soil. SESSION. 



149 



AMOUNT OF WATER-SOLUBLE PLANT FOOD ELEMENTS IN SOIL. 



We have tried to fix in your minds the very important fact that 

 within the surface four feet of every well managed field there are car- 

 ried, in the aggregate and on the average, something more than 120 to 

 160 tons of the essential plant food elements. We desire now to empha- 

 size another even more important fact, namely, the great bulk of these 

 large amounts of plant food elements is locked up in the form of sub- 

 stances which dissolve with extreme difficulty in water, and, while 

 it is true that none of- the plant food materials become available to 

 crops until they are dissolved in the soil moisture, it is nevertheless 

 fortunate that the great reservoir of plant food materials is filled with 

 form so difficulty soluble. Wcve this not so, the percolation of rain 

 through the soil would carry away so completely the plant food materials 

 that even in regions of abundant rainfall the land would be deserts 

 and only in the arid regions, where leaching cannot occur and where 

 irrigation must be resorted to, could crops be grown without the heaviest 

 and ever repeated applications of fertilizers. 



The smallest cube represented on the chart shows, relatively, the 

 aggregate volume of the essential plant food elements which were 

 recovered from 8 soil types, 4 of which were good and 4 poor, by leach- 

 ing quickly through thin layers of them 55 times their dry weight of pure 

 water, a layer of water which would cover the surface to a depth of 

 some 16 to 18 inches. This cube of plant food soluble in water derived 

 from one cubic foot of surface soil is a little less than .1 of an inch 

 on an edge; constitutes .1148 per cent of the dry soil, thus making 

 a weight of a little more than half an ounce per foot of surface soil, and 

 3,660 pounds or 1.83 tons per acre. The amounts of the respective plant 

 food elements so recovered are given in the next table. 



Plant food elements removed from the surface foot of 8 soils with 

 pure water: 



Amount in pounds per acre of surface foot. 



Good soil. 

 Poor soil.. 



Number of crops of wheat required to remove the above amounts. 



22.7 

 20.1 



293.8 

 80.4 



286.3 

 101.0 



1.1 

 .5 



17.3 



7.2 



241.6 

 118.6 



