July 9, 1920] 



SCIENCE 



23 



to determine their relation to the requirements 

 of the plant. 



Various chemical methods have been pro- 

 posed from time to time for analyzing the soil. 

 Most of these have been based upon the fan- 

 tastic claim that they determine the " avail- 

 able" plant food in the soil. Dr. Hopkins 

 early realized the futility of such a claim, and 

 concerned himself only with the determination 

 of the total amounts of plant food within the 

 soil. He used chemical analysis as a means of 

 taking an invoice of these substances within 

 the soil, just as the merchant takes an invoice 

 of the goods upon his shelves. "Whether or not 

 the farmer makes the proper use of this ma- 

 terial, depends largely upon him and the kind 

 of farming he carries on, just as it depends 

 upon the business ability of the merchant 

 whether or not his business is successfiil, but 

 in both cases an accurate invoice of stock with 

 which he must work is as absolute a necessity 

 for the fanner as for the business man. 



The chemical analysis of the soils of Illinois, 

 carried on with this idea in mind, soon showed 

 a marked variation in the amounts of the vari- 

 ous essential plant foods present in the soil. 

 The brown silt loam of Campaign county, for 

 example, contains over 9,000 pounds of mag- 

 nesium, 10,000 pounds of calcium, 35,000 

 pounds of potassium, and only 1,000 pounds 

 of phosphorus in the plowed surface soil. As 

 measured by this accurate soil invoice, phos- 

 phorus is the most limited element in the soil, 

 and, as measured by the crop requirements 

 also, it is found that phosphorus is the most 

 limited of plant foods in this typical corn belt 

 soil. There is sufficient calcium present, for 

 example, for the production of a 100 bushel 

 crop of corn for 90 centuries. There is suffi- 

 cient magnesium for 13 centuries, sufficient 

 potassium for 18 centuries, while there is suffi- 

 cient phosphorus for only 62 years, even if it 

 could be utilized by the plant, and provided a 

 maximum crop of 100 bushels of com were pro- 

 duced and all material except the grain is re- 

 turned to the soil. These illustrations are 

 typical, and are very significant in emphasiz- 

 ing the importance of phosphorus to crop pro- 



duction, and indicate clearly its marked defi- 

 ciency in the soil. 



The fifth element, nitrogen, is very impor- 

 tant. It is used by the plant in large quanti- 

 ties, and when purchased upon the markets of 

 the world it is the highest priced of all mate- 

 rials. A hundred pounds of nitrogen are re- 

 quired for the production of 100 bushels of 

 com, and nitrogen at present is selling for 

 $.30 a pound. The maintenance of the ni- 

 trogen supply of the soils is, in the language 

 of Dr. Hopkins, " the most important practical 

 problem confronting the American farmer." 

 It is quite evident that the farmer can not 

 afiord to purchase commercial nitrogen for 

 the production of hie connnon farm crops. A 

 tax on com of $.30 per bushel for this purpose 

 is absolutely prohibitive. The farmer must, 

 therefore, depend upon legume nitrogen which 

 is obtained by legumes such as clover, alfalfa, 

 soybeans, etc., by the aid of symbotic bac- 

 teria from the inexhaustible supply in the air, 

 provided the soil conditions are favorable to 

 their growth and development. It is, there- 

 fore, necessary that a legume occur in the ro- 

 tation and that the legume hay or chaff pro- 

 duced must be carefully conserved and returned 

 to the soil, either as farm manure or green 

 manure crops. It is of importance, also, that 

 the utmost use be made of legume cover crops 

 grown in connection with the production of 

 wheat, and other cereals, and in the develoi)- 

 ment of this use of legume cover crops the re- 

 search work of Dr. Hopkins is particularly 

 outstanding. Sweet clover was a favorite crop 

 with him for this purpose, and he was among 

 the first to call attention to its great possibili- 

 ties. Unfortunately, legumes, so essential for 

 soil improvement, can not be successfully 

 grown on many soils in Illinois, as they now 

 exist, because of the acid soil conditions which 

 frequently absolutely prevent their growth. 



A limestone soil is a rich soil, is an age old 

 truth. Soils which have become famous every- 

 where for their persistent fertility are lime- 

 stone soils. This is true of the soils of the 

 far western United States, the bluegrass re- 

 gions of Kentucky, the valley of the Nile, the 

 black soils of India and Russia. Limestone, 



