Soih 69 



PLANT-FOOD IN THE SOIL 



The method by which plants secure their food from 

 the soil has been known less than a century. From the 

 time of the ancient Greeks and Romans down to the 

 beginning of the nineteenth century, investigators sought 

 to find some one substance in the soil that was the real 

 food of plants. At different times it was thought to be 

 fire, water, niter, oil, and many other materials. During 

 this period all plant-food was supposed to come from the 

 soil ; it was not known that the greater part of it comes 

 from the air. 



Of the ten elements required by plants, seven, in ad- 

 dition to those obtained from water, come from the soil. 

 These are potassium, phosphorus, calcium, magnesium, 

 iron, sulfur, and nitrogen. A number of non-essential 

 elements, including sodium, chlorine, and silicon, are also 

 taken up by most plants. All crops require the same 

 elements for their growth, although they do not use them 

 in the same proportion. Sugar-beets and potatoes use 

 relatively large quantities of potassium, the grain crops 

 require considerable phosphorus, while alfalfa and clover 

 use more calcium. 



Soils are made up largely of insoluble material of no 

 food value to plants. The amount of actual plant-food 

 in the soil is comparatively small, but since plants do not 

 use large quantities of this food, the supply of most of the 

 elements is sufficient for crop production. Only a small 

 part of the total plant-food of the soil is available during 

 any one year. Roots penetrate every part of the surface 

 soil, but they can absorb only the material that is in solu- 



