C2 Agricultural Chemistry. 



surface layer of dry soil is called an "earth mulch" and serves 

 the same purpose as a covering of straw or like material. 



Another important result of tillage is that the soil is thorough- 

 ly exposed to the influence of the air. The nitrification processes 

 are greatly facilitated, with the production of nitrates -and car- 

 bon dioxide. The disintegration and solution of mineral par- 

 ticles will take place from the mechanical and chemical actions 

 brought into play. It will also prevent the formation of such 

 compounds as sulphide of iron, known to be injurious to vege- 

 tation. Oxygen is also necessary for the germination of seeds, 

 and the aeration of soils by tillage is necessary for this important 

 start in the plant's development. 



By means of tile drainage the many chemical reactions going 

 on in a soil are carried down to a greater or less extent into the 

 subsoil; for as the water level is lowered the air enters from 

 above to fill the spaces in the soil. By drainage, the depth to 

 which the roots penetrate, and consequently the extent of their 

 feeding ground, is increased. This helps them to withstand 

 drought. They will not be so easily affected by the extreme dry- 

 ing of the surface of the soil that takes place in times of little 

 rainfall. Roots will not grow in the absence of oxygen and will 

 rot as soon as they reach a permanent water level. 



In a water-logged soil denitrification is active and nitrates 

 present are destroyed, a part of the nitrogen being evolved as 

 elemental nitrogen and returned to the atmosphere. The soil 

 may in this way, suffer a considerable loss of plant food by lack 

 of drainage. 



Losses caused by drainage. The water draining from land 

 always carries with it dissolved matter. The substances chiefly 

 removed by the water will be calcium carbonate, and the nitrates, 

 chlorides and sulphates of calcium and sodium. When heavy 

 rain falls these substances are washed into the subsoil and partly 

 escape by the nearest outfall into the springs, brooks and rivers. 

 The loss of nitrates during a wet season may be very consider- 



