26 



PRINCIPLES OF AGRICULTURAL CHEMISTRY 



age of the plant. That is to say, the young plant takes up more 

 mineral matter in proportion to the quantity of organic matter it 

 elaborates, than the older plant. 



The nature of the soil also affects the changes in the ash of 

 a plant during its growth. When an insufficient quantity of plant 

 food is supplied by the soil, the plant may withdraw ash ingre- 

 dients from its older parts for use in continuing its growth. As 

 we have seen, in the presence of an excess of plant food, the 

 plant may take up more ash than it needs. These facts should 

 be borne in mind in considering the changes in the ash ingre- 

 dients during the growth of plants. The composition of the 

 plant at different stages on one soil, under one set of conditions, 

 may be quite different from the same plant grown on another 

 soil under another set of conditions. When the plant approaches 

 maturity, ash may be washed out by rain. 



For the reasons just given, the following general statements 

 may not apply to individual cases. With the entire plant, the 

 percentage of potash in the ash decreases almost invariably, 

 while, as a rule, silica increases, magnesia increases, lime de- 

 creases, and phosphoric acid decreases in some cases and in 

 others increases. 



The above statements are made with respect to the entire plant. 

 Exceptions occur with certain parts of the plant, as, for example, 

 the ears of oats. 



An example is presented in the following table. Samples of 

 the entire wheat plant were collected at different periods of 

 growth and subjected to analysis by Pierre: 1 



EFFECT OF STAGE OF GROWTH ON ASH OF PLANTS. 



1 Wolff's Ashen Analysen. 



