138 Plant Physiology 



Knowledge of the ash content is of interest physiologi- 

 cally when related to plant behavior or work. 



73. Composition of the ash. - - A detailed analysis of 

 the constituents of the ash indicates that through absorp- 

 tion the plant obtains, as a rule, more or less of all of the 

 soluble mineral elements of the soil. Whatever occurs 

 in the soil solution is apt to be found in the plant to at 

 least a slight extent, although the plasmatic membranes 

 of the root-hairs show a certain definite selective absorp- 

 tion, as already indicated. Commonly eleven elements 

 are found in the ash, as follows: phosphorus, potassium, 

 calcium, magnesium, sulfur, iron, sodium, chlorine, silicon, 

 manganese, and aluminium ; and, generally speaking, 

 the soil is the only source of these elements. (Nitrogen, 

 likewise derived from the same source, is, of course, a part 

 of the volatile product.) 



Chemical analysis cannot determine with any degree of 

 exactness what the plant actually requires from the soil ; 

 but it is important because it gives a general indication of 

 the relation of plant to soil solution, it sheds some light 

 upon the general problem of nutrition, and it makes pos- 

 sible an exact computation of the amounts of mineral 

 nutrients which various crops remove from the soil. The 

 table on the opposite page compiled by Kedzie shows the 

 percentage composition of the ash of familiar crops. 



From these data it is obvious that there are certain 

 general relations worthy of recollection, such as these : 

 the seed is relatively rich in phosphorus and magnesium, 

 and usually deficient in calcium ; stems and leaves may 

 contain much calcium, and often a high per cent of silicon ; 

 while the fleshy roots here noted show the highest potas- 



