— 42 — 



The presence of drought is brought about as follows. Towards the 

 advent of spring the field becomes covered with weeds which appear 

 here after the cultured plants are cleared away. These weeds absorb the 

 whole of the insignificant remainder of useful water, held in the small 

 interstices of the soil particles in the form of small drops, which the more 

 exacting cultured plants could not use. The soil on such a field turns 

 out to be dry to a depth of 41/2 feet or more. Autumn, winter and 

 spring (the non-vegetation period) contributes so little water to the soil, 

 that at the period of spring sowing only the superficial soil layer becomes 

 moistered to 40 or 50 cm. The roots of the cultured plants rapidly pe- 

 netrate the whole of this layer with a dense network, and, meeting fur- 

 ther with an insurmontable barrier in the shape of the intermediate dry 

 layer, cease their downward growth, and give out a much more than 

 normal number ot side roots, which expend the reserve useful water in 

 a shorter time than a normal number would. In consequence of the fai- 

 lure of water in the soil the weaker plants commence to perish, and the 

 more hardy become prematurely ripe having reached the deeper horizons 

 of the humid so 1 layer with their root-systems. 



The intermediate dry layer, lying under the upper periodically humid 

 layer, does not contain sufficient useful water, and the mineral salts the- 

 rein remain in an insoluble condition. When a favourable moist spring 

 arrives and the dry layer moistens, the plants use up in assimilating 

 mineral salts alone, much more useful water than they would on rich 

 soils where the mineral salts are already dissolved. 



Consequently drought is preceded by two factors: 



1) A part only of the root - inhabited soil layer being in a moist 

 condition at the period of spring sowing, instead of the v/hole layer of 

 110 to 130 cm. 



2) A chronic perennial dryness of the intermediate dry so.l layer 

 lying under the periodically humid layer; and as a result of their dry- 

 ness, the absense therein of the chemical processes for transforming 

 insoluble mineral matter into soluble and 



3) A deep position of the permanent humid layer owing to which, 

 the intermediate dry layer is of very great thickness. 



