432 UTILIZATION OF MINERAL SALTS 



solutions with an osmotic pressure of less than 2 atmos. are employed in 

 solution culture work. (2) Either the various solutions used in any one 

 investigation must be buffered to the same pH, or else the influence of dif- 

 ferences in their pH must be evaluated. (3) IMost species of plants grow 

 better in aerated than unaerated solution cultures. A solution is an abnormal 

 environment for the roots of most species. The oxygen concentration of 

 such aqueous media is relatively low, and carbon dioxide may accumulate 

 as a result of root respiration. Aeration of solution cultures is therefore 

 often necessary and almost always desirable. (4) Provision must be made 

 either for a slow constant renewal of the solution, or else for its frequent 

 replacement with fresh solution if its composition is to be kept even ap- 

 proximately constant. The various ions in a solution are not absorbed at equal 

 rates. Neither is the intake of water proportional to the intake of ions. 

 Furthermore ions, and perhaps certain organic compounds, often diffuse from 

 the roots into the solution. The net result of all these factors is to cause 

 a rapid change in the proportion of elements in any solution culture unless 

 adequate provisions are made to offset these effects. (5) Considerable diffi- 

 culty is often experienced in keeping the iron in solution cultures in a soluble 

 form. In general the more alkaline the solution the more readily iron passes 

 out of solution. In solutions with a pH of about 6.0 or higher special precau- 

 tions must be taken to prevent iron deficiency from becoming a limiting factor 

 in plant growth (Hopkins and Wann, 1926 and others). Often the develop- 

 ment of iron deficiency in a solution culture can be avoided only by introduc- 

 ing fresh portions of a dilute solution of an iron salt at frequent intervals. 



For many types of investigations "sand cultures" are preferred to solu- 

 tion cultures. Suitable vessels are filled with a pure quartz sand which is 

 kept moistened with a nutrient solution. Provision must be made for fre- 

 quent renewal of the solution, or better, for a continuous supply of solution 

 to the vessel. This is frequently accomplished by means of "drip cultures" 

 in which solution from a reservoir flows into the sand, usually one drop at a 

 time, provision being made for the drainage of excess solution from the 

 vessel (Fig. 97). An important advantage of sand over solution cultures is 

 that in the former the roots grow in a much more nearly normal environment. 

 IVIaintenance of definite quantitative relations among the various ions is more 

 easily accomplished in solution cultures than in sand cultures, however. Hence 

 for certain types of investigations the former are more useful than the latter. 



Employment of the technique of sand and solution cultures has led to 

 many advances in our knowledge of the mineral salt relations of plants. 

 Most of the available information regarding the essentiality of various elements 

 in plant development has resulted from investigations in which a solution or 



