152 ACTION OF SOIL ON FOOD OF PLANTS IN MANURE. 



strikingly the action of manure was exhibited, this 

 more abundant crop did not equal that in the experi- 

 ment previously mentioned of the unmanured plot kept 

 for a considerable time under culture. Upon compar- 

 ing the amount of phosphoric acid contained in the two 

 fields, we find that as the sheep pastures, 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 contained 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 

 ingredients, 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 draw their food 

 chiefly from the upper layers of the arable surface soil. 



If the action of the mineral constituents depends 

 upon the sum of effective particles in certain places in 

 the soil, the action rises with the number of particles by 

 which the sum has been increased in these very places. 



A more accurate acquaintance with the composition 

 of arable surface soil, and its relation to the nutritive 

 substances, together with a consideration of the nature 

 and requirements of plants, must gradually lead to a 

 comprehension of many other phenomena in agriculture, 

 which hitherto are quite unexplained, and to many 

 farmers are absolute mysteries. Although we know 

 most accurately the general laws of the growth of 

 plants, as far as these stand in connection with soil, air, 

 and water, yet in many cases it is extremely difficult to 

 discover the causes that render a soil unproductive for 

 one culture-plant, e.g. peas, while the same soil is fruit- 

 ful for other plants which require the same nutritive 

 substances as peas, and often in still greater quantity. 

 If the ground is rich enough in nutritive substances for 

 these other plants, why is it that they do not act in the 

 same way upon the peas ? What causes prevent the 



