CHAPTER IV 



ABSORPTION OF ASH-CONSTITUENTS 



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

 hydrogen, oxygen and nitrogen, every organ 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: 



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

 of these elements of 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 espe- 

 cially in the work of Knop and Nobbe. 1 



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

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

 potassium, calcium, magnesium and iron, and sometimes chlorine also. 



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

 tion, in the following proportions by weight: one part of KNO3, one part of 

 KH 2 PO$, one part of MgS0 4 , and four parts of Ca(N0 3 ) 2 . 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 obtain their nitrogen from the soil, as has been seen in the preceding 

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

 concentration must be very low ; as long as the plants are still young, o. 1 per cent . 



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

 1868. P. 572-663. » 



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