454 INVESTIGATION BY CULTURE EXPERIMENTS 



ing crops as needed. In other words, the crops growing on brown 

 silt loam soils are enabled to withstand drouths that would pro- 

 duce very severe damage on such a soil as the lower Illinoisan 

 gray silt loam on tight clay. Of course even the brown silt loam 

 becomes much less absorbent and less retentive of moisture where 

 the surface soil is allowed to become deficient in humus. 



As a general average (the late Wisconsin being disregarded) 

 the brown silt loams contain in the surface soil of an acre (2 mil- 

 lion pounds) about 4800 pounds of nitrogen, 1200 pounds of phos- 

 phorus, and 34,000 pounds of potassium, amounts which, if they 

 could be drawn upon at will, would furnish the nitrogen for 100 

 bushels of corn (grain only) every year for 48 years, the phosphorus 

 for 70 years, or the potassium for 1790 years. For four tons per 

 acre of clover hay each year, the nitrogen, if drawn only from the 

 surface soil, would be sufficient for 30 years, the phosphorus for 

 60 years, and the potassium for 280 years. 



These data are for very large crops, and take into account only 

 the plant food in the surface soil to a depth of 6f inches, but these 

 crops are not loo large to try to raise, and the fertility of the surface 

 soil must be maintained if we are to maintain a permanent, profit- 

 able agriculture. We may reduce the crop yields to the lowest 

 limit of profit on land valued at $150 to $200 an acre, but still the 

 absolute limit in years is short for the nitrogen and the phosphorus 

 in this most common prairie soil of the corn belt; and, if such crops 

 of corn and clover as are mentioned above had been removed from 

 this land from the time Columbus discovered America until now, 

 every pound of phosphorus contained in the soil to a depth of four 

 feet would have been required for the crops grown. 



So far as the author has been able to learn, the oldest soil experi- 

 ment field in the United States with an authentic record of its 

 origin and with a present continuation of the experiments origi- 

 nally inaugurated is on the campus of the University of Illinois, 

 or rather it is surrounded by the University campus. In the 

 biennial report for 1879 and 1880, on page 232, and under date of 

 March 10, 1880, is the following: 



"The Farm Committee then submitted the following report: 



"To the Hon. Board of Trustees of the Illinois Industrial University: 



