158 Journal or the Department or Agriculture. — Aug., 1922. 



can l)e explained partly by the fact that the plant has more space in 

 which to develop and also by the fact that the soil does not become 

 so compacted by irrigation waters on the outside of the beds so that 

 air containing oxygen can enter the soil more readily and a better 

 root system can be developed. In the furrow method the soil is kept 

 more or less loose and friable all the time. The t.i/pe of soil is of 

 importance in choosing the method of applying water to the lands. 

 Sandij soils : Sandy soils should preferably be irrigated by the flood- 

 ing method unless the water is very scarce, for in such soils the idea 

 should be to retard the rate at which the water enters the soil. By 

 Hooding tliis water quickly over the surface the air cannot escape 

 very readily, and therefore prevents the water from entering too 

 rapidly. If the furrow method is used on sandy soils, the water will 

 soak away at the upper end of the furrow unless the slope be very 

 steep or unless a very strong stream of water be turned into the 

 furrow. In either case there will be considerable danger of washing 

 and forming " sloots." Ilie nature of the water: Waters that will 

 not Xif'netrate into the soil readily can best be applied by the furrow 

 method, for if such waters are applied by flooding they will tend to 

 stand over the whole surface of soil and thereby In-ing about injurious 

 effects on the soil and the crop. 



Water and Crop EELATiONsriiP. 



In the preceding pages an attempt has been made to point out 

 the physical relationships between soil and irrigation waters. A few 

 notes will now be given on the relationship between the crop and 

 irrigation waters. 



(1) Optimum Moisture for Plant Growth. — It has been found 

 that most crops grow best in a soil containing 40 to 60 per cent, of the 

 maximum capacity of the soil to hold water against gravity. This 

 maximum capacity is attained when the soil has about 90 per cent, 

 of its pore or air space filled with water. This pore space makes up 

 about 30 to 40 per cent, as a rule of the volume of the soil in situ, so 

 that the volume of the soil occupied by water at maximum capacity 

 will be 90 per cent, of 35 equals 31.5 per cent, filled with water. 

 Now 40 to 60 per cent, of 31.5, say 50 per cent, of 31.5, equals 

 15.75 per cent, of the volume of the soil in situ, should be filled with 

 water in order to get optimum water-content for plant growth. In 

 a foot section of soil we shall, therefore, need 15.75 per cent, of 

 12 inches equals 1.98 inch of water. In other words, in order to 

 raise a perfectly dry soil to optimum moisture, where the pore space 

 is 35 per cent., we should have to apply the equivalent of 1.98 inch 

 per foot depth of soil. Soils normally contain moisture in them at 

 the time that it becomes necessary to irrigate so that we can reckon 

 on about the equivalent of an inch or more in each foot of soil at the 

 time of irrigation. In order, therefore, to bring the soil to optimum 

 moisture for plant growth about .75 inch will probably have to be 

 applied for each foot (in depth) of soil that it is desired to moisten. 



To wet a soil six feet in depth we shall require ,75 x 6 = 4.5 in. at 

 each irrigation, whereas to wet a soil that is only two feet deep will 

 require 1.5 Inch. Obviously a deep soil can receive heavier irriga- 

 tions at longer intervals than a shallow one in order to raise the soil 

 to optimum moisture-content. This means that the water will be 



