No. 6. DEPARTMENT OF AGRICULTURE. 459 



in order that it may be perfectly dry, it must be driven out by 

 heating the soil up to the temperature of boiling water for a consid- 

 erable period of time — at this temperature (212 degrees Fahr.) the 

 water vaporizes and escapes into the air. Naturally, this form of 

 water in the soil is not a useful one for plants, as it exists in soils 

 after plants have perished for lack of moisture. 



Rainfall the Source of Soil Water. 



The water in soils is due to the rainfall; the water-level in the 

 earth rising in seasons of greatest rainfall, or wet periods, and falling 

 in dry periods, hence, while the water that plants obtain is due to 

 rainfall, it does not follow that it shall occur immediately preceding 

 or during the season of the plant's growth. For example, there are 

 places where the rainfall in the course of a j'ear is so great as to 

 provide such a total excess of water as to cause the soil to be thor- 

 ougly saturated, or to keep the level so near the surface as to prevent 

 the growth of plants, yet luxuriant crops are grown in these places. 

 This is explained on the ground, first, that v.'hile the average rain- 

 fall is so great as to cause the soil to be thoroughly saturated, the 

 time of the rainfall is such that during the crop-making season a 

 sufficient amount only is provided for the plants; that is, that which 

 falls during the season that crops are not growing partly sinks into 

 the soil and partly runs off, and by the time the crop-season is ready, 

 the water level is lowered sufficiently to enable the plants to throw 

 their roots deeply, and they are fed with the capillary water drawn 

 from the reservoirs below, which were filled by the preceding rains; 

 and second, though the rainfall may be, on the average, too great, 

 the soil is of such a character, open and porous, as to enable the rapid 

 percolation of it to lower levels, and the climate such as to cause 

 a rapid evaporation of it into the atmosphere. The moisture that 

 may be obtained from rainfall is, therefore, influenced by two con- 

 ditions, the time of its precipitation and the character of the soil 

 upon which it falls. 



An Even Distribution of Rainfall Desirable. 



The water from the immediate rainfall to be most useful to plants 

 should be distributed evenly throughout the growing season; in such 

 cases, a minimum precipitation would provide for a maximum pro- 

 duction, if the soil is of such a character as to retain and hold that 

 which falls upon it for the use of the plants. Therefore, w^hile rain 

 is the primary source of water to plants, needed amounts may be 

 obtained from underground supplies. That is, the season of rainfall 

 may occur during the time when plants are not usually growing, 

 but the w^ater sinks into the soil which holds it as in a reservoir, 



