60 SCIENCE AND SOIL 



plowed soil if it represented the average composition of the solid 

 crust of the earth. Corn is the most important American crop, 

 and the common farm practice is to retain on the farm the corn- 

 stalks (stover), so that the plant food removed in the grain is of 

 the greatest consideration. 



While there is probably no cultivated soil whose composition 

 is exactly the same as the average of the earth's crust, and while 

 100 bushels of corn per acre is about four times the average yield 

 for the United States, nevertheless the data given in Table 8 pre- 

 sent the broadest possible conception of the great problem of soil 

 fertility in relation to permanent agriculture; because all soils 

 are made essentially from the earth's crust, and, if some are richer, 

 others are certainly poorer, than this general average. Likewise 

 the loo-bushel yield of corn is of immediate interest, for it has been 

 produced, and can be produced throughout the corn belt in 

 normal seasons with good farming on the richest and best-treated 

 soils; and the production of large yields is an essential considera- 

 tion, both from the standpoint of profitable farming and for the 

 future support of a rapidly increasing population. 



There are natural agencies which may operate under different 

 conditions to enrich, deplete, or maintain the fertility of the soil. 



In the formation of residual soils from the leaching of disinte- 

 grating and decomposing rock materials, as illustrated in Tables 4 

 and 5, the percentage of a given plant-food element may increase 

 or decrease or remain constant, depending upon whether the 

 compound in which that element occurs is proportionately less 

 or more soluble than the bulk of the material. Thus in the decom- 

 position of gneiss (Table 4), it is evident that the alkali bases, as 

 potassium, magnesium, calcium, and sodium, were leached out 

 much more rapidly than the iron, aluminum, silicon, and phos- 

 phorus; and consequently the per cent of phosphorus doubled and 

 the per cent of potassium and magnesium markedly decreased, 

 while the calcium practically disappeared. On the other hand, in 

 the formation of residual clay from limestone (Table 5), the per 

 cent of phosphorus decreased distinctly and the per cent of calcium 

 very greatly, while most of the elements in the silicate minerals, 

 including potassium and sodium, very markedly increased in per- 

 centage. 



