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 withont difficulty compound 
an artificial soil that should include every element of 
plant-food in abundances, and yet be perfectly sterile. The 
potash of feldspar, the phosphoric acid of massive apatite, 
the nitrogen of peat, are nearly innutritious 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 Zuider Zee, p. 362, give in a 
single statement their ultimate composition. We are in- 
formed how much phosphoric acid, 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 experience also shows that 
a soil of such compusition is fertile; but the analysis does 
not necessarily give proof of the fact. A nearer approach 
to providing the data for estimating what a soil may sup- 
ply to crops, is made by ascertaining what it will yield to 
acids, 
