154 JOUKNAL 01- THE DEPARTMENT OF A(iKI( L i/1 UltE. AlG., 1922. 



irrigation scbcnies have been perfected, only 1-lOtli to l-otli (jf the 

 arid and semi-arid lands will be under inioation. showinji that it 

 is not the available surface of land that will restrict the further 

 extension of irrigatJon, but the lack of water itself. Tliis means that 

 every effort should be made to obtain the biggest possible yield per 

 unit of water used. This point will be referred to later. Since so 

 large an amount of the earth's surface can never be brought under 

 irrigation, it will remain for the dry farmer where possilde to render 

 productive the rest of the arid and semi-arid lands. 



Irrigation mul Drjj Fan/iuig. — Irrigation and diy farjiiing must 

 go hand in hand in bringing the thirsty desertlauds to productivity, 

 for by applying the principles of dry farming to irrigation practice 

 the biggest total yield per unit of water will be obtained and the best 

 and .safest utilization of the available water supply will be made. 



Plant Grou'tli and Irrigation. — There are some five essential 

 factors in the growth of plants, namely, oxygen (from air), water, 

 food, temperature, and light. There must, of course, also be no 

 injurious or harmful substances. In order to get the best response to 

 irrigation water, it is necessary always to study the factors given 

 above. Perhaps some of us have wondered at the nuuvellous response 

 of our crops and the veld to a shower of rain, whereas a similai 

 quantity of water applied artificially in the normal way may result 

 in hardly any response at all. The words of the late Professor F. H. 

 King will be read with interest in this connection. He says: " The 

 ideal application of water to a field is realized completelj- in the 

 gentle, continuous rain which falls at just the rate which permits 

 each drop to enter the soil where it falls without ever saturating 

 completely any portion of the root zone above its caj)illary capacity, 

 but carrying all parts of it up to that limit. If any method of apply- 

 ing- water to the field is devised which can approach this ideal in all 

 its effects upon the soil, a vast increase in yield will be possible and 

 a large increase in the duty of water will be secured. With such a 

 method of distributing water expeditiously and economically, many 

 more acres of land in the arid regions could be reclaimed and most 

 ■of the losses from seepage and alkalis would be avoided." The reason 

 then why the rainfall results in such a I'csponse would seem to lie in 

 the fact that the soil does not become saturated, which means that 

 the air (containing oxygen) is not all driven out of the soil, thereby 

 ■depriving the roots for a time of one of the essential factors in their 

 growth. The rain also contains, as a rule, a larger quantity of oxygen 

 dissolved in it than does irrigation water. This oxygen will help to 

 take the place of that driven out of the soil by the water. Another 

 explanation is found in the effect of a large quantity of water applied 

 at one time to the surface of the soil. 



Effect of Water on the Soil. 



{a} Un the size of the soil particles : Soils formed under 

 arid and semi-arid conditions are largely only slightly altered 

 rock-powder containing varying quantities of organic material. 

 When irrigation waters are led on to such soils certain changes 

 take place, due largely, to the action of water and carbon- 

 dioxide on the feldspars. As a result, the particles become partly 



