for January, 1920 



411 



through which a particular substance has to pass are each 

 of them handled by a separate organism which cannot 

 perform upon any other stage but its own. 



Space will not permit any attempt to trace bacterial 

 work in connection with all the above compounds, and 

 moreover all of it is not yet known ; but with regard 

 to protein, bacterial action is fairly well understood 

 and many of the species specializing in this connection 

 have been isolated and cultivated in the laboratory, and 

 named. 



Protein is the most complex compound entering into 

 [jlant structure, so much so that definite formulas have 

 not yet been applied to it, although chemists are well 

 acquainted with its chemical composition. While protein 

 is known as a nitrogenous compound it rarely contains 

 more than twenty per cent of nitrogen. The fact con- 

 nected with the work of bacteria which specialize upon 

 nitrogen are more fully known than they are in connec- 

 tion with other things, as these nitrifying bacteria have 

 been studied for a great number of years being the 

 first which was discovered. The importance of nitrogen 

 is further emphasized by the fact that it is never part of 

 the original disintegrated rock which is the basis of the 

 soil. Nitrogen is also the most expensive article of plant 

 food to purchase, and it is also the one which is the least 

 easy to retain in the soil. 



The decomposition of the organic nitrogen contained 

 in protein involves several processes ; amnToniiuii com- 

 pounds are first produced which take at least two stages, 

 each of which is under the charge of a distinct species 

 of organism. These compounds are then seized upon 

 by others and turned into nitrites which are changed by 

 still different ones into nitrates. It is only in the latter 

 form that soil nitrogen is available. The last two 

 changes are accomplished very quickly, which is fortu- 

 nate, as when ammonia or nitrates are in contact w'ith 

 the roots of plants for any length of time, harm mav 

 result. 



The artificial application of nitrate to the soil is gen- 

 erallv in the form of nitrate of soda, and its availability 

 is shown bv the quickness of its effects : although when 

 a plant is obtaining from the soil all the nitrogen it re- 

 quires it is possible to apply nitrates without producing 

 anv appreciable result, and in this case the application 

 may, to a great extent, be looked upon as waste. 



.Another set of nitrifying bacteria have the power of 

 fixing the free, or gaseous, nitrogen existing in the soil, 

 on their own account. 



The fixing of free nitrogen from the atmosphere is 

 greatly intensified when a leguminous crop is grown. If 

 wc pull up a root of clover, peas, or beans, we find little 

 nodules present, which, when examined under the micro- 

 scope, are seen to contain numerous bacteria. These 

 organisms are busily at work seizing nitrogen from the 

 air anrl building it up with compounds of use both to 

 themselves and to the plant. The necessary energy comes 

 not from decaving plant residues as is the case with other 

 bacteria, but from the iuices of the living plant : the or- 

 ganism is a parasite living upon its host, but it is one of 

 the few parasites that gives more than it takes. 



.All the various bacteria require food to Iniild up the 

 enersry necessarv to carry on their work. With the ex- 

 ception of those inhabiting the root nodules, they get their 

 food out of the soil which they obtain principally from 

 decomposed plant residues, although there is no doubt 

 they can to some extent make use of food, such as phos- 

 jjliates. etc., having a direct mineral origin. 



In addition to food, these bacteria require an ample 

 supjily of oxygen, although in the case of one species it 

 appears that it can work in the absence of this gas. or 

 at all events can do with much less than the others. 



THE STERILIZIXI.; EFFECT OF K.MN. 



From time immemorial practical men have felt that 

 rain had a fertilizing etTect. Ancient and medi.-eval 

 writers attributed this effect to some aerial spirit or 

 celestial nitre washed down. Leibig put it down to am- 

 monia. But since Liebig's statement, vast numbers of 

 analyses have been made of rain from all parts of the 

 world, and all agree in proving that there is not enough 

 ammonia or anything else in rain to make any difiference. 

 Everyone is, however, aware of the great benefit of rain ; 

 of what a brilliant green the lawn puts on immediately 

 after it, although a day or two before it may have been 

 parched from drought and of a brown color. That the 

 beneficial efifect of rain is due to something more than 

 the actual water supplied is well shown by the fact that 

 no amount of artificial watering will do as much good. 

 What then is the cause of the marked efifect of rain? 



During some soil investigations it was discovered that 

 soil possesses two atmospheres ; one the free atmosphere 

 in the soil spaces, and another atmosphere dissolved in the 

 moisture contained in the soil particles. This free at- 

 mosphere is much like our own, except that it contains 

 more carbon-dioxide and it is eminently suited to soil 

 organisms. The dissolved atmosphere is, however, en- 

 tirely diflferent inasmuch as it is almost entirely devoid 

 of oxygen, especially in dry weather, and consists mainly 

 of carbon dioxide and nitrogen. The fact that it exists 

 in such close proximity to the free atmosphere indicates 

 that the oxygen is used up more rapidly than it is re- 

 newed, and this shows that the plant roots and micro- 

 organisms are more or less constantly in need of more 

 oxygen. So far as is known there is no process in the 

 soil that will hurry up this renewal of dissolved oxygen, 

 and plants and bacteria arc. in its absence, restricted in 

 growth by the lack of it. 



Rain is practically a saturated solution of oxygen, and 

 when it falls upon the soil it not only supplies water, but 

 also renew^s the stock of dissolved oxygen, and thus gives 

 the plants and the micro-organisms a new lease of ac- 

 tivity. 



The great value of maintaining a plentiful supplv of 

 oxygen is one of the reasons why keeping the soil well 

 aerated by frequent cultivation between plants does so 

 much good. When a hard crust is allowed to exist the 

 work of bacteria is greatly checked, and in a compacted 

 soil their activities are almost brought to a standstill. 

 Also soil in this condition allows water to evaporate very 

 quickly and plants suffer from drought sooner than if 

 the surface of the soil is kept loose. 



r'RE\l-:XT.\TIVE FOR C.M'SES ir.ARMFl'L TO SOIL. 



On the other hand bacteria cannot carry on their work 

 in a soil that is continually wet and water-logged, be- 

 sides which such a condition prevents plants developing 

 root action. Hence the necessity for subsoil drainage 

 where such does not naturally exist. 



While for many reasons a ]ilentiful su])ply of humus 

 is a necessar}' condition of fertility, this hunnis is the 

 cause of soils becoming acid : the acidity is detrimental 

 to the beneficial soil population, especially those engaired 

 in nitrification, and at the same time it acts favorably 

 in assisting the growth and multiplication of those organ- 

 isms which work harm. 



To prevent soil acidity lime must be added, and its ad- 

 dition to the soil is also required because most of the 

 l)eneficial bacteria cannot exist without it. ajiart from any 

 question of acidity. In a previous article I pointed out 

 other ways in which lime is valuable in forming a fertile 

 soil, and it was then stated that either carbonate of lime 

 (ground limestone), or oxide of lime (quick lime) could 

 he used, as it has been for a long time supposed that both 



