20 NUTRITION OF PLANTS 



The supply of the first three elements comes from the air and water, 

 the remainder are taken up in the form of salts from the soil. 

 Nitrogen is of the greatest importance, being an essential 

 constituent of every organic body. It constitutes the chief bulk of. 

 our atmosphere, but plants (other than Leguminous species) are 

 unable to assimilate it in a free state, that is, unless it is first 

 chemically combined with another element. Plants most probably 

 absorb their nitrogen from the soil in the form of nitrate or 

 ammonia compound. Besides nitrogen, the other elements likely 

 to be deficient in the soil are phosphorous and potassium. These 

 are often supplied in the form of phosphates and potash salts. 



Soil bacteria and Nitrification. It is now known that the 

 appropriation of nitrogen by plants is clue to the agency of bacteria, 

 which exist in all fertile soils. It is considered that the presence 

 of active bacteria in the- soil is as necessary to plants as the working 

 of the yeast plant is to the brewer. These microscopic organisms, 

 or ferments as they are called, perform the useful function of con- 

 verting the nitrogenous organic matter in the soil into nitrates, or 

 soluble plant-food. The process is accomplished by the action of 

 two separate groups of bacteria, and is termed nitrification ; one 

 group of these organisms convert ammonia into nitrites, and the 

 other change nitrites into nitrates. Although both organisms are 

 always present in fertile soils, the one cannot perform the work of 

 the other. A favourable degree of warmth and moisture are 

 essential to the active operations of these bacteria ; at low tem- 

 peratures their work is retarded, and at a certain degree of cold- 

 ness it practically ceases. Hence an open porous soil, assisted by 

 good tillage and mulching, encourage their activity and thereby 

 tend to promote oxidation of the organic matter in the soil. 



Nitrogen-collecting Bacteria. It is well-known that on the 

 roots of certain Leguminous plants there usually occur nodules 

 or tubercles, which vary in size from that of a pin's head to a pea. 

 These nodules contain a species of bacteria \vhich abstract and fix 

 the free nitrogen of the air. The nitrogen thus becomes stored up 

 in a combined form in the roots and stems of such Leguminous 

 plants, and when the roots of these are left in the ground, or the 

 whole crop dug in as green-manure, the soil is considerably 

 enriched thereby with nitrogen. Only Leguminous plants (chiefiy 

 those of the sub-tribe Papilionaceae) obtain their nitrogen in this 

 way, all other plants, so far as is known at present, depending for 

 their supply on nitrates formed by the soil bacteria from organic 



