152 ACTION OF SOIL OX FOOD OF PLANTS IN MANURE. 



hitherto are quite unexplained, and to many farmers are 

 absohite mysteries. Although we know most accurately 

 the general laws of the growth of plants, as far as tliese 

 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 cuhure-plant, e.g. peas, 

 while the same soil is fruitful 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 

 tliat they do not act in the same way upon the peas ? 

 What causes prevent the latter from appropriating the 

 nutritive substances, which the ground offers to other 

 plants in a perfectly available condition ? FinaUy, how 

 comes it that this very soil, after a few years, will again 

 yield a remunerative crop of peas, although by inter- 

 vening harvests we have rather impoverished than en- 

 liched its store of nutritive substances ; and that peas, 

 when sown among oats, barley, or summer corn, will 

 often yield a higher crop than when they grow alone 

 upon a field, and have not to share with other plants the 

 store of mineral constitueiits ? 



Analogous facts are observed in the cultivation of 

 clover. In many districts, a field, after producing many 

 clover crops, will become almost unfruitful for that plant. 



In such cases, manuring fails in restoring to the field 

 the power of producing clover ; but after several years, 

 during which the same field continues to give remune- 

 rative crops of cereal and tuberous plants, the soil again 

 becomes for a while fruitful for clover. 



For a considerable number of our cultivated plants we 

 have a pretty accurate knowledge of specific manuring 

 agents, i.e. those which have a peculiarly favourable 

 influence upon the majority of fields. Farm-yard manure, 



