Decembeb 18, 1908] 



SCIENCE 



867 



Other literature is also cited in this an- 

 cient volume, showing that the investiga- 

 tion of this problem of toxic excreta from 

 plant roots was a live question seventy 

 years ago. 



That crop rotation has great value has 

 been recognized for centuries and nowhere 

 has its importance been more clearly shown 

 than on the oldest soil experiment fields at 

 the University of Illinois, where, after 30 

 years of crop rotation, 58 bushels of corn 

 per acre are stiU produced as an average 

 of the last three crops, while less than 25 

 bushels is the average for the same three 

 years on land where corn has been grown 

 every year for 30 years. The value of crop 

 rotation must be attributed to the assistance 

 thus rendered in retarding the development 

 and multiplication of injurious insects and 

 fungous or bacterial diseases and possibly 

 in avoiding injury from poisonous plant 

 excreta, and to the addition of organic 

 matter, which supplies some nitrogen and 

 hastens the liberation of other essential 

 elements; but the effect of crop rotation is 

 always to reduce, and never to augment, the 

 total supply of mineral plant food in the 

 soil and subsoil. 



The bank must receive deposits as well 

 as honor checks and drafts; the merchant 

 must purchase stock as well as sell goods; 

 and likewise, if we are to remove continu- 

 ally plant food from the soil in large crops, 

 we must give back to the soil with intelli- 

 gence based at least upon the mathematical 

 facts. 



This is truly the age of science, but sci- 

 ence means knowledge; it does not mean 

 theory or hypothesis. One dollar taken 

 from 100 dollars leaves not 100 dollars, but 

 only 99 dollars. This is a scientific fact 

 which no theory or hypothesis can nullify. 

 Likewise when a crop removes 20 pounds 

 of phosphorus from the soil it leaves that 

 soil 20 pounds poorer in phosphorus than 



before the crop was grown. The rotation 

 of crops or the application of salt or some 

 other stimulant may liberate another 20 

 pounds of phosphorus from the soil and 

 thus enable us to grow another crop the next 

 year, and possibly this may be repeated for 

 several or many years, but meanwhile the 

 total supply of phosphorus in the soil is 

 growing smaller and smaller year by year, 

 until ultimately neither crop rotation nor 

 soil stimulants can liberate sufficient phos- 

 phorus from the remaining meager supply 

 to meet the needs of profitable crops. 



It is certainly safe teaching and safe 

 practise to return to the soil as much or 

 more than we remove of such plant-food 

 elements as are contained in the soil in 

 limited amounts when measured by the 

 actual requirements of large crops during 

 one lifetime. 



The average prairie soil of more than 

 20 counties in southern Illinois contains 

 such a limited supply of phosphorus that 

 60 such crops as we raise on our best 

 treated land in the corn belt would require 

 every pound of phosphorus contained in a 

 12-inch stratum of the southern Illinois 

 soil; while two centuries of such crops, if 

 they could be grown, would completely 

 exhaust the soil of its phosphorus content 

 to a depth of 40 inches. 



These are the oldest prairie soils in the 

 state, both agriculturally and geologically. 

 They are also the poorest prairie soils in 

 the state in the total supply of every valu- 

 able plant-food element. In harmony with 

 universal experience, these soils do not im- 

 prove, but continually deteriorate with time 

 and use where no adequate return of plant 

 food is made. These soils are not renewed 

 by deposits from overflow or by the removal 

 of the depleted surface by erosion, and 

 without the positive addition of deficient 

 plant food the future condition of these 

 soils must be the same as the present eon- 



