No. 6. DEPARTMENT OF AGRICULTURE. 807 



between the two takes place, under the action of sunlight, by which 

 these elements are combined in such a way as to produce starch. 



Starch therefore accumulates in the leaf as a result of this process 

 known as assimilation. 



The various changes w'hich the starch undergoes, and the manner 

 in which it contributes to the nutrition of the plant, is a matter be- 

 yond the limits of our subject. But sullice it to say that 98 per cent, 

 of the organic portion of the plant is manufactured by the process 

 here indicated, so that it may be said that in the main the plant gets 

 its food from the air and from pure water. But these elements alone 

 will not suflQce to maintain plant life; in fact no plant can grow 

 without that vital substance within its cells known as protoplasm. 



Plants grow by a multiplication of their cells, and cells empty 

 of protoplasm are dead. 



Protoplasm, besides containing the elements, carbon, hydrogen and 

 oxygen, also contains about 16 per cent, of nitrogen. Most agri- 

 cultural plants also contain in their dry water-free state from one- 

 half to two per cent, of nitrogen in the form of proteids. 



Plants obtain their nitrogen mainly from the soil, and so import- 

 ant is this element to their growth that a soil may be said to be rich 

 or poor as its contents is high or low in nitrogen. In fact the prob- 

 lem of agriculture to-day is to supply to the soil an abundant store 

 of this essential element. 



The nitrogen of the soil \b, in the main, stored away in its humus 

 content, hence soils rich in humus are also rich in nitrogen. Thus it 

 is nitrogen which the agriculturist seeks when he migrates to the 

 prairie loams rich in humus, or when he reclaims the forest to possess 

 a virgin soil. In fact it is the nitrogen problem with which the 

 soil bacteriologist is more concerned than with any other, and its 

 importance and bearings will be made more apparent as we proceed. 



As has been intimated, from four to seven per cent, of the dry 

 weight of the plant is composed of inorganic or mineral matter. In 

 this portion we recognize, as most important, potash, soda, magnesia, 

 lime, iron, phosphoric acid, sulphuric acid, chlorine and silica. 



These occur usually in abundance in all soils, although not always 

 in an available form; in fact many of them exist in an insoluble state 

 and need first to be digested or rendered soluble before they can be 

 absorbed by the plant. 



Soil bacteriology is partly concerned with those processes in the 

 soil by which stores of mineral food are unlocked to growing crops. 

 But to understand these processes in full it will be necessary to con- 

 sider for a moment the question of the origin of soils, and thus trace 

 each step in the operation. 



