8o PLANT PRODUCTS 



the balance of the ingredients than their absolute amounts 

 (see p. 8). Exactly what balance is necessary for any set of 

 circumstances is only approximately known, and the actual 

 cultivator will need to experiment for himself on his own 

 soil. 



Nitrification in Soils. — The air in the soil differs f roin 

 ordinary air in that it contains less oxygen and more carbonic 

 acid, owing to the oxidation of organic matter in the soil by 

 the action of the air. As two volumes of oxygen produce two 

 volumes of carbon dioxide, this change does not effect the per- 

 centage of nitrogen. Some small quantities of nitrogen may 

 be taken out of the air by nitrogen fixing bacteria, and some 

 small quantities of nitrogen may be added by de nitrification. 

 The atmosphere in the soil and the ordinary atmosphere above 

 the surface diffuse into one another. The rate at which 

 this diffusion will take place is lessened by compression, 

 but is fairly independent of the fineness or coarseness of 

 the particles of the soil. The effect of rolling the soil will 

 be to first compress the soil, prevent diffusion taking place, 

 and, therefore, increase the percentage of carbon dioxide. 

 When the percentage of carbon dioxide in the soil-air 

 increases, the amount of carbon dioxide dissolved in the soil 

 water will also increase, since the amount dissolved depends 

 upon the partial pressure of the carbon dioxide. 



As the amount of carbonic acid dissolved in water increases, 

 so the solvent action of the soil water increases at the same 

 time. Rolling, however, by checking the diffusion of fresh 

 air into the soil, lowers the percentage ot oxygen and dis- 

 courages oxidizing bacteria. The ultimate effect of rolling 

 the soil is, therefore, to increase the supply of phosphorus 

 and potassium to the plant, and decrease the supply of 

 nitrogen. Opening up the soil by harrowing produces the 

 opposite effects. These effects are, however, very temporary, 

 since secondary results, due to bacterial life, quickly come 

 into play. In addition to the soil atmosphere considerable 

 quantities of gas are occluded on the surface of the soil 

 particles. Ferric hydrate is particularly powerful in this 

 respect. Peat, and all other forms of organic matter, are 



