THE SOIL 27 



ure and weigh it again in one week, and in two weeks. What has be- 

 come of the water? 



5. Fill two glass jars with moist, well-packed soil. Smooth the surface 

 of one and let it form a crust. Keep the second covered to the depth 

 of an inch with loose soil. Weigh. At the end of a week, weigh again 

 and examine the condition of the soil in each. 



SOIL VENTILATION 



Uses of Air. As has been already said, it is as important to 

 admit air to the soil as to remove surplus water. Few plants can 

 flourish unless air is admitted to their roots. The air is of benefit 

 in several ways. Plants need the oxygen to breathe; the carbon 

 di ox'ide works in the soil, preparing plant food. 



Under- ventilated Soils. There is a great dif- 

 ference in the natural ventilation, or airing, of 

 soils. Stiff clay soils are poorly ventilated. 

 They are so compact, or close, that they do 

 not admit the air freely; in wet seasons or 

 regions the water stands and chokes the air SOIL PARTICLES 

 spaces. Such soils need drainage. As water AND SPACES 

 is drawn out, air is drawn in by suction. Drainage allows roots to 

 penetrate deeper and range more freely ; these roots open channels for 

 water and air. Tillage, or cultivation, also helps ventilate the soil. 



Over-ventilated Soils. On the other hand, coarse, sandy soils 

 are often too thoroughly ventilated. Air and water pass freely 

 through and carry away plant food; because of this loss of plant 

 food, sandy land is often poor. Such a soil needs to be kept firm, 

 except for a soil- mulch to protect it against loss of moisture by 

 evaporation. 



