504 INVESTIGATION BY CULTURE EXPERIMENTS 



determine the conditions under which the supply of nitrogen will 

 be maintained. Of course it requires no new investigations to show 

 that sufficiently large applications of manure will maintain the 

 supply of nitrogen, whether the crops are rotated or grown continu- 

 ously, as at Rothamsted, with wheat, barley, or mangels. 



NOTE. In passing from this extended consideration of the field experi- 

 ments conducted in various parts of the United States, the reader will per- 

 haps be interested to note the following correspondence in relation to the 

 application of science to practical farming: 



" GILMAN, ILLINOIS, November 23, 1909. 



" DEAR DOCTOR HOPKINS : Am sending you a few comparative figures, 

 which I trust may interest you. I have no doubt you can see more in them 

 than I can, but I see much that gives encouragement for the future: 



" COMPARATIVE YIELDS OF CORN FROM TREATED AND UNTREATED 

 LAND: 1909 CROP (Bushels per Acre) 



Corn on clover sod; land cultivated 30 years, with no manure and no 



pasture: Untreated 



Same kind of land: Treated with | ton raw rock phosphate 

 Same: Treated with k ton phosphate and 3 tons limestone 



Second-year corn after clover : Untreated 



Same: Treated with i ton per acre of phosphate .... 



65.1 bushels 

 81.9 bushels 

 84.1 bushels 



70.0 bushels 

 77.6 bushels 



" On the clover sod there seems to be about a normal difference in yield. 

 Much of the last field, including the check strip, has had two lo-ton appli- 

 cations of manure in 6 years. 



" Kind regards, 



(Signed) " F. I. MANN." 



"UNIVERSITY OF ILLINOIS, URBANA, December i, 1909. 



" MR. F. I. MANN, Oilman, Illinois. 



" DEAR SIR : I thank you for your letter of November 23, giving the 

 1909 results on your 200 acres of corn from the methods of soil improvement 

 which you have been practicing for several years. I note that the cumula- 

 tive effect of the system is apparently becoming evident. Where phosphorus 

 produces a ton more clover per acre (as you reported last year), the increased 

 clover and added phosphorus must increase the following corn crop. 



"Two of our old plots here at the University yielded exactly the same 

 (64 bushels) as an average of three corn crops (1895-1897) before we began 

 applying limestone and phosphorus to one of them. This year the untreated 

 clover sod produced 32.8 bushels, and the treated land yielded 77.6 bushels, 

 per acre. Where limestone without phosphorus was applied, the yield was 

 38 bushels, and, with limestone, phosphorus, and potassium, 83.7 bushels. 



" Very truly yours, 



(Signed) " CYRIL G. HOPKINS." 



