2O PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Quantity of Plant Food Needed by Plants. The experiments 

 by Wolff already cited show the minimum requirements of the 

 oat plant, but this method has not been applied to other plants, 

 and we are dependent upon other methods for ascertaining the 

 needs of the plant. The only other method which has been used 

 is to make the analysis of the plant. 



This is not altogether a safe guide: first, because plants may 

 take up an excess of plant food; and secondly, near maturity 

 material is easily washed from the leaves and other parts of the 

 plant by rain, dew, etc., as shown by LeClerc and Breazeale. 1 For 

 estimating the draft of the plant on the soil, analysis is of course 

 more satisfactory. Such analyses are also used for calculating 

 the manurial value of feeds. Most of the estimations of the 

 mineral matter in plants have been made by analyses of the ash. 

 Nitrogen is determined on a separate sample of the original plant 

 material. 



The Ash of Plants. When plant substance is burned, the 

 greater portion of it passes off as volatile bodies. The residue is 

 termed the ash. The ash is sometimes spoken of as the mineral 

 part of the plant, or the inorganic part. These terms are not cor- 

 rect. The ash is merely that portion of the plant which forms 

 compounds not volatile at the temperature of the combustion. A 

 portion of it may have been present in the plant in the form of 

 inorganic bodies, and a portion has undoubtedly been in organic 

 combination. 



The term crude ash is applied to the ash as secured by burning. 

 Pure ash, or carbon free ash, is the crude ash less the free carbon, 

 carbon dioxide, and sand contained in it. 



Under ordinary conditions, all of the nitrogen and hydrogen, 

 most of the carbon and oxygen, a considerable part of the sul- 

 phur, and a small portion of the potash and chlorine pass off dur- 

 ing combustion. The ash consists chiefly of carbonates, oxides, 

 sulphates, phosphates, silicates, and chlorides of potash, lime, 

 magnesia, and soda. Unburned carbon is usually present, and in 

 rare cases cyanides and sulphides are found. The fact that a 

 1 Yearbook, U. S. Department Agriculture, 1908, p. 389. 



