PLANT INDUSTRIES 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 im cultivated soils extensive 
underground drainage will in times of heavy rains hasten the 
removal of surplus water and at all times will facilitate 
better aération 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. 
