30 CHAPTER III 



It will be readily appreciated from the above that soilc 

 rich in humus, as well as those of fine texture, will not only- 

 have a higher percentage of available water than sandy soils 

 without humus, but that they will require irrigation at less 

 frequent intervals. In general sandy soils should receive 

 water frequently, a small amount at a time. Clay soils and 

 those rich in humus should require relatively large but less 

 frequent applications, since their available water content is 

 high, and percolation both slow and small. 



The nature of the sub-soil is also an important factor. A 

 sandy soil resting on a clay sub-soil will lose water by percola- 

 tion very slowly. Again, a clay soil resting on a sandy sub- 

 soil will require more frequent irrigations than a clay soil on 

 a clay sub-soil, but the former will have a very much better 

 mechanical condition and will suffer less from water-logging. 



A point often disregarded is, that when grown on a 

 highly fertile soil plants have a smaller water requirement 

 than on a soil of low fertility. For this reason soils should be 

 cultivated where practicable as soon after irrigation as possible, 

 as this enables the various physical, chemical and biological 

 changes to take place more rapidly. Working with maize, 

 Widstoe* obtained the following results : — 



Pounds of Water Transpired for One Pound of Dry Matter. 



In every case, excepting the abnormal infertile sand, the 

 careful stirring of the soil enabled the plant to produce one 

 pound more of dry matter with a smaller quantity of water than 

 when the soil was not cultivated. Apart from this, with in- 

 creasing richness in available plant-food, the rate of evapora- 

 tion of water from the soil tends to decrease. 



(d) The Surface Conditions and Topography of the 

 Land.— -On this will depend the method of water distribution. 

 " Flooding " and " furrow " irrigation are the two methods 

 commonly employed. The former method is used as a rule on 

 gently sloping land, while furrows are used where the slope is 

 more decided, where the water supply is limited, and for cer- 

 tain crops like potatoes, tobacco, etc. "^ Flooding is conducted in 

 two ways. One way is to cover the field with a thin sheet of 



