7 6 



SOIL CONDITIONS AND PLANT GROWTH 



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 

 with acid and alkali to break up the compound particles completely, 

 the other made on the untreated soil, where the breaking up is only 

 partial : 



Fine gravel 

 Coarse sand 

 Fine sand 

 Silt 



Fine silt 

 Clay 



8-6 



11*2 



43*2 



I3-3 



36-3 



The existence of these compound particles puts out of the question 

 any complete quantitative interpretation of a mechanical analysis. The 

 properties of a soil are not the sum of the properties of the separate 

 fractions clay, fine silt, silt, etc. because in a normal soil these frac- 

 tions, which we may regard as the ultimate particles, are largely bound 



