BUREAU OF PLANT INDUSTRY. 187 



to water and thus to prevent the accumulation of salts in harmful 

 quantities if sufficient irrigation water is applied to leach the sur- 

 face soil, at least occasional! v. On the other liand, if the dissolved 

 salts in (he irrigation water are chiefly salts of sodium, the effect on 

 the soil is to make it colloidal, in which condition it absorbs water 

 so slowly that the leachinjr out of the. dissolved salts is difficult or 

 impossible. 



In the light of these results it has been possible to recommend the 

 treatment of irrigated land to prevent injury when the irrigation 

 water is deficient in calcium and magnesium salts and to correct con- 

 ditions of impermeability that have developed. 



The application of calcium sulphate, or gypsum, in small quantities 

 serves to prevent the injurious effects on the soil of the irrigation 

 water that is deficient in calcium and magnesium salts. The same 

 treatment is often beneficial, slowly and to a limited extent, in cor- 

 recting conditions of impermeability that already exist. When the 

 condition of impermeability is serious the action of the gypsum is 

 often very slow, because of the fact that it is not very soluble. For 

 such conditions it has been found that aluminum sulphate is more 

 effective than calcium sulphate in flocculating the soil colloids and 

 making the soil more permeable to irrigation water. 



It may be remarked that the calcium-sodium ratio of a stream, is 

 a much more constant factor at any given point than is the total 

 salt content. When irrigation water contains more sodium and 

 potassium than calcium and magnesium, there is danger that its 

 continued use may cause the land to become hard and impermeable 

 to water. Some of the important supplies of irrigation water in 

 the United States' carry more sodium and potassium than calcium 

 and magnesium, and difficulties of hardness and impermeability of 

 the soils are developing from the use of such water. 



RELATION OF SOIL SOLUTIONS TO THE GROWTH OF PLANTS. 



In the field study of the comparative effects of different crop plants 

 on the yields of other crops following in the rotation, some notable 

 differences have been obtained in the *' carry over " or residual fer- 

 tilizing effects of hairy vetch, crimson clover, cowpeas, and soy 

 beans on the small grains when tobacco, potatoes, and corn are the 

 intervening crops. Moreover, in these and in other cropping com- 

 binations the generally unfavorable effect of corn, as compared with 

 [)otat()es and tobacco, on the yields of crops following in the rota- 

 tion has been a striking feature. These " crop effects " do not seem 

 t) be fully explainable on the basis of relative draft on the soiFs 

 supply of plant-food elements. For example, the tobacco crop re- 

 moves much larger quantities of the plant-food elements than the 

 potato crop, but better yields of potatoes are obtained after tobacco 

 than after potatoes themselves. 



The nutrition of plants from the standpoint of ionization phe- 

 nomena has been investigated, and the results indicate that plants 

 are not restricted to the concentrations of the soil solutions in imme- 

 diate contact with the roots or brought to the vooti^ by the move- 

 ment of water, but may draw upon concentrations of solutions located 

 at some little distance from the roots. 



