140 IRRIGATION PRACTICE 



an average of many experiments, crops were produced on 

 unmanured soils with a transpiration ratio of 782; and on 

 manured soils with a transpiration ratio of 572. Bouyoucos 

 has concluded, from a series of carefully conducted tests, 

 that the greater the concentration of the soil solution, 

 that is, the more substances it holds in solution, the 

 smaller the transpiration ratio. Fertile soils are usually 

 more soluble than infertile ones, and the soil solution of 

 fertile soils is usually more concentrated. The same 

 investigator has also shown that as the soil solution 

 becomes richer in soil constituents, the cell sap of the 

 plant becomes more concentrated, and that this may be 

 the reason that less water enters the plant daily when 

 the concentration of the soil solution is high. 



Different substances influence the transpiration ratio 

 differently. Acids, for instance, tend to accelerate trans- 

 piration, and to increase the transpiration ratio. Alkalies 

 have the opposite effect. This is of importance to the 

 irrigated sections, since under arid conditions alkaline, 

 rather than acid, soils are naturally produced. Lime, 

 phosphoric acid, potash and nitrates tend, especially, 

 to reduce the water-cost of dry matter. Of first impor- 

 tance are the nitrates. The richer the soil is in nitrates, 

 the more surely will the water-cost of the crop be reduced. 

 This law appears and reappears in investigations on all 

 manner of soils, from all parts of the world. The main- 

 tenance of an abundance of nitrates in the soil is undoubt- 

 edly of prime importance in reducing the water needs of 

 crops. Increasing the concentration of the soil solution 

 reduces the transpiration ratio only when the substances 

 held in solution in the soil moisture are true plant-foods. 

 Bouyoucos showed that a solution of common salt, or 

 sodium sulfate, or other substances, not direct plant- 



