52 DRAINAGE AND VENTILATION 



distance between the drains will vary with the closeness of the soil 

 texture and the depth at which they are laid. Their distance apart 

 varies from fifteen to one hundred feet. In Illinois, in rich black 

 loam lands, the drains are usually from one hundred to one hun- 

 dred and fifty feet apart. The size of the tile should not be smaller 

 than three inches. Every drain should be placed on a true grade, 

 and its outlet should end above the water line in order to prevent 

 clogging by silt. The average cost of tiling in many places does 

 not exceed $11.50 per acre, while the productive value of the 

 land will be nearly doubled. 



Soil Ventilation. The presence of air in the soil is just as neces- 

 sary to the life of ordinary plants as water. In fact but few plants 

 can flourish whenever the free oxygen of the air is excluded from 

 them. It has been proved by experiments that seed will not ger- 

 minate when oxygen is absent. If the soil is flooded, root breath- 

 ing is checked and the plants are drowned just as effectively as an 

 animal would be under water, because not enough free oxygen can 

 reach them. Oxygen is needed in the soil by the germs which form 

 the nitrates so essential to the plant. Likewise it is needed to 

 prevent the destruction of the nitrates after they have once been 

 formed. 



Underventilated Soils. When we examine soils of different 

 kinds, we find that they vary greatly in their natural ventilation. 

 Compact soils like clay are poorly ventilated, and in wet seasons 

 water fills and chokes all their air spaces. Good drainage and 

 proper tillage are the only available remedies suggested. We can 

 easily prove the value of ventilation by planting a few garden seed 

 in two samples of the same soil arranged as follows: In a tomato 

 can of common soil place a few tomato or cabbage seed. Make a 

 number of holes in the bottom of a second can, fill the can with soil, 

 and plant seed as before. Keep the top soil loose in this can, but 

 do not disturb the soil in the other can. Watch for several days 

 and note developments. In a little while you will find that the 

 plants in the first can have stopped growing. This is because 

 the plant does not receive proper ventilation. Cultivation pre- 

 vents the soil from baking and hardening and also helps to give it 

 better ventilation. 



Another thing which contributes towards soil ventilation is the 



