66 



SOIL CONDITIONS AND PLANT GROWTH 



the composition of the soil solution. As might be expected from 

 the known absorptive properties of clay and of humus, drainage water 

 contains mere traces of NH 4 and PO 4 , and only little K ; it contains 

 chiefly carbonic acid, SiO 4 , Cl, SO 4 , NO 3 , Ca with some Fe, Mg and 

 Na. Typical analyses are given in Table XXIX. : 



TABLE XXIX. ANALYSES OF DRAINAGE WATERS FROM CULTIVATED FIELDS: PARTS 

 PER MILLION OF SOLUTION. 



It will be observed that the total concentration of the Rothamsted 

 drainage water varies from -02 to -05 per cent 



Organic Matter. 



The distinguishing characteristic of soil is that it contains part of 

 the complex material synthesised by plants. This material affords 

 energy to numerous micro-organisms, and is gradually converted by 

 them into simple substances appropriate for plant nutrition. We may 

 look upon its constituents as taking part in a perpetual cycle : in one 

 stage nourishing the growing plant and storing up the energy of 

 sunlight, in the other stage nourishing micro-organisms and liberating 

 energy. In addition, it has important physical effects on the soil. 

 Unfortunately, not much is known of the highly complex components 

 of the plant and even less is known about the important organic 



1 A. Voelcker's analyses of five samples collected between 1866 and 1869 (289). 



2 Von Seelhorst's analyses of samples collected weekly, or fortnightly, from a field 

 between August, 1899, and August, 1900 (261). 



