50 Physics of the SoiL 



directions, leaving the sub-soil exposed, and where crops 

 are always smaller. On the other hand, where two fur- 

 rows are thrown together to form the "back-furrow" and 

 the depth of soil increased crops are notably more vigorous. 



We do not yet know just why a sub-soil when exposed 

 to the surface is less productive than the true soil, but the 

 difference seems in some way to be associated with the 

 larger per cent, of the extremely minute particles which 

 sub-soils contain. 



In arid regions where the rainfall is not sufficient for 

 crop production it seldom occurs that the deeper soil is 

 markedly different in productiveness from that at the sur- 

 face. Soil taken from the bottom of cellars and even from 

 depths as great as 30 feet is found quite as productive 

 when placed upon the surface as the top soil. So gener- 

 ally true is this that when it is desirable to level fields foi 

 purposes of irrigation in arid climates the soil from the 

 higher places may be scraped to the lower levels without 

 fear of lessening the productiveness of the fields. 



66. Uses of Soil. In the agricultural sense the most im- 

 portant use of soil is to act as a storehouse of moisture for 

 the use of plants; and the productiveness of any soil is in 

 a very large degree determined by the amount it can hold, 

 by the manner in which it is held and by the readiness and 

 completeness with which the plant growing in it is able to 

 withdraw that water for its use as needed. 



In the second place, the soil is a storehouse from which 

 plants derive the ash ingredients of their food, the lime, 

 the potash, phosphoric acid and other materials of this class, 

 all of which are derived from the slow decay and solution 

 of the soil grains. 



Besides these the soil is a laboratory in which a great 

 variety of microscopic forms of life are at work during 

 the warm portions of the year, breaking down the dead 

 organic matter of the soil, converting it into nitric acid 

 and other forms available to higher plants, and the student 

 must never forget that the magnitude of the crop taken 



