CHEMICAL DETERMINATIONS CONCERNING THE SOIL. 131 



The reason is that the particular food needed by the plant has 

 not been given to it. Owing to this fact farmyard and other 

 vegetable and animal manures are justly valued, and this one 

 thing more than any other has been in their favour. They 

 contain all the necessary constituents required by the plant, so 

 that in using them the want is inevitably supplied. You may, 

 however, give large quantities of material in this way which are 

 not at all helpful because not in demand, but which may some- 

 times be rather injurious. Farmyard manures possess a large 

 number of valuable properties, but they are not in all cases the 

 most desirable to use. 



During the last few years a discovery has been made which 

 was at first received with considerable doubt ; it has now, however, 

 become established, and gives a greater amount of hope to 

 Agriculture (which is the twin sister of Horticulture) than any 

 discovery made in recent years. It lies in the notable fact that 

 leguminous plants have the power, by means of certain organ- 

 isms, of obtaining abundant quantities of combined nitrogen, 

 the nitrogen itself being derived without cost from the air. 

 This combined nitrogen is the most valuable of manures, and 

 gives, broadly speaking, the largest help to fertility. From this 

 discovery the intelligent cultivator has the power of obtaining 

 unlimited supplies of nitrogen absolutely free of cost. Recently 

 I examined a Vetch, the roots of which were covered with small 

 nodules ; these nodules occur on the roots of leguminous plants 

 generally in greater or less abundance. They are produced in 

 response to certain forms of bacteria, which may be seen by means 

 of a good microscope and proper preparations. Having a root with 

 nodules, place it in water, and allow the particles of earth around 

 it to become thoroughly saturated ; then apply the nodules to a 

 piece of glass; add a little red ink or methy-aniline violet dye, 

 which will stain them, so as to render them easily visible. These 

 nodules are enlargements of the root, presenting much surface 

 to the surrounding soil, by which surface nitrogen, as ammonia 

 or nitrates, can be absorbed. 



It is necessary to remember a law of excretion which may 

 be thus expressed. Every excreted product is injurious to the 

 organism excreting it. For example, when a plant absorbs 

 carbon-dioxide it must get rid of the oxygen, which stands in 

 the relation of an excreted product, or it will soon begin to 



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