244 BOTANY PART I 



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 ( 26 ). 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 contains 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 MgS0 4 . 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 sub- 

 division, as what is known as " Thomas-meal." Superphosphate is 

 obtained by the treatment of potassium phosphate with sulphuric acid. 

 The Soil and Plant Geography. 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 other species. Plant geography ( 27 ), 

 however, shows that the composition of the soil exerts a great influence 

 on the distribution of plants. This depends, on the one hand, on the 

 fact that different plants make different demands on the amount and 

 solubility of the essential food-materials, and, on the other, upon the 

 presence in the soil of substances other than the indispensable salts ; 

 the influence of these non-essential substances is different upon different 

 species of plants. For example, CaCo 3 has a poisonous effect on some 

 plants, and NaCl upon others, while other plants can endure large 

 doses of these substances. 



The effect of the soil upon the distribution of plants does not depend merely 

 upon its chemical nature. The physical properties of soils play an important role. 

 Further, a plant may be absent from a locality, which, so far as the nature of the 

 soil is concerned, would be suitable, because its seeds have never been brought to 

 the spot. 



(e) Gases 



While water and salts are, as has been seen, as a rule absorbed from 

 the soil, the air contains substances which are necessary to the success- 

 ful existence of the plant, and must be termed food-materials. These 

 are carbon dioxide and oxygen. They are, as a rule, obtained from 

 the atmosphere. Only submerged water plants obtain them from the 

 water, in which case they are absorbed in the same way as other 

 dissolved substances. 



Oxygen. When a plant is deprived of oxygen, all vital manifesta- 



