170 



Twld may be good for a number of years, but then it begins to grow less 



aad less, and we may add, poorer and poorer in quality. What is the 



5r«ason of this ? It is probable that the combinations of phosphoric acid are 



/5i«ariy exhausted ; these were not so abundant as many other substances 



. at the •commencement, (see Table I., ) but more of them in proportion, than 



; aaything else, has been taken away. Second, suppose that the farmer has 



'sold all his grain, but has been careful to return the straw as manure.; 



5ie does not see why the land should run down, and in fact it does not so 



•quickly as in the first case ; still, after a time, it also begins to show 



:5narks of exhaustion. The different classes of the ash of plants given 



; ■above explains this at once. In the straw, as will be seen, he has returned 



, 'chiefly silica to the soil, whereas it is chiefly phosphoric acid which he has 



Vbeen selling off in the grain. 



The same thing would result in the exclusive cultivation of any other 

 _^Tain, with this difference only : that some other element would be ex- 

 iiausted. Some soils bear this severe treatment longer than others ; but 

 ■sooner or later they must succumb to such management. If turnips or 

 •potatoes alone were cultivated, instead of the phosphoric acid becoming 

 ^exhausted, it would be the potash and soda, and whatever amount of 

 phosphoric acid might be applied, the difficulty would still exist unrelieved. 



JThe effect of cropping, as deduced from the above facts, can be none 

 folhei- than to impoverish the soil. From Table I., it will be seen that 

 silica, iron, alumina, and organic matter, form a large per cent, of the 

 snaterial of the soil, nearly 90 lbs. in 100. There is no fear then of 

 ^exhansting these materials, as alumina and iron are not usually constit- 

 xieTits of plants. The danger of exhaustion is with reference to the ma- 

 terials or elements so abundant in seeds and in roots, which are found in 

 .the 5 or 10 lbs. which remain to make up the 100. The quantities of these 

 «iibstances are usually small in the soil to commence with, and, as they 

 ■are fhe xery ones constantly being carried away, are soonest exhausted. 



'Whenever one of these important elements is gone, or reduced to a 

 •ismall quantity, the crop will begin to fail and become shriveled in its 

 kernel, to weigh less per bushel and to be of an inferior quality. 



It has become a proverb in some parts of England, and has been imported 

 to this country, that 'lime enriches the fathers and impoverishes the sons.' 



But from the above facts it is easy to explain the proverb. It many 

 Itimes happens in cultivated fields, that the lime becomes first exhausted, 

 «3!f in other instances, the lime and sulphuric acid have both been wholly, 



