SOIL. 3 



should not be understood that they are usually taken 

 directly from it by the plant, nor do they enter the 

 plant in the condition of a simple element. Some 

 plants that belong to the pea family are able, under 

 certain conditions, to assimilate more or less nitrogen 

 directly from the air, but the other three elements find 

 their way into the plants through the soil. Because 

 the above named elements make up so much of the 

 weight, it should not be inferred that the other ten or 

 eleven elements that are derived from the soil are of 

 only minor importance. Although their combined 

 weight is only from one to five per cent., the absence 

 of any one of these may materially interfere with the 

 growth of the plant. 



AYHY LAND BECOMES POOR. 



It will be clear from what has been said that if crops 

 are repeatedly removed from the soil, some element or 

 elements will become exhausted, or, as we ordinarily 

 say, the laud becomes poor. 



Different crops do not remove an equal amount of 

 the elements, and even different specimens of the 

 same crop vary considerably in the amount of any one 

 element that they take up from the soil. Some plants 

 take a great deal of nitrogen from the soil and re- 

 turn only a portion of it ; others take only a small 

 amount of nitrogen from the soil and give much back ; 

 the former make land poorer, the latter make it richer 

 in nitrogen. The same is true in regard to plants 

 using potash and phosphoric acid. When a piece of 

 land becomes tired of one crop, it is often able to pro- 

 duce some other in fair quantity. To keep soil in 

 such condition that it will produce a crop in paying 

 quantities, we must keep the before named fifteen ele- 

 ments present in sufficient quantity we must fertilize. 



