140 ACTION OF SOIL OX FOOD OF PLANTS IN MANURE. 



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 re- 

 tained more firmly in the arable top layer. 



But as a sandy soil gives up at harvest more nutritive 

 substances in proportion to what it contains, than a fruit- 

 ful loam, a more speedy exhaustion is the consequence ; 

 its power of production does not last long, and can only 

 be sustained by frequent manvu^ing, to supply the con- 

 stituents 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 opera- 

 tions of tillage becomes less marked. 



The same causes wliich 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 httle or no result, 

 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 mil- 

 lion square decimetres, the absorption numbers express 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). Volker, Henneberg, and Stohmann, 

 in experiments made upon different soils to determine 

 their absorption numbers for ammonia, observed that the 

 earth retained a greater quantity from a concentrated than 

 from a dilute solution of ammonia or salts of ammonia ; 

 whence it follows, as a matter of course, that the am- 

 monia is divided between the water and the soil, and that 



