74 



SOIL CONDITIONS AND PLANT GROWTH 



Table XXXII. — Nitrates in Arable Soils (Top g") at Different Seasons 

 OF THE Year (Russell). 



Roughly speaking each 1000 parts of nitrogen in arable soils may be 

 grouped as follows: soluble in alkali 500, unstable compounds 10, 

 ammonia i, nitrates i to 12. 



The Constitution of the Soil. 



The components of the soil do not form a mere casual mixture. 

 A much more intimate mingling prevails, amounting almost to a loose 

 state of combination, from which the separate substances are only ex- 

 tracted by drastic mechanical means, or gentle chemical treatment. 

 The soil colloids and the calcium carbonate appear to be responsible 

 for the formation of the compound particles, and as soon as they are 

 altered by treatment first with acid and then with alkali the particles 

 fall to pieces and the silt, clay, etc., can be readily separated by sedi- 

 mentation processes. No method has been devised for measuring the 

 size of the compound particles, but that they are large is shown by the 

 following analyses of the same soil, one made after the usual treatment 



