186 ANNUAL REPORTS OF DEPARTMENT OF AGRICULTURE. 



yam {I>io:<corea alata) is so similar to the white or common potato 

 that an important future is predicted for it in all those re^dons vv-herc 

 it can be grown successfully. About .'^50 persons are this year grow- 

 ing yams, experimentally and for market, from propagating ma- 

 terial supplied by the department. The chayote, a subtropical vege- 

 table of the squash family, is now produced in commercial quantities 

 in some of the Southern States, and the department is cooperating 

 with dealers in northern cities who desire to bring this new vegeta})le 

 to tlie attention of the American public. 



CUCUMBER MOSAIC. 



The investigations of cucumber mosaic during the past year were 

 along lines developed as a result of the work of previous seasons. It 

 has been proved that the common milkweed is an important source 

 of mosaic infection for cucurbits in the field and is probably more 

 important than the wild cucumber as a source of infection in many 

 localities, because the milkweeds are found in the immediate vicinity 

 of cucumber fields, while the wild cucumbers are often at such dis 

 tances from the fields that infection is not certain to occur in all 

 seasons. If aphids are present in the fields the proximity of the 

 milkweed plants is almost certain to result in infection of the 

 cucumber crop, since the aphids feed on both the cucumber and milk- 

 weed. 



PLANT NUTRITION. 



QUALITY OF IRRIGATION WATER IN RELATION TO IMPERMEABLE SOILS. 



It has become increasingly apparent that the quality of the water 

 used in irrigation is an important factor in the success or failure of 

 agricultural production on irrigated land. On many of the older 

 irrigated areas and even on some of those but recently put under 

 irrigation extensive tracts of land haA^e become relatively unproduc- 

 tive ])ecause of troubles associated with the accumulation of the so- 

 called alkali salts. These difficulties are sometimes manifestly due 

 to alkali; that is, to the fact that the quantity of soluble salts in 

 the soil is so great that the soil solution becomes too concentrated 

 to permit crop plants to grow. It has been found also that the 

 alkali salts have other injurious effects, particularly as regards the 

 movement of water through the soil. 



In view of the fact that the application of irrigation water con- 

 tinually adds soluble salts to the soil it is important that some of the 

 water applied should percolate through the soil to carry away the 

 dissolved salts. Unless this occurs it is inevitable that salt will 

 accumulate in the soil as a result of irrigation. A system of irri- 

 gation practice to be continually successful requires that the soil be 

 kept in such a condition that the irrigation water may readily perco- 

 late into it and at least a part of the water pass on through the 

 surface layers of soil to carry away the dissolved salt. 



It has been demonstrated by investigational work, both in the 

 laboratory and in the field, that the permeability of the soil to water 

 is influenced by the quality of the irrigation water with respect to 

 its dissolved salts. When the dissolved salts are chiefly salts of 

 calcium and magnesium the effect on the soil is to keep it permeable 



