CHAPTER VII 



PLANT FOOD IN THE SOIL 



There are four things that render land barren : the want of juices necessary 

 for the nourishment of plants ; the having bad juices that tend to hurt and 

 destroy plants ; the being so solid as to hinder the tender roots from extend- 

 ing themselves in search of their food ; and the being so porous and spongy 

 as to be easily hurt by rain, heat, or cold. VERGIL 



76. Three important elements of plant food. The three ele- 

 ments of plant food which must be given most attention by the 

 farmer are nitrogen, phosphorus, and potassium. These are im- 

 portant because they are most likely to be lacking in available 

 form in a soil which has been cropped for some time, and because 

 plants use them in somewhat larger quantities than they use 

 the other soil elements except calcium. These are, therefore, 

 the three elements which are commonly used in commercial fer- 

 tilizers, and the ones most commonly mentioned in discussions 

 of soil fertility. It is to these elements that the farmer must 

 give most careful attention in maintaining soil fertility. 



77. Available and unavailable plant food. Plants take their 

 food from the soil, dissolved in the soil water. As the roots 

 penetrate the soil and take in water they also take in plant food 

 in solution. 



Plant food-material which will dissolve in water and may be 

 taken into the plant is known as available plant food, and the 

 food that is insoluble or in solutions which cannot be taken up 

 is known as unavailable. As most of the plant food in the soil 



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