188 



NITROGEN, PHOSPHORUS AND POTASSIUM 



added to it. It is a soil condition that good soil management 

 maintains and poor management destroys. " Helpful soil organ- 

 isms" perpetuate themselves, and they remain in the soil so long- 

 as the farmer maintains soil conditions favorable to them. But 

 as regards the " plant-food elements," a virgin soil may become 

 depleted, and crops consequently fail. Moreover, some soils are 

 unproductive because they especially lack some one essential 

 element (Chapter VI). The only way to restore a depleted supply 



FIG. 124. Sixty-four bushels of potatoes for nine dollars. When a necessary fertilizer 

 was added to this soil a 64 per cent increase in yield was obtained. 



of elements, or to nullify the effect of a lack of any one or two ele- 

 ments, is to add plant-food material to the land (Fig. 124). It is 

 entirely possible for the farmer to add fertilizing elements to the 

 soil, and to maintain in the soil a sufficient available supply, so 

 that his efforts concerning moisture conservation, aeration, 

 tillage, etc., shall not be in vain. Compare Figures 125 and 126. 



Of the elements essential to plant growth, four are much dis- 

 cussed in relation to crop production, viz., nitrogen, phosphorus, 

 potassium, and calcium (lime). 1 Since the liming of soils is so 



1 Commonly, the word "lime" is used instead of calcium, though it is the 

 oxide of calcium (CaO). 



