the southern States of America, for instance, as also 

 in many parts of Hungary, it is so deep as to make the nature 

 of the substrata quite indifferent. The composition of the soil, 

 however, apart from chemical considerations, is equally im- 

 portant. Deep sands like those about Berlin, which sometimes 

 are carried oflP together with the whole crop which they sustain, 

 and plastic clays which are turned by the heat of sunnner into 

 stone, are the extremes. Between these there are innumerable 

 gradations, varying in the degree of fineness and looseness, of 

 very different value in proportion as they are suflficiently reten- 

 tive to allow of no waste of chemical matters, and loose enougii 

 to permit the free penetration of the rootlets. 



b. The capabilities of soil, as regards temperature, is the next 

 point to be considered. According to Schubler, the temperature 

 depends very little on its chemical constituents, but principally 

 on its colour, and on the moisture it contains. The darker it is, 

 the higher in general its temperature ; consequently vegetable 

 mould has in this respect a very decided influence. Humus, 

 however, is an active agent in point of temperature, not only 

 as regards its colour ; its loose structure continually saturated 

 with oxygen favours extremely the process of decomposition 

 of the organic substances in the soil ; or in other words, a con- 

 tinued combustion which must elicit a quantity of heat equivalent 

 to the results of combustion. How important this is will appear 

 from the following consideration : — If we assume the deptli of 

 very good soil, and rich in humus, to be eiu:ht inches, there will 



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