Scientific Agriculture. 



270 



[March, 1909. 



buds, seeds, flowers, growing tips, and, in 

 fact, in all parts rich in the living plant 

 substance, protoplasm. Hence a steady 

 supply of nitrogen must be obtained in 

 the food of plants, although in this res- 

 pect the plant is far less wasteful than 

 the animal and may use the same nitro- 

 gen over and over again once it has been 

 absorbed. The source of the plant's 

 supply of nitrogen is almost solely from 

 the nitrates of calcium, magnesium, 

 potassium, and sodium which are con- 

 tinually being produced in the presence 

 of the requisite bases in every fertile 

 soil during the decomposition of its 

 nitrogenous humus. The nitrogen of 

 this humus is oxidized in the presence of 

 free oxygen by nitrifying bacteria, 

 which are present in all ordinary soils. 

 Ultimately nitric acid is formed, usually 

 with ammonia and nitrous acid as inter- 

 mediate products, and this nitiic acid 

 combines with the alkaline bases present 

 in the soil to form nitrates. 



This process is a continuous one, and 

 takes place most rapidly when the soil 

 is warm, moderately moist and well 

 aerated. It stops, or becomes very slow, 

 if the soil is at all acid, completely dry, 

 very cold or in a swampy, badly aerated 

 condition owing to an excess of water. 



When a soil dries up in which nitri- 

 fication has been active, the nitrates may 

 sometimes be so abundant as to form an 

 efflorescence on the surface of the soil- 

 Since all these nitrates are, however, 

 soluble in water, any heavy shower of 

 rain will tend to wash out the excess of 

 nitrates from the soil before the roots 

 have time to absorb them. Hence there 

 is far less danger of loss, and a more 

 permanent effect is produced when the 

 nitrogen needs of the plant are supplied 

 by the bacterial oxidation of humus 

 manures applied to the soil, than when 

 the same amount of nitrogen is directly 

 applied in the form of a dressing of 

 Chilian saltpetre (sodium nitrate). 



Although the air, which contains four- 

 fifths by volume of nitrogen, represents 

 an enormous total amount of this ele- 

 ment, it is not directly of use except in a 

 few special cases. Ordinary green plants 

 can make no use whatever of the free 

 nitrogen of the air. It enters the plant, 

 and is found dissolved in the sap in 

 every part, but cannot be assimilated or 

 used as food any more than horses can 

 digest the sand which they often 

 swallow when grazing. Every thunder- 

 storm, however, and also certain slow 

 chemical processes of oxidation like 

 that of phosphorus in moist air cause 

 the oxygen and nitrogen of the air to 

 combine, ultimately forming nitric acid, 

 which is washed down by the first 

 shower of rain, In the same way, when- 



ever the air contains ammonia derived 

 from manure heaps, or from decaying 

 animal or vegetable matter, this is 

 washed down by the rain, and oxidised 

 to nitric acid and nitrates in the soil. 



The amount of combined nitrogen avail- 

 able for the plant's use which reaches 

 the soil in this manner, although quite 

 appreciable, is never more than a small 

 fraction of that removed from the soil 

 by drainage and by the crops. Under 

 the most favourable circumstances it 

 does not represent more than a tenth 

 or twelfth of the annual loss of nitrogen 

 from a well-drained, cultivated soil with 

 an average rainfall, and is usually 

 considerably less. 



Certain organisms exist, however, in 

 most soils which have the power of 

 assimilating the free nitrogen of the air 

 and ultimately enriching the soil with 

 combined nitrogen available for the 

 plant's use. The most important of 

 these orsranisms is Clostridium Pasteuri- 

 anum, an an&erobic bacterium, which is 

 unable to exist in ordinary soils unless 

 supplied with free nitrogen and unless 

 associated with certain other micro- 

 organisms. The latter shield it from the 

 oxygen of the air, which is poisonous to 

 this organism. If all the required con- 

 ditions were fulfilled, soils containing 

 this bacterium might gain from 20 to 

 30 lbs. of nitrogen ( = 120 to 180 lbs. of 

 sodium nitrate) per acre per annum. As 

 a matter of fact, the actual gain due to 

 the presence of this and similar micro- 

 organisms appears usually to be com- 

 paratively small, partly because the con- 

 ditions are rarely the best possible and 

 partly because denitrifying bacteria are 

 usually also present which set free the 

 combined nitrogen of the soil and may 

 in some cases cause a loss instead of a 

 gain to occur. In addition, the con- 

 ditions which favour the fixation of 

 nitrogen by soil bacteria not dependent 

 upon leguminous plants are, in general, 

 not the most suitable ones for the 

 development of ordinary crops. 



It is well known that leguminous plants 

 differ from ordinary ones in having the 

 power of assimilating the free nitrogen 

 of the air, and hence can exist on the 

 poorest and sandiest soils where nitrates 

 are practically absent or very deficient 

 in amount. They have, however, this 

 power only when peculiar tubercles are 

 developed on their roots which contain 

 nitrogen fixing bacteria derived from 

 the soil or originally present on the 

 coats of the seed. These root tubercle 

 bacteria penetrate the young roots and 

 there give rise to tubercular swellings, 

 in which the bacteria are nourished and 

 carry on the assimilation of free nitrogen 

 for the benefit of the plant bearing 



