32 CHAPTER III 



farmer through his head-gate."* The duty of water varies 

 according to (1) the rainfall ; (2) porosity of the soil ; (3) fer- 

 tility of the soil ; (4) the frequency of irrigation and amounts 

 applied ; and (5) the average humidity of the air. 



Crops require to be irrigated when the soil moisture has 

 reached the pomt of lento-capillarity, i.e., the point at which 

 the capillary movement of the soil moisture becomes too slow 

 to satisfy the requirement of the crop. At and below this 

 point the plant is obliged to expend an unnecessary amount of 

 energy in securing water. At this stage the plants become 

 flaccid and a slight change of colour in the leaves and stalks is 

 apparent. In general irrigation should be curtailed during the 

 early development of crops, so that a deep root development 

 may be stimulated. In the case of grain crops, irrigation should 

 be liberal from the time of flowering to just before maturity, 

 because during this stage translocation of food material and 

 various anabolic processes which involve the use of water are 

 most active. In South Africa the most common error of farmers 

 who have abundance of water at their disposal is over-irrigation, 

 particularly during the early stages of growth. Often, too, 

 the fact is overlooked that, although increased irrigation may 

 give increased yields, it is apt to do so at an increased water 

 cost. Where the amount of water available is limited, and 

 land relatively cheap, the yield per acre is not as important as 

 the yield from a given quantity of water. Widstoe"" found 

 that the amounts of dry matter in pounds produced by the same 

 quantity of water (30 acre inches) spread over one acre and 

 four acres respectively were as follow : — 



Crop. One acre. Four acres. 



Wheat 6,951 22,180 



Maize 15,294 43,028 



Lucerne 8,133 3,272 



Potatoes 3,660 10,920 



The approximate amount given per irrigation may be taken 

 as 3 inches for a light, 5 to 6 inches for an average, and 8 inches 

 for a heavy irrigation. 



The irrigation of specific crops is dealt with in later chap- 

 ters in which the various crops are discussed. It should be 

 borne in mind that for all crops, where possible, the soil should 

 contain sufficient moisture at the time of planting to ensure ger- 

 mination and a vigorous seedling growth. 



Effect of Irrigation on the Crop. — Liberal irrigation 

 tends to delay maturity, to increase the percentage of carbo- 



