PLANT-FOOD MATERIALS IN SOILS 93 



These analyses show in some cases more, and in others 

 less, of the various constituents below the surface foot, 

 with the exception of nitrogen, which is always less in the 

 subsoil. The fact that the greater part of the roots of 

 most plants is in the surface soil makes the draft greater 

 on that layer, but the total volume to a depth of four feet, 

 or even more, may be considered to be the feeding ground 

 of crops. 



107. Upward movement of plant-food materials. — There 

 is another way in which the soil to a considerable depth 

 may contribute to the nourishment of crops. This is by 

 furnishing plant-food materials that are carried upward 

 by ascending currents of moisture, or that are absorbed by 

 roots from the lower depths and deposited near the surface 

 when the plants die. To what extent the upward movement 

 due to moisture is operative is something of a question ; 

 in humid regions probably very slightly, in semi-arid and 

 arid regions it is doubtless of considerable moment, as indi- 

 cated by the existence of alkali crusts. 



108. Plant nutrients compose a small part of the soil. — 

 Another point brought out by. Table 17 is the very small 

 proportion of the soil that is represented by plant-food ma- 

 terials. For instance, the sum of all of the nitrogen, phos- 

 phoric acid, lime, magnesia and potash is not much more 

 than two percent of the total weight of the soil, and it would 

 be easy to find analyses that would show much less. Some 

 of the very important substances are present only in tenths 

 or even hundredths of a percent. The great bulk of the 

 soil contributes nothing to plant growth other than to furnish 

 mechanical support and to store air and water for the use 

 of roots. 



109. Relation of composition to productiveness. — The 

 productiveness of a soil is not necessarily directly propor- 

 tional to the quantity of plant-food materials that it con- 



