WHAT PLANT FOOD 18 33 



31. Roots Take in Plant Food. When the chemist 

 analyzes a plant, he finds many things besides the carbon 

 which is taken from the air and the water with which it is 

 combined to make starch. He finds compounds of nitrogen, 

 phosphorus, potassium, calcium, and other substances. 

 Now these elements, with the exception of nitrogen, are not 

 to be found in the air in appreciable quantities, and the 

 nitrogen of the air is not in a form in which most plants can 

 use it. Phosphorus and potassium and the other things are 

 to be found in the soil, for soil is simply decayed or disinte- 

 grated rock, and these elements are a part of all rocks. The 

 surface soil which is penetrated by the roots also contains de- 

 caying organic matter, and it is from this that plants obtain 

 their supply of nitrogen. Nitrogen, phosphorus, potassium, 

 and the other elements can not be taken in by the roots of 

 plants and pass through their tissues in a solid state, hence 

 they must be in a soluble form so that they can be carried by 

 the water which is drawn in by the roots. Most of the com- 

 pounds of these elements are not soluble in pure water, but 

 the water in the soil contains some carbon dioxid given off 

 by the roots, and this carbon dioxid is an efficient aid in 

 dissolving the material in the soil particles. These com- 

 pounds of potassium, nitrogen, phosphorus, etc., are known 

 as plant food. 



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, hence only the most 

 elementary statements regarding them will be made here, 

 and no attempt will be made to explain the functions or 

 forms of these elements. Nitrogen, though present in the 

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



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