WATEK-CULTURE METHOD HOAGLAND AND ARNON 483 



adversely. Should plants be iDJured, however, by the presence in the 

 water of high concentrations of elements like zinc, changing solutions 

 will not prevent injury. Because of the wide variation in the composi- 

 tion of water from different sources, no specific directions to cover all 

 cases can be given. 



CHANGES OF NUTRIENT SOLUTION 



As the plants begin to grow, nutrient salts will be absorbed and 

 the acidity of the solution will change. More salts and acid may be 

 added, but to know how much, chemical tests on the solution are 

 required. When these cannot be made, an arbitrary procedure may 

 be adopted of draining out the old solution every week or two, imme- 

 diately refilling the tank with water, and adding salts and acid, as 

 at the beginning of the culture. The number of changes of solution 

 required will depend on the size of plants, how fast they are growing, 

 and on volume of solution. Distribute the salts and acid to different 

 parts of the tank. In order to effect proper mixing, it may be well 

 to fill the tank at first only partly full (but keep most of the roots 

 immersed) and then after adding the salts and acid, to complete the 

 filling to the proper level with a rapid stream of water, which should 

 be so directed as not to injure the roots. 



TESTING AND ADJUSTING THE ACIDITY OF WATER AND NUTRIENT 



SOLUTION 



Ordinarily some latitude is permissible in the degree of acidity (pH) 

 of the nutrient solution. For most plants a moderately acid reaction 

 (from pH 5.0 to 6.5) is suitable. If distilled water is used in the prep- 

 aration of nutrient solutions, no adjustment of its reaction is necessary. 

 If tap water is used, a preliminary test of its reaction should be made 

 and if the water is found alkaline, it should be acidified before adding 

 the nutrient salts. 



As already stated the reaction (pH) of the nutrient solution is subject 

 to change as the plant grows. The reaction of the culture solution 

 should be tested from time to time and corrected, if found alkaline. 



The chemicals required for testing acidity of water or nutrient solu- 

 tion are: 



1 . Bromthymol blue indicator. — This can be obtained with directions 

 for use, from chemical supply houses, in the form of solutions or im- 

 pregnated strips of paper. Strips of other test papers covering a 

 wide range of acidity are also now available on the market and may be 

 found, by the amateur who understands their use, very convenient for 

 adjusting the acidity of water as well as that of the nutrient solution. 



2. Sulphuric acid. — Purchase a supply of 3 percent (by volume) 

 acid of chemically pure grade. (Concentrated, chemically pure sul- 



114728—39 32 



