138 IRRIGATION PRACTICE 



The physical and the chemical properties, as well as 

 the depth of the soil, are of importance in determining the 

 water-cost of dry matter. The deeper the soil is, the 

 smaller is the transpiration ratio. This is to be expected, 

 for the deeper the soil the more complete will the root- 

 development be; and the more extensive the root-system 

 is, the more easily may water and the mineral foods be 

 obtained. Thus, carbon assimilation and all the other 

 vital functions of the plant are stimulated into action. 

 Leather grew crops in jars of different sizes, and, almost 

 invariably, found that the crops grown in large jars 

 were produced at the smallest water-cost. While the 

 depth of the soil is an incidental factor, it may at times 

 be of considerable importance. There is, throughout the 

 irrigated region, a tendency to use very large quantities 

 of seed. Unless the soil is deep and easily penetrated, the 

 mass of roots, resulting from the large quantity of seed, 

 may not find sufficient space in which to develop prop- 

 erly. The resulting crowding and overcrowding lead to 

 immense numbers of stunted individual plants, that do 

 not always possess the vigor to use water to the best 

 advantage. This is of particular importance wherever 

 hardpan is near the surface, or where a heavy clay under- 

 lies the top soil. Moderate quantities of seed should be 

 sown, even under irrigation. 



The relation of the physical composition of a soil to 

 the water-cost of the plant grown has not been thoroughly 

 investigated. It was observed, however, long ago, that 

 in the use of water, a loam soil is more economical than 

 a sand soil. It seems that, with given conditions of 

 fertility, the finer the soil the smaller is the transpira- 

 tion ratio. Soils rich in clay or fine sand are naturally 

 more economical of water than are those containing coarse 



