Amount of Water Required by Crops. 139 



166. Drainage May Increase the Available Soil-Moisture 



When the subsoil is too close and too fully saturated with 

 water to permit the roots of crops to penetrate it, as is the 

 case where drainage is needed, the roots of plants are forced 

 to develop in so limited an amount of soil that when a dry- 

 ing time comes, and when the demands of the crops for 

 moisture are large because of rapid growth, capillarity 

 from below is not able to supply the moisture as fast as 

 needed, and the result is the zone of soil occupied by the 

 roots becomes so dry that growth is impeded. 



On the other hand, where a field is well drained the roots 

 are extended through much larger volumes of soil ; the lo- 

 cal demands are thus less urgent and the water need not 

 move so far by capillarity before the plant comes in pos- 

 session of it. Under these conditions the moisture of the 

 surface four feet of soil is in close reach of the roots and 

 capillarity may still add to this supply from below. 



167. The Amount of Water Required by Crops. It has 



been determined by careful and extended observations in 

 this country and in Europe that almost any one of the cul- 

 tivated crops withdraws from 300 to 500 tons of water from 

 the soil for each ton of dry matter produced. In Wiscon- 

 sin the amounts of water lost from the soil by evaporation 

 during the growing season and through the plant are given 

 in the table below : 



Table showing the mean amount of water used by various 

 plants in Wisconsin in producing a ton of dry matter. 



