THE EXPERIMENT STATION 39 



forty distributed on different soil types over the state. On these fields 

 such problems as relate to plant-food requirements, proper systems 

 of crop rotation, drainage, prevention of soil erosion, subsoiling, and 

 dynamiting are being investigated. As an indication of the growth 

 in popularity of these fields it may be said that when this work first 

 started it was difficult to obtain land for the purpose, sometimes even 

 by offering a fair rental price, but after a few of these fields had been 

 established and had begun to demonstrate their value, the popularity 

 of this work grew to such an extent that finally the University came 

 into the position of being able to demand some form of permanent 

 tenure, either by deed or by permanent lease, before it would accept a 

 piece of land for these experimental purposes. 



A SYSTEM OF PERMANENT FERTILITY 



As the results of his investigations accumulated, Dr. Hopkins 

 gradually evolved a philosophy or doctrine of soil fertility intended to 

 apply to the normal soils of Illinois and of similar areas. His domi- 

 nant idea was to provide a system of soil fertility that would result in 

 a permanent agriculture. He recognized that the continual removal 

 of crops from the land must finally lead to soil exhaustion unless the 

 materials taken from the soil be restored. The common practise of 

 applying commercial fertilizers returns to the soil some of the neces- 

 sary elements, but not in amounts proportionate to the quantities re- 

 moved by crops, thus producing the effect of a stimulation of the soil 

 rather than that of a sufficient supply of plant food. By restoring 

 the elements to the soil somewhat in excess of the amounts in which 

 they are removed by cropping, the soil is not only maintained in its 

 natural fertility but is actually built up and made more productive. 

 In order to accomplish this effect the elements must be secured in the 

 most economic form, which ordinarily calls for raw materials rather 

 than those that are treated or manufactured. Thus in ordinary crop- 

 ping systems, raw rock phosphate should be used and the natural bio- 

 logical process of the soil will make the phosphorus available. Nitro- 

 gen should be secured from the air through the growth of legume 

 crops. Potassium is abundant in most normal soils and the problem 

 of supplying this element is usually a matter of liberation from the 

 minerals naturally present in the soil rather than the addition of pot- 

 assium salts. In order that these natural biological processes may 

 function properly, the soil must be kept supplied with an excess of 

 basic material, and for this purpose natural crushed limestone serves 

 best. 



