CHAPTER XII 



NITROGEN, PHOSPHORUS AND POTASSIUM IN 

 RELATION TO SOIL FERTILITY 



An Important Controllable Factor. " Sufficient available 

 plant-food elements'' is a fifth positive factor determining soil 

 fertility (Chapter VII). The maintenance of fertility may be 



FIG. 123. Feed the soil and you feed the crop. When this peat soil was supplied with the 

 necessary elements of plant food this splendid crop of corn was the result. 



accomplished in a large measure by maintaining in the soil a good 

 available supply of the important elements (Fig. 123). 



Outside the irrigated sections the source of "water" is the rain- 

 fall; thus, so far as the farmer is concerned, the water problem is 

 mainly a question of conserving and controlling this moisture for 

 crop use. As regards " air, " it is free and abundant and the supply 

 of carbon dioxide contained in it remains practically the same. 

 With drainage and good tillage a lack of air in the soil need never 

 be a cause, either directly or indirectly, of low yields. "Good 

 tilth" can neither be bought nor sold, leached out of the soil or 



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