98 MISSOURI ACfelCULTURAL REPORt. 



through symbiosis, as in the case of the nodule formefs of the clover 

 roots. 



With increase of light green plants made their appearance — green 

 algae on the surface of the sands and lichens on the rocks of the ris- 

 ing mountains, just such lichens and algae as we find today in shaded, 

 rocky ravines, and in sandy places where light is dim and moisture 

 abundant, obtaining the mineral elements of their sustenance from 

 sources inaccessible to the plants we cultivate. 



Having run their allotted course of existence, each generation of 

 these humblest of organisms left behind in the sands of the shore, or 

 of the slowly crumbling surface of the rock, a little additional plant food, 

 wrested from the face of the rocks and sands, and from the inert nitro- 

 gen of the air converted into a form available to plants of a higher 

 order and stored in a substance much more easily soluble than the bare 

 rock, and yet not so readily washed away as are the mineral salts, a 

 substance to which we apply the term humus. 



In due time the higher plants made their appearance, feeding upon 

 the stores provided for them by the soil organisms, and returning these 

 stores to the soil again in their own decay. To some extent, no doubt, 

 these higher plants have increased the available plant food in the soil 

 by direct action of their root fluids upon the particles of sand and clay ; 

 but it seems probable that this action is comparatively insignificant, and 

 that the actual accumulation of both mineral and nitrogenous plant food 

 in the soil in available form, is still largely the work of soil organisms. 

 This view is supported by the fact that the productiveness of a soil is, 

 up to a certain limit, proportionate to its humus content. The washed 

 sands of the seashore, the wind-sifted sands of the dunes, or the clays 

 brought up from deep excavations, support but a meager growth of vege- 

 tation, until manure or fertilizers carrying soluble plant food, both nitro- 

 genous and mineral, are added. 



If the function of humus were only that of a nitrogen purveyor, 

 we should be able to restore the fertility of a soil depleted of hunuis by 

 the addition of fertilizers carrying nitrogen only ; but experience has 

 taugiit us that such soils demand some of the mineral elements of 

 fertility quite as urgently as nitrogen. 



For example : Twelve years ago at the Ohio Experiment Station 

 a soil, depleted of its hunuis by many years of tenant husbandry, was 

 put under a 5-year rotation of corn, oats and wheat, one year each, 

 followed by two years in clover and timothy. One plot in this experi- 

 ment has been fertilized with nitrate of soda, 160 pounds per acre an- 

 nually on each of the cereal crops, or 480 pounds for each 5-year rota- 



