368 HOW CROPS FEED. 



nourishment of crops, are often the chief factors of fer< 

 tility on account of their indirect action, or because they 

 supply some necessary physical conditions. Thus humus 

 is not in any way essential to the growth of agricultural 

 plants, for plants have been raised to full perfection with- 

 out it ; yet in the soil it has immense value practically, 

 since among other reasons it stores and supplies water and 

 assimilable nitrogen. Again, gravel may not be in any 

 sense nutritious, yet because it acts as a reservoir of heat 

 and promotes drainage it may be one of the most import- 

 ant components of a soil. 



What the Soil must Supply, — It is not sufficient that 

 the soil contain an adequate amount of the several ash-in- 

 gredients of the plant and of nitrogen, but it must be able 

 to give these over to the plant in due quantity and pro- 

 portion. The chemist could without diffi'culty compound 

 an artificial soil that should include every element of 

 plant-food in abundance, and yet be perfectly sterile. The 

 potash of feldspar, the pl)osphoric acid of massive apatite, 

 the nitrogen of peat, are nearly innutritions for crops on 

 account of their immobility — because they are locked up 

 in insoluble combinations. 



Indications of Chemical Analysis. — The analyses by 

 Baumhauer of soils from the Ziuder Zee, p. 362, give in a 

 single statement their ultimate composition. We are in- 

 formed how much phosphoric aci<l, potash, magnesia, etc., 

 exist in the soil, but get from the analysis no clue to the 

 amount of any of these substances which is at the dispo- 

 sition of the present crop. Experience demonstrates the 

 productiveness of the soil, and experien(>e also shows that 

 a soil of such composition is fertile ; but the analj^sis does 

 not necessarily give proof of the f ict. A nearer approach 

 to providing the data for estimat ing what a soil may sup- 

 ply to crops, is made by ascertaining what it will yield to 

 acids. 



