172 



Nature-Study Agriculture 



How crops 



of food 

 material 

 that t 



Two ways 



the soil of 

 nitrogen 



How lack of 



affects 

 plants 



Supplying 



From this list it would appear that some crops exhaust 

 tne l an d of a given element much faster than other 



crops do ; for example, a beet crop removes a large 



' 

 amount of potassium. Apparently alfalfa and beans, 



rec l u i r i n g more nitrogen than any other crops men- 

 tioned, would very soon exhaust the soil of this 

 valuable food. But it will be remembered that both 

 alfalfa and beans belong to the family of legumes and 

 are able to take nitrogen from the air as well as from 

 the soil. 



Nitrogen. Excepting the legumes, plants take nitro- 

 & en mam ty fr m tne humus of the soil, and not from 



the air. The soil more often lacks nitrogen than any 



. _ T . 



other plant-food element. Nitrogen is a substance 



that readily leaches out of the soil, its compounds being 

 very soluble, like salt or sugar. Not only is nitrogen 

 drained away by water, but under some conditions it 

 escapes in large quantities into the air as a gas. Ex- 

 perimenters found that in a field growing grain con- 

 tinually, six times as much nitrogen was lost from the 

 soil each year as was required to make the crop. Twenty- 

 five pounds were removed with the grain, and one hun- 

 dred and fifty pounds escaped into the air and in the 

 drainage. 



Where the supply of nitrogen is low, plants will be of 

 a pale green color and their growth will be poor. More 

 t ] ian anv other plant-food element, nitrogen promotes 

 the growth of leaves and stems. Nitrogen may be sup- 

 plied to the soil either by raising a leguminous crop such 

 as clover or field peas and plowing it in, or by adding 

 a fertilizer containing nitrogen. 



