PLANT INDUSTKIES 321 



301. Water drainage. The amount of water in the soil va- 

 ries and depends upon many factors. The coarse soils (gravel 

 and coarse sand) soon become dry when there is little rainfall 

 and no replenishment of water from below. Fine-grained soils 

 which contain a good supply of organic matter may become 

 filled with water and may retain this water for a long time. 

 They may become so filled with water as almost entirely to 

 exclude the air of the soil, thus suffocating the roots of plants. 

 But some air remains in all soils, even below ponds and 

 streams, and some water plants can live with their roots in 

 such water-logged soils. From 50 to 60 per cent of the total 

 water-holding capacity of soil is found to be best for the growth 

 of cereals. Our agricultural plants, except rice, cannot endure 

 water-logged soils, and drainage has often been necessary to 

 bring about a more favorable water-and-air content of culti- 

 vable lands. 



Natural drainage may be either on the surface or under- 

 ground. It is evident that in cultivated soils extensive 

 underground drainage will in times of heavy rains hasten the 

 removal of surplus water and at all times will facilitate 

 better aeration of the soil. The annual rainfall in the United 

 States varies from ten inches or less to more than sixty 

 inches per year, and when any considerable part of this rain- 

 fall comes in a short period of time, non-enduring plants will 

 suffer and sometimes be drowned unless adequate drainage is 

 supplied. 



North America still has immense tracts of waste swampy 

 land that only needs drainage and tillage to make it highly 

 productive soil. It is estimated that the United States has 

 nearly 100,000,000 acres of swamp land, much of which is 

 drainable, and in many places trained engineers are devising 

 the needed drainage systems, and the land is being reclaimed. 



302. Influence of cultivation on water supply. In earlier 

 times agriculturists advised against cultivating corn and other 

 crops during times of drought, because they thought that, if 

 constantly stirred, the soil would lose its moisture more rapidly. 



