50 ORIGIN AND KINDS OF SOILS CONSIDIiRED. 



on the banks of upland rivers. Mechanically, they are of a texture easily 

 penetrated by all the implements of culture, and not liable to become hard 

 on the surface, and crack after heavy rains followed by drought; chemically, 

 they contain clay, sand, calcareous matter, and humus ; and with reference 

 to vegetation, produce abundant crops in all ordinary seasons, with moderate 

 supplies of manure. 



160. In general, much more depends on the texture of a soil and its capacity 

 for retaining or parting with water and heat, than on its chemical composition. 

 Soils have been found consisting chiefly of clay, others chiefly of calcareous 

 earths ; some, in America, without calcareous earths ; and all producing good 

 crops for a series of years. Nevertheless, it has been found that no soil will 

 remain fertile for many years that does not contain lime in some form natu- 

 rally, or is not liberally supplied with manure containing animal matter, 

 one ingredient of which is lime in a state of phosphate or sulphate. 



167. Subsoils. Next in importance to the texture of a soil, is the nature 

 of the subsoil or substratum on which it rests ; because on the texture and 

 other circumstances of this subsoil depends, in a great measure, the capacity 

 of the surface-soil for retaining or parting with water or heat. The worst 

 subsoils are those of clay kept moist by subterraneous water ; and the best, 

 those of clay resting on gravel or porous rock ; because these retain a useful 

 degree of moisture, and admit of increasing the surface-soil to any depth 

 which may be required for culture. Sandy and gravelly subsoils, with but 

 a thin coating of surface-soil over them, are not sufficiently retentive of 

 moisture ; and chalky subsoils are generally cold. 



168. The surface of soil* has, perhaps, as powerful an influence on their 

 natural fertility as the subsoil ; because on the inclination of the sur- 

 face depends, in a considerable degree, the moisture retained by the soil, 

 and consequently its fitness for the growth of plants. Too steep a slope 

 throws off the rain with too great rapidity, and thus deprives the soil of a 

 sufficient supply of water during dry seasons ; while a flat surface will 

 retard its drainage and occasion loss of heat by evaporation. The colour of 

 the surface of a soil exercises some influence on its heat. A dark-coloured 

 soil will be sooner heated by the rays of the sun than a light-coloured soil ; 

 but it will also part with its heat more rapidly when the sun does not shine. 

 A white soil, such as we sometimes find on chalky or marly subsoils, is the 

 longest of all soils in being warmed, because by all white surfaces the rays 

 of light and heat are reflected, while by all black surfaces they are absorbed. 

 Hence, taking into consideration colour, texture, and aspect, a dark sandy 

 soil, on a surface exposed to the south or south-east, must be the warmest of 

 all soils j and a moist white clay of compact texture, similarly exposed, the 

 coolest. It may be thought that such a soil would be colder on a surface 

 exposed to the north than on a southern exposure ; and this will be the 

 case when the soil is in a dry state, but not when it is supplied with moisture 

 from the subsoil ; because, in the latter case, the cold, produced by evapo- 

 ration, is great in proportion to the warmth of the atmosphere. The aspect 

 is not only of importance with reference to the influence of the sun in 

 warming or cooling the soil, but also as to its effects in maturing the produce 

 which grows on it. 



169. The plants which grow on a soil are the surest indications, to a prac- 

 tical botanist and cultivator, of the actual state of that soil with reference to 

 culture ; though they do not always indicate the improvement of which the 



