370 INVESTIGATION BY CULTURE EXPERIMENTS 



been much less effective than where liberal provision is also made 

 for phosphorus and nitrogen. 



The sodium sulfate and the magnesium sulfate produced more 

 than three fourths as much increase as the potassium sulfate, and 

 may have been profitable in themselves, but even if they cost 

 nothing, they would not overcome one half of the deficit standing 

 against the treatment with nitrogen and phosphorus. 



In the last five-year average, potassium pays a profit of $3.93, 

 but meanwhile the deficit on plot n has increased to $10.43, the 

 soil having become so deficient in decaying organic matter that 

 only half a crop can be produced with the amount of potassium 

 liberated from the immense supply still remaining in the soil, 

 even though phosphorus and nitrogen are supplied in available 

 form. The other sulfates have become only half as effective as the 

 potassium salt, and the fact that sodium produces the same effect 

 as magnesium strengthens the common belief that their chief 

 action is to liberate potassium from the insoluble silicates. 



Under these conditions, it ought not to be expected that decaying 

 organic matter of itself would liberate sufficient potassium from the 

 soil for the production of maximum crops. However, any system 

 under which the organic matter content of the soil can be main- 

 tained in optimum amount will necessarily return to the soil in 

 the organic matter most of the potassium taken from the soil. 

 On the other hand, all of the crops taken from plot 2 during the 

 55 years have removed in both grain and straw only 2330 pounds 

 of potassium (based upon Rothamsted analyses), or only one 

 fifteenth as much as was contained at the beginning in 2 million 

 pounds of the fine surface soil. In other words, the total supply of 

 potassium contained in 2 million pounds of the soil would be 

 sufficient for such crops (grain and straw) for 800 years. 



The 55 crops from plot 2 have removed about 650 pounds of 

 phosphorus, and 2 million pounds of the surface soil of plot 3 

 (unfertilized) now contain only 980 pounds of phosphorus soluble 

 in strong nitric or hydrochloric acid, after ignition, and reported 

 by Doctor Bernard Dyer * as total phosphorus. Here we find that 

 the phosphorus actually removed in 55 crops from plot 2 is two 



1 Bulletin 106, Office of Experiment Stations, United States Department of 

 Agriculture. 



