CHAPTER IV 



ABSORPTION OF ASH-CONSTITUENTS 



§1. Cultures in Artificial Media.— Besides the four elements, carbon, 

 hydrogen, oxygen and nitrogen, every oigan of the plant contains many other 

 elements, the so-called, ash-constituents. The four constituents just named 

 volatilize and are lost during incineration, but more or less ash always remains. 

 According to Knop, the average amount of ash left after burning plant tissue 

 is about 5 per cent, of the original dry weight. The following elements have 

 been found in the ash of plants: 



Sulphur 



Phosphorus 



Chlorine 



Bromine 



Iodine 



Fluorine 



Boron 



Silicon 



Potassium 



Sodium 



Lithium 



Rubidium 



Magnesium 



Calcium 



Strontium 



Barium 



Zinc 



Mercury 



Aluminium 



Thallium 



Titanium 



Tin 



Lead 



Arsenic 



Selenium 



Magnesium 



Iron 



Cobalt 



Nickel 



Copper 



Silver 



Experiments with plant cultures in artificial media show that only a few 

 of these elements of the ash are essential to normal growth. Cultures may 

 be prepared by using either a neutral solid medium to which various salts are 

 added, or by dissolving the respective salts in water and employing the solu- 

 tion thus formed. Clean quartz sand, ground pumice or ground charcoal 

 may be used as solid media, or even finely divided platinum-wire, but the latter 

 is very expensive. Quartz sand with various salts is most frequently used. 

 The method of water-cultures has been well worked out in many researches 

 dealing with the necessity of various substances for plant growth, but es- 

 pecially in the work of Knop and Nobbe."^ 



The study of artificially controlled cultures has shown that plants need the 

 following elements in salts, for normal growth: nitrogen , sulphur , phosphorus, 

 p otassium , c alcium., ma gnesium and iron^. and sometimes c hlorine al so. 



These essential elements may be supplied to the plant as salts in water solu- 

 tion, in the following proportions: one part of KNO3, one part of KH2PO4, 

 one part of MgS04, and four parts of CaCNOa)^. A trace of ferric phosphate 

 is also added. The addition of a nitrogen compound to the culture medium is 

 necessary although nitrogen is not one of the ash-constituents, for plants ob- 

 tain their nitrogen from the soil, as has been seen in the preceding chapter. 

 This particular nutrient solution is known as Knop's solution. The concentra- 

 tion must be very low; as long as the plants are still young, o.i per cent, suffices, 



1 Knop, Wilh., Der Kreislauf des Stoffes. Lehrbuch der Agrikulturchemie. Leipzig and St. Petersburg. 

 1868. P. 572-663.* 



76 



