16 



TABLE T. The weight and aretage composition of ordinary crops, fyc. Continued. 



From the above table we can judge of the quantity of the different 

 soil constituents which various crops absorb from an acre of ground, 

 and how certain plants demand some one particular ingredient more 

 than others. In general, we may say that the cereal crops apparently 

 possess a capacity for feeding on silicates not enjoyed by other crops, 

 and contain a less amount of nitrogen than either the root or legumi- 

 nous crops ; nevertheless, they respond the most readily to nitrogenous 

 manures. The amount of phosphoric acid is the most constant of all 

 the constituents of crops, being concentrated in the grain. The root 

 crops contain a large amount of potash, and are the most exhausting 

 to the soil in consequence ; they take up more nitrogen than do the 

 cereals, besides other ash constituents, as phosphoric acid. The legu- 

 minous crops contain about twice as much nitrogen as do the cereals, 

 and the potash and lime occur in large proportions. Silica is nearly 

 absent. They respond most readily to potash manures. 



The growth of forests is far less exhausting to a soil than are most 

 ordinary farm crops, especially where the leaves from the trees are left 

 to manure the ground by their decay. 



PERMANENT FERTILITY. 



The investigations of Messrs. Lawes and Gilbert* in regard to the ex- 

 haustion of land by the same crops grown year after year on the same 

 field, left unmanured, which they have been carrying on at Kothamsted, 

 England, for the past forty years, lead them to conclude that all lands 

 left an manured for a longer or shorter number of years have a certain 



*.Tonrn. Agric. Soc. 



