THE WORK OF ROOTS 131 



much water as another on the same land and for the 

 same crop ; house-plants are more often killed by in- 

 judicious watering than by any other cause. The 

 greatest harm is done by over -irrigation, which not 

 only drowns the roots but ruins the soil -tilth. When- 

 ever it is possible, irrigation should be done by means 

 of underground pipes . or drains : these deliver both 

 water and air to the roots precisely where they are 

 needed, and, if a good surface mulch be maintained, 

 there is almost no loss by evaporation. This method is 

 especially adapted to greenhouses and intensive horti- 

 culture generally. It is stated that by this system, as 

 practiced in the open, only a twentieth part of the or- 

 dinary amount of water needs to be given and the tilth 

 of the soil is kept in the best condition, while the sur- 

 face of the land is left free for tillage. 



The amount of water which the soil should contain 

 to give the best results will vary somewhat, according 

 to circumstances (some plants require much more than 

 others) . In general the soil should not contain more 

 than 60 per cent of its water-holding capacity, i. e., at 

 least two-fifths of the spaces should be occupied by air. 

 The water -hoi ding capacity of a soil may be deter- 

 mined by saturating it with water, draining off the 

 surplus, weighing a portion of it and then drying it on 

 a water- bath to constant weight. The loss in weight 

 livided by the dry weight gives the percentage 

 of water (i. e., water-holding capacity of the soil). 



