34 FIELD CROP^ 



ELEMENTS OF PLANT FOOD AND THEIR USES 



32. Nitrogen. In order to understand the nature and 

 uses of the different elements of plant food, we must know 

 something of chemistry and of soils. Only the most ele- 

 mentary statements regarding them will be made here, how- 

 ever, and no attempt will be made to explain the functions or 

 forms of these elements. Nitrogen, though present in the 

 air, cannot be used by plants in the form in which it occurs 

 there. Plants can utihze ''combined" nitrogen only; that 

 is, nitrogen combined with some other element or elements. 

 The bacteria which live on the roots of certain plants have 

 the power of taking nitrogen from the air and changing it 

 into a form in which it is available for the use of the plants 

 on which these bacteria live. When the roots or any portion 

 of such plants decay in the soil, the nitrogen in them is made 

 soluble, and a portion of it becomes available for any plants 

 that may subsequently grow in the soil. The air is, there- 

 fore, one great source of nitrogen. Another is decaying 

 vegetable matter in the soil, which is* acted upon by other bac- 

 teria and changed to the nitrate form, in which plants can 

 use it. These bacteria are able to work only in warm, moist 

 soil which contains plenty of air. They thus supply another 

 argument for good tillage and drainage. 



33. Phosphorus. Phosphorus is a part of nearly all 

 rocks from which soil is formed, though in many soils the 

 quantity is so small that it is soon reduced below the needs 

 of crops. In sour, or acid, soils the supply of phosphorus 

 is largely in an insoluble form that cannot be used by plants. 

 Ordinarily this condition can be corrected by applying lime, 

 but on soils which are very acid, as marshes and other low, 

 wet lands are likel}' to be, the application of lime is not 

 practical and it is necessary to supply phosphorus in an 

 available form in order to grow crops. It is obtained from 

 deposits in the soil in certain sections and from stock- 

 yards where large numbers of animals are slaughtered, as 



