THE SURFACE SOIL AND MINERAL MATTERS. 151 



Dr. ZoL'Uer found by analysis that this lield contained, 

 per hectare, to a depth of G inches, only 727 kilogrammes 

 ( = 14 cwt.) of phosphoric acid. 



The plot manured with phosphoric acid produced six 

 times more corn and five times more straw than the un- 

 manured plot. It will be observed that, however strik- 

 ingly the action of manure was exhibited, this more 

 abmidant crop did not equal that in the experiment pre- 

 viously mentioned of the unmauured plot kept for a con- 

 siderable time imder culture. Upon comparing the 

 amount of phosphoric acid contained in the two fields, 

 we find that as the sheep pasture, to the depth of 6 inches, 

 contained only half as much as the other (tilled but 

 unmanured), the dressing with superphosphate was only 

 just sufficient to make the sheep-meadow, to the depth of 

 8 or 10 centimetres ( = 3 to 4 inches), equal to the other 

 unmanured plot, in respect of the phosphoric acid con- 

 tained in it. 



These considerations explain how it is that by the 

 absorption of nutritive substances in the upper layers of 

 the soil a supply of these constituents or manuring ingre- 

 dients, small in comparison to the total store in the 

 ground, exercises so remarkable an action in the increase 

 of produce, in the case of plants which di'aw their food 

 chiefly from the upper layers of the arable surface soil. 



If the action of the ndneral constituents depends upon 

 the sum of effective particles in certain places in the soil, 

 the action rises with the number of particles by wliicli 

 the sum has been increased in these very places. 



A more accurate acquaintance with the composition of 

 arable surf^ice soil, and its relation to the nutritive sub- 

 stances, together with a consideration of the nature and 

 requirements of plants, must gradually lead to a compre- 

 hension of many other phenomena in agriculture, wliicli 



