168 FAKM-YAKD MANURE. 



Oat crops will therefore exhaust the soil the most 

 speedily ; after 12f years the harvest will sink to three- 

 fourths of the original amount. 



JSTone of the causes tending to diminish or increase 

 the crops have any influence on this law of exhaustion 

 of the soil by cultivation. Whenever the stock of nu- 

 triment has been lowered to a certain point, the ground 

 ceases to be productive, in an agricultural sense, for 

 cultivated plants. 



For every cultivated plant such a law exists. This 

 state of exhaustion will inevitably take place, even 

 though only a single one of the various mineral con- 

 stituents required for the nutrition of the plants has 

 been withdrawn from the soil by a succession of crops ; 

 for the one constituent which fails or is deficient ren- 

 ders all the rest ineffective. With each crop, each 

 plant, or portion of a plant, taken away from a field, 

 the soil loses part of the conditions of its fertility, that 

 is, after a course of years of cultivation it loses the 

 power of again producing this crop, plant, or part of a 

 plant. A thousand grains of corn require from the soil 

 a thousand times as much phosphoric acid as one 

 grain. ; and a thousand straws demand a thousand 

 times as much silicic acid as one straw. When, there- 

 fore, the soil is deficient in the thousandth part of 

 phosphoric or silicic acid, the thousandth grain or 

 the thousandth straw will not be formed. If a single 

 stalk of corn is taken away from a field, the conse- 

 quence is that the field no longer produces one straw 

 in its room. 



Hence it follows that a hectare of ground, contain- 

 ing 25,000 kilogrammes of the ash-constituents of 

 wheat, uniformly distributed, and presented to the 

 roots of the plants in a perfectly available condition, 

 can, up to a certain point, continue to give in succes- 

 sion remunerative crops of various cereal plants, with- 

 out receiving any restoration of the mineral constitu- 

 ents taken away in the corn and straw, provided that 

 the uniform mixture of the soil be maintained by care- 

 ful ploughing and other suitable means. The succes- 



