ii. 1 SOURCES OF PLANT FOOD. 2 1 



solid matter cannot enter into a plant so 



long as it retains its solid form ; but it may be 

 received when it has become a liquid, by being 

 dissolved in water, or when it has taken the form 

 of gas. It may therefore be taken as a rule, that 

 the inorganic matter in plants is obtained only 

 from those portions of the soil which are 

 soluble, or capable of becoming soluble. There are, 

 however, two bodies carbonic acid and am- 

 monia which are of necessity associated not only 

 with the inorganic bodies, but are also present with 

 the organic group. They are, moreover, to a certain 

 extent exceptional, for plants not only receive these 

 and water with the soluble matters obtained from 

 the soil, but they also receive them from the stores 

 existing in the atmosphere. 



CHAPTER III. 

 FERTILITY OF THE SOIL. 



36. In addition to the physical and chemical classi- 

 fication of soils, we have another point of character 

 which is distinctly recogniz^"ancT determined by the 

 cultivation of the land, viz., the fertility or barrenness 

 of the soil. We can explain by physical: and chemical 

 investigations, the causes _which jnfluence the produc- 

 tive powers of land, and in many cases these researches 

 indicate the means whereby those powers may be 

 increased or maintained. In the first place, a clear 

 distinction must be drawn between those portions of 

 the soil which are capable of yielding nour- 

 ishment to vegetation, and those which cannot do 

 so. A soil may contain large supplies oi 

 every ingredient which a crop requires, and may 

 still be unable to yield them to the plant. 

 The great truth must be fully realized, that it is 



