ELEMENTS OF PLANT FOOD 35 



there. Plants can utilize " 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 on the soil. The air is there- 

 fore one great source of nitrogen. Another is decaying vege- 

 table 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 the quantity in many 

 soils is so small that it is soon reduced below the needs of 

 crops. In soils which are sour or acid, the supply of phos- 

 phorus is largely in an insoluble form so that it can not be 

 used by plants. Ordinarily this condition can be corrected 

 by applying lime, but on soils which are very acid, such as 

 marshes and other low, wet lands are likely to be, the appli- 

 cation of lime is not practical and it is necessary to supply 

 phosphorus in an available form in order to grow crops. 

 Phosphorus is obtained from deposits in the soil in certain 

 sections and from stock yards where large numbers of ani- 

 mals are slaughtered, as bones are very rich in this element. 

 The ordinary forms of phosphate fertilizers are raw bone 

 meal, raw rock phosphate, and acid phosphate. 



34. Potassium. Potassium or potash, the latter the form 

 of potassium to which reference is usually made, is the third 

 great element of plant food. Like phosphorus, it is present 



