Irrigatiox. 159 



iised more economically in the deep soil, for it has been deiinitely 

 proved that the loss of water from the soil by evaporation is much 

 ■greater than from a free water surface where the surface is kept more 

 or less constantly moist by too frequent irrigations. For example, 

 Fortier, working in America, found that, where the surface of the 

 soil was kept constantly moist, 4.75 inches of water were lost per week 

 as against 1.88 inch from a free water surface. This only goes to 

 show how desirable it is to choose a deep soil so that the lands may 

 be irrigated at longer intervals, thereby reducing surface evaporation 

 to a minimum. Let us consider now the cjuantity of water used per 

 season and the crop yield. 



(2) Quantity of Water and Yield. — Those familiar with irriga- 

 tion practice know it is an established fact that the crop yield is not 

 proportional to the quantity of water applied, decrease in yield jnay 

 often accompany further increase in water supplied above a certain 

 point. This point where decrease in yield is first observed will depend 

 largely on the type of soil, for crops are mucli better able to stand 

 heavy applications of water on deep porous soils than on heavy imper- 

 vious clays. In the accompanying tables are given some data which 

 bear out the point mentioned. Tliese data are taken from certain of 

 the Utah Agricultural Experiment Station publications. A study 

 of Table I will make clear the fact that the yield of a crop is not 

 proportional to the quantity of water applied. At first the increase in 

 water results in a fairly large increase in yield. Later on, however, 

 when the water is increased from, say, 30 to 50 inches per acre, the 

 addition of twenty extra inches results in only a Aery small increase, 

 and in some cases, e.g. in the mealie. we have a decrease in yield. A 

 glance at the yield of crop per inch of water applied makes the point 

 even more clear, for the table shows that there is a very rapid decrease 

 in the yield of crop per inch of water as the applications become 

 greater. This means that the crop uses the water less economically 

 when it is heavily irrigated. 



Besides this, there are the dangers, already enumerated, to the 

 physical conditions of the soil, and to these we must add the extra 

 danger of " brak." In Table II we see that a given quantity of water 

 applied to a small area of land produces about one-third the total 

 yield of dry matter that is produced by the same quantity of water 

 spread over four times the area of land at the rate of a quarter the 

 quantity of water per acre. This is undoubtedly a point worthy of 

 consideration in the arid and semi-arid sections, where water and not 

 land is the limiting factor in plant growth. The available land is 

 much greater than the available water, so that yield per inch of water 

 should always be the first consideration. It is not to be assumed that 

 on all farms results similar to those given in the tables will be 

 obtained. The results there reported were obtained on a very deep 

 well-drained soil. On shallow or impervious soils the decrease in yield 

 following large applications of water would be even more pronounced. 

 The decrease is to be accounted for, probably, owing to the fact that 

 the larger quantities of water tend to replace and exclude air to a 

 great extent so that one of the essential factors in plant growth, 

 namely, oxygen, becomes deficient and limits or restricts further 

 increase in yield. 



Again, the lack of oxygen results in an increase in the activities 

 or reduction of anaerobic bacteria, with the result that a substance 



