54 THE PEINCIPLES OF AGRICULTURE 



and in passing down it gives up some of its 

 heat, and the soil temperature is thereby raised. 

 In the summer the rain is the cooler, and the 

 soil parts with some of its heat. On lands 

 which have been thoroughly under- drained, crops 

 are far better able to withstand drought than 

 those on land which needs drainage. 



78. Few cultivated plants can thrive with 

 their roots in free water. When the free water 

 is near the surface, it is injurious in several 

 ways : it limits the feeding space ; it makes the 

 soil cold in spring ; it occupies the space which 

 should be filled with air ; it causes plant-food to 

 be locked up ; it dilutes the plant -food in solu- 

 tion; it prevents the action of micro-organisms; 

 it causes the rainfall to be carried off largely by 

 surface drainage. Thorough under- drainage tends 

 to remove all these unfavorable conditions. If 

 there is no effective under- drainage, either by 

 natural or artificial channels, the water must 

 escape by surface evaporation. 



Sd. The capacity is increased hij proper tillage 



79. Tillage enables soils to hold moisture by 

 two means : by increasing the depth of the soil 

 in which the plants can grow (that is, by in- 

 creasing the depth of the reservoir), and by 

 Increasing the capillary power of the soil. We 



