CHAPTER VI 



CROPS AS FEEDERS ON THE PLANT-FOOD ELEMENTS 



IN THE SOIL 



The elements most commonly studied in relation to crop pro- 

 duction are nitrogen, phosphorus, potassium and calcium. These 

 four will receive careful consideration in further study. All crops 

 are feeders on these important elements and hence they remove 

 from the soil varying amounts. In this respect a soil actually 

 gives up a part of itself in producing crops. 



Soil Particles Are Not Plant Foods. — It was thought at one 

 time that crops consumed or digested soil grains, especially the 

 finest ones.^ This thinking led some of the very early agricultural 

 writers to advocate the reduction of soil to very fine dust so as to 

 enable plants to obtain the amount of soil sufficient for good yields. 

 It was an idea similar to this, no doubt, that gave to the word 

 ^'manure'^ its original meanings, viz., "to till" and "to work 

 by hand." 



Forms in Which Plants Secure Their Elements, — We have 

 learned that plants secure the elements they require in the form of 

 carbon dioxide, water and soluble salts. The soluble salts are 

 derived largely through the decay of organic matter and the 

 mineral soil particles. 



Amotmt of Elements Removed From Soil by Crops. — The 

 accompanying table gives the amounts of nitrogen, phosphorus, 

 potassium and calcimn removed by the more common crops. 

 These amounts are based on crop yields that may be secured 

 on a productive soil. The amounts contained in the grain, 

 stalk and straw are given where called for. These figure repre- 

 sent the averages of many hundreds of crop analyses made in 

 many laboratories. 



It is to be observed that it requires approximately a pound of 

 nitrogen to grow a bushel of oats or barley, and about one and one- 

 half pounds per bushel of corn (Fig. 25.) 



A fifteen-bushel wheat crop would take from the soil about one- 

 half the amounts removed by a thirty-bushel crop. The draft 

 made on the elements by a seventy-eight-bushel corn crop would 



1 Jethro Tull's theory— England, 1730. 



61 



