Plants in Relation to Man 469 



the second season, seed is planted and the surface soil culti- 

 vated, if necessary, to conserve moisture. 



This practice is common also in many fruit-growing districts, 

 as in California. During the dry season, when fruits need water 

 for full development and irrigation is not available, the surface 

 soil is thoroughly and frequently tilled, with the result that 

 the soil moisture is conserved for the trees and is not lost by 

 direct evaporation. 



225. Soil air. Soil air is an essential constituent of pro- 

 ductive soil. The farmer and gardener recognize this and seek 

 by cultivation to ventilate or aerate the soil sufficiently for 

 growing plants. The relative quantity of air in the soil is, 

 like that of water, dependent upon the size of the soil par- 

 ticles. The finer the texture of the soil, the larger the number 

 of interstices and consequently the more air contained. The 

 coarser the particles, the less the number of interstices and 

 consequently the less air contained. How can you test this? 

 But the amount of air in a soil is not the only important factor. 

 It is the circulation of the air in the soil which is important, 

 and which likewise depends upon the size of the interstices. 

 The larger the interstices, the better aerated or ventilated 

 the soil is likely to be. Thus a coarse silt or loam is better 

 aerated although it contains less air than a very fine silt or 

 clay. 



Measuring the soil's capacity for air. It is estimated that 

 an average grade of soil holds about one half its own volume 

 of air. This estimate may be tested with different kinds of 

 soil of normal moisture and condition. 



Exercise. Put fifty cubic centimeters of each sample soil into 

 glass jars and shake down well to get about the same compactness as 

 in nature. Pour from a graduated beaker containing fifty cubic cen- 

 timeters of water just enough to reach the surfaces of the soil. How 

 much water is required ? This represents in each its air capacity while 

 holding its usual amount of soil water. 



Now dry out each sample thoroughly, and again saturate with 

 water. The amount of water now taken is practically equivalent to 



