ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



with more or less clay or loam ; and as silicious sand 

 has a very limited power of absorbing potash and the 

 other mineral constituents of plants, the ingredients of 

 the supplied manure, which have become soluble, 

 spread sooner and penetrate deeper into a sandy soil, 

 which also gives back comparatively more of them than 

 any other soil. In many cases, therefore, a stiff loam 

 may be improved by sand ; as, on the other hand, the 

 addition of loam to a sandy soil will cause the nutritive 

 substances, supplied by the manure, to remain nearer 

 the surface or to be retained more firmly in the arable 

 top layer. 



But as a sandy soil gives up at harvest more nutri- 

 tive substances in proportion to what it contains, than 

 a fruitful loam, a more speedy exhaustion is the conse- 

 quence ; its power of production does not last long, 

 and can only be sustained by frequent manuring, to 

 supply the constituents which have been removed. 

 Exactly in the same degree, as the manure acts more 

 beneficially in restoring the productive power, the 

 effect of the mechanical operations of tillage becomes 

 less marked. 



The same causes which restore to an exhausted loam 

 a large portion of its lost productive power, if the land 

 is but sufficiently broken up by the plough, are at work 

 in a sandy soil also ; but they produce little or no re- 

 sult, because the sand is deficient in those substances 

 which the action of the plough is intended to render 

 available. 



As the surface of a hectare (2-J acres) represents 1 

 million square decimetres, the absorption numbers ex- 

 press the number of kilogrammes of potash, phosphoric 

 acid, and silicic acid, which, when applied on a field, 

 will spread from the surface downwards to a depth of 

 10 centimetres (about 4 inches). Yolker, Henneberg, 

 and Stohmann, in experiments made upon different 

 soils to determine their absorption numbers for am- 

 monia, observed that the earth retained a greater 

 quantity from a concentrated than from a dilute solu- 

 tion of ammonia or salts of ammonia ; whence it fol- 



