SECT. II PHYSIOLOGY 209 



tions to the contrary, it is still probable that, in addition to carbonic 

 acid, other acids or acid salts may be excreted by the root ; this would 

 render possible further absorption of mineral substances which are 

 difficult to dissolve. Plants with marked excretion of acids (Maize, 

 Rye, Oats) make less demand on an already opened up good soil than 

 do those with weaker (Wheat, Barley) or scarcely noticeable excretion 

 of acid (Meadow Grasses). The most marked power of decomposing 

 mineral substances is shown by certain Lichens which live upon 

 granite and are able to dissolve the mica (-''). The solution of solid 

 rock by the plant may most readily be shown by allowing the roots 

 to grow against smooth polished slabs of marble ; the course of the 

 roots is indicated by the etching of the surface. 



Some soils, especially those containing nmcli clay, lime, or liumns, have the 

 property of retaining potassium and ammonium salts, and in less degree salts of 

 calcium and magnesium, as well as phosphates ; these substances are not easily 

 washed out of the soil but can be obtained by plants. This is spoken of as the 

 power of absorption of the soil for the substances in question. This does not hold 

 for all salts ; thus, for instance, sulphates and nitrates are not absorbed. Absorp- 

 tion is completely wanting in a pure sandy soil. It is impossible to enter here 

 into the causes of the absorption in various cases. 



Since the plant thus continues to absorb nutrient salts from the 

 soil, this must become poorer in the particular substances unless the 

 loss is repaii'ed in some way. In nature this results from the fallen 

 and dead parts of plants returning to the soil, and the salts contained 

 in them becoming available for further life. In agricultural practice, 

 however, a large proportion of the vegetation is removed in the crop, 

 and the salts it contains are thus lost to the ground ; at the most a 

 fraction may be returned to the soil in the dung of grazing animals. 

 The effect of manure in increasing growth, which has for ages been 

 known to practical men, depends at least in part on the salts contained 

 in it. Since, however, the amount of salts thus returned to the soil 

 is insufficient to meet the loss, artificial manuring is required in 

 agricultural practice (-"). The first place among manures must be 

 given to those which contain nitrogen, potassium, and phosphoric acid. 

 Nitrogenous substances which are used besides guano (which also con- 

 tains phosphoric acid) are Chili saltpetre, ammonium sulphate, calcium 

 cyanamide, and calcium nitrate ; the two last have recently been 

 artificially prepared from atmospheric nitrogen. Potassium is present 

 in the Stassfurt waste salts, of which kainite is the most important 

 since it also contains MgSO^. As an important source of phosphorus, 

 the so-called Thomas slag may be mentioned ; this substance is formed 

 in working ores containing phosphorus, and consists of triple phosphate 

 of calcium. It can only be utilised by plants when in a state of very 

 fine subdivision, as what is known as " Thomas-meal." 



From what has been said it might be concluded that a soil 

 capable of supporting one kind of plant must be able to support any 



