igoo] Shutt — Soils. 47 



From the data presented it is evident that the amounts 

 per acre of mineral plant food of immediate agricultural value are 

 very small, compared with the amounts of total plant food present. 

 Nor must we suppose that the whole of these supplies — small as 

 they are — can be secured by any crop, for its root system occupies 

 necessarily a more or less restricted area and does not envelope 

 every soil particle. A poor physical condition of the soil and lack of 

 sufficient moisture are factors that still further prevent the utiliza- 

 tion of this available plant food. One of the chief functions of 

 mechanical processes for disturbing- soil is to hasten the conversion 

 of inert material into these more valuable compounds. The principal 

 object — indeed, in most instances the only object — in applying 

 manures and fertilizers is to add to this store of available plant 

 food. The quantity of soluble food so added is insignificant, com- 

 pared with that already present in an insoluble state, but the in- 

 creased yields resulting, fully corroborate the statement that a 

 soil's productiveness should be measured by the amounts of its 

 plant food which are more or less available, rather than by the 

 amounts of that shown by extraction by a method of analysis 

 employing strong mineral acids. This view can scarcely be unduly 

 emphasized; it explains, as we shall see, in a large degree, the value 

 of the clover crop as a fertilizer, which we shall now consider. 



We have already mentioned that the legumes — of which 

 clover is a prominent member — have a source for their nitrogen 

 other than and additional to that present in the soil. Like other 

 plants, they are unable to absorb free nitrogen of the air through 

 their leaves ; like in other plants, that which they absorb through 

 their rootlets must be as nitrates. In what way, then, is the in- 

 disputable fact that they can make use of atmospheric nitrogen to 

 be explained ? The careful researches of Hellriegel, Wilfarth, 

 and other chemists have shown that the legumes obtain the nitro- 

 gen of the air existing in the interstices between the soil particles 

 tht^ough the agency of certain micro-organisms present in the soil. 

 These bacteria, whose special function is the assimilation of free 

 nitrogen, attach themselves to the roots of the growing clover or 

 other legume, forming thereon nodules or tubercles. These 

 nodules, swarming with countless hosts of the germs, are to be 

 found in sizes varying from a pin's head to a pea, and frequently 



