22 THE SOILS AND CROPS OF THE FARM. 



Burning a plant causes it to divide into two 

 classes of substances; those which pass into the air 

 as gas or vapor, and those which remain as ash. The 

 first may be called the combustible; the second the 

 incombustible or mineral part of the plant. 



If the burning has been thoroughly done, the ash 

 will be found to be but a small part of the plant, 

 either in bulk or weight; generally not more than five 

 per cent, or one part in twenty of the total dry mat- 

 ter. The parts of the plant differ much in the per- 

 centage of ash or mineral matter. The leaves have 

 most; the stems next, and the seeds least. 



Plants obtain from the soil, by means of their 

 roots, all the mineral or ash matter found in them. 

 The soil also furnishes most of the water they con- 

 tain. The air furnishes to plants through their leaves 

 nearly all the matter, except water, which passes off 

 as gas when the plants are burned. 



The soil proper gives to plants but a small part 

 of their bulk or weight. Several wagon-loads would 

 be required to remove soil equal to a great tree in 

 weight or bulk. Had the matter of which the tree is 

 composed been mainly taken from the soil, the sur- 

 face would have been lowered so that the tree would 

 have stood in a hole. Instead of this the surface of 

 the soil about a large tree is often above the general 

 level. If thoroughly burned all the wood in the tree 

 would leave but a comparatively small quantity of ash. 



Chemical KlemeiitN in Plantis. — The chem- 

 ical elements usually found in plants are thirteen 

 in number — three gases and ten solids. Of these, 

 ten — three gases and seven solids — are essential 

 to the life and growth of plants. The other three 



