ASH. II 



93 



as salts or as oxides. In the following summary we will follow in general the 

 statements of Ad. Mayer (1895, II, i) : — 



1. The nitrates, sulphates, and chlorides of the metals above mentioned 

 undergo transposition in the soil, so that the metals in these salts are exchanged 

 for others occurring there. These latter go into solution whilst the former are 

 absorbed. Thus potassium is more readily absorbed than ammonium, and that 

 in turn more readily than magnesium, &c., so that we can affirm that in the 

 series potassium, ammonium, magnesium, sodium, calcium, the succeeding 

 element can be ousted by its predecessor from its combination ; more especially 

 is it the case that frequently on the addition of salts of alkalis to the soil calcium- 

 salts pass into solution. No rule is, however, without its exception, for, on 

 the contrary, potassium can be turned out of combination by ammonium and 

 sodium. This replacement of bases takes place most commonly if double sili- 

 cates of aluminium are present in the soil whichin addition to aluminium contains 

 another base as well. The accessory base suffers replacement while the aluminium 

 remains permanently united with silicic acid. Humates act like double silicates 

 but less actively. 



2. Oxides of alkalis and alkaline earths, as also their hydrated oxides, are 

 absorbed in the first instance through the agency of humic acids, although no 

 replacement takes place as in the first case. Double silicates and even pure 

 caolin can act like humic acids. 



3. As already noted phosphoric acid is apparently the only acid firmly 

 retained, and that, too, whether it occurs as a salt or a free acid ; its retention is 

 effected by carbonate of lime or even more firmly by hydrated ferric oxide. 



The question of the causes of absorption, so often discussed by agricultural 

 chemists, more especially as to whether absorption is to be regarded as a chemi- 

 cal or as a physical question, need not concern us ; what we know is that ab- 

 sorption takes place only in clay, chalk, or humus-containing soils, but not in 

 quartz sand ; we are more concerned with the ejects of absorption than with its 

 causes. Soils formed by disintegration of rocks which are originally poor in 

 salts suitable to act as plant nutrients, are thus gradually rendered more fertile, 

 more especially since the substances absorbed by the soil are largely protected 

 from being washed away by rain, although they are still capable of being taken 

 up by the plant. Protection from removal by water is, of course, not absolute, 

 but, according to Peters (i860), it requires in most cases a very considerable 

 amount of water to remove an absorbed substance from the soil ; for example, 

 28,000-36,600 parts of water are required to remove one part of potassium. 

 Finally, it is of the highest importance that the individual substances so 

 absorbed, if they be firmly combined, should be distributed in the soil in a state 

 of minute subdivision. It will be seen later that in such a condition they are 

 much more easily accessible to the plant. 



Plants in the soil encounter an extremely dilute solution of nutritive salts, 

 and, in proportion as they take up from it the more important of these salts 

 for their own use, other salts previously absorbed will pass into solution. At the 

 same time we must not assume that the plant's supplies are obtained only from 

 the substances in solution ; the plant is also able to make use of solids which it 

 brings into solution. Although such solution phenomena are well seen in the lower 

 plants, especially in lichens, still to avoid prolixity, we will confine our attention 

 to the special characters of the root-system in the higher plants so far as they 

 concern the absorption of minerals. In our earlier consideration of the root as the 

 organ of absorption of water, opportunity was taken to speak of its mode of 

 distribution in the soil. As a general rule the root meets with water and nutritive 

 salts in the same place, and there are few adaptations known which are especially 

 concerned in the absorption of salts only. Among these, the intimate fusion of 

 the roothair and the soil particles may be especially noted, and that subject we 



