of increasing the growth of plants, always contain one or att 

 of these three substances, potash, phosphoric acid and nitrogen. 

 A luxuriant plant growth is not caused merely by the 

 manure or fertilizer as such, but by the potash, phos- 

 phoric acid and nitrogen contained in it. It must be kept 

 clearly in mind that plants need all three of these plant 

 foods ; no one of them can do the work of the other two, 

 nor any two the work of the other one. However rich a 

 soil may be in any one of them, or in any two, if one is 

 totally lacking, no plant can grow ; if one of these sub- 

 stances is present, say only in sufficient quantity for a 

 quarter crop, only a quarter crop will be grown, even if 

 the other two substances be present in great excess of the 

 needs of a full crop. 



Once the nature of plant food is well understood, plot 

 experiments become simple. The idea is to determine 

 which of the three elements of plant food the soil needs. 

 This the experimenter learns by "putting the question" to 

 the soil itself. He applies different fertilizing materials on 

 different plots, and he gets his answer in the quantity and 

 quality of crop the plots produce. 



Part III includes a description of the Experiment 

 Farm at Southern Pines, North Carolina. The State 

 Horticultural Society of North Carolina is conducting 

 there an elaborate and extensive series of experiments with 

 fertilizers to determine the needs of different horticultural 

 Crops in the field. This enterprise is managed with great 

 thoroughness and the experiments are directed by trained 



