70 SOIL PHYSICS AND MANAGEMENT 



implements dynamite must bo resorted to. In planting trees on 

 hardpan land dynamite may be used and tbus allow tbe roots their 

 usual penetration. If the hardpan is caused by sodium carbonate, 

 it may be necessary to apply gypsum to destroy this carbonate and 

 thus break up the hardpan. 



Humid and Arid Subsoils. The subsoils of arid regions do 

 not differ materially from the surface and subsurface because the 

 fine particles are not moved downward to any extent by percolating 

 water. In addition to this, soluble substances are present, which 

 flocculate the collodial clay and prevent its movement downward. 

 The arid subsoils do not possess the " raw " or unproductive nature 

 that characterizes the humid ones. In arid regions very deep plow- 

 ing may be done immediately preceding the planting of the crop 

 without detriment; in fact, it is of great benefit, because it allows 

 deeper root penetration and greater moisture retention. In the 

 process of leveling, preparatory to irrigating, the soil is sometimes 

 removed to a depth of several feet without injurious effect on the 

 crop that follows. In humid regions the farmer must be careful 

 not to turn up much of the "raw" unweathered material, just 

 preceding time of planting the crop, but if deep plowing is done 

 sufficient time should be given for the soil to " weather " before 

 the crop is put in. This is probably partly due to biological 

 conditions. 



The color differences do not obtain in the arid regions because 

 the organic matter is derived almost entirely from roots, and these 

 penetrate very deeply ; so there is no great accumulation of organic 

 matter in the surface stratum. Oxidation of iron has not generally 

 gone very far because of lack of moisture, hence arid soils are not 

 usually highly colored. 



Plow Sole. Where plowing takes place at a somewhat uniform 

 depth for a long time, the tramping of horses and the sliding action 

 of the plow in the bottom of the furrow have a tendency to form a 

 compact layer or plow sole. The washing of the fine material from 

 the loose, plowed soil down on the furrow bottom tends to increase . 

 the tightness of the plow sole. In order to break up this stratum 

 and prevent the formation of another, plowing should be done at 

 variable depths, and when the moisture condition is such that pud- 

 dling will nrt take place. 



