476 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 8 



plants. This absorption of nutrient salts causes not only a decrease 

 in the total amounts of salts available, but a qualitative alteration 

 as well, since not all the nutrient elements are absorbed at the same 

 rates. One secondary result is that the acid-base balance (pH) of the 

 solution may undergo changes which in turn may lead to precipitation 

 of certain essential chemical elements (particularly iron and man- 

 ganese) so that they are no longer available to the plant. Also to 

 be considered are the effects of salts added with the water (discussed 

 later). 



For these various reasons, the maintenance of the most favorable 

 nutrient medium throughout the life of the plant involves not merely 

 the selection of an appropriate solution at the time of planting, but 

 also continued control, with either the addition of chemicals when 

 needed or replacement of the whole solution from time to time. Proper 

 control of culture solutions is best guided by chemical analyses of 

 samples of the solution taken periodically and by observations of the 

 crop. Further investigation will determine if successful standardized 

 procedures requiring only limited control and adjustments can be 

 developed for a given crop, locality, and season of the year. 



The plant physiologist, in his experiments, prepares his solutions 

 with distilled water for the purpose of exact control. The commercial 

 grower, or the amateur, is usually limited to the use of domestic or 

 irrigation water which contains various salts, including sodium salts, 

 such as sodium chloride, sodium sulphate, and sodium bicarbonate, 

 as well as calcium and magnesium salts. Most waters suitable for 

 irrigation or for drinking can be utilized in the water-culture method, 

 but the adjustment of the reaction (pH) in the nutrient solution 

 depends on the composition of the water. Some waters may contain 

 so much sodium salt as to be unfit for making nutrient solutions. 

 Even with a water only moderately high in salt content the salt 

 may concentrate in the nutrient solution with possibly unfavorable 

 effects on the plant, if large amounts of water have to be added to 

 the tanks and the solutions are not changed. Also we have had ex- 

 perience with a well water which was highly toxic because it contained 

 too high a concentration of zinc, apparently derived largely from circu- 

 lation through galvanized pipes. The water was, however, not inju- 

 rious to tomato plants when used on a soil, because of the absorbing 

 power of the soil for zinc. 



As already indicated, the successful growth of a crop is dependent 

 on sunlight and temperature and humidity conditions, as well as on 

 the supply of mineral nutrients furnished by the culture medium. 

 Complex interrelations exist between climatic conditions and the 

 utilization of these nutrients. The relation of nitrogen nutrition and 

 climatic conditions to fruitfulness has often been stressed. In some 

 localities, deficient sunshine may prevent the production of profitable 



