for September, 1920 



313 



nitrification is carried on by the ceaseless activities of several 

 species of micro-organisms classed as bacteria, and recent in- 

 vestigations at New Jersey and Rothamsted seem to suggest that 

 in addition to bacteria, minute plants classed as ferments, molds, 

 etc., also assist in the process of nitrification. 



The fascinating story of the nitrifying organisms is now 

 becoming an old one and we have previously in these lessons 

 briefly outlined their work, but the story bears retelling, es- 

 pecially for the bcnetit cf the many new readers of this journal. 

 The nitrogen found in the soil is derived chiefly from the 

 accumulated remains of animals and plants, but as nitrogen, it is 

 useless to be used again as plant food. Not until the nitrogen 

 ha.i been changed into a nitrate does it become available. This 

 change can only take place by the work of micro-organisms, and 

 the passage of soil nitrogen into a nitrate involves several stages 

 each complete in itself, and each stage is the work of a distinct 

 species of bacteria. 



Roughly speaking, nitrogen is first broken up into ammonia, a 

 process involving several changes. If the ammonia were left 

 to itself it w-ould be partly lost into the atmosphere and partly 

 changed into carbonate of ammonia, a substance of which the 

 plant may possibly utilize indirectly small quantities, but which 

 in excess is harmful to plant life. But the ammonia is not left 

 to itself. It is at once seized upon by another set of bacteria — 

 quite distinct from those bringing about the existence of the 

 ammonia — which change it to nitrite; this in turn is acted upon 

 by another micro-organism, also quite distinct from any other, 

 and changed to nitrate. Strictly speaking the bacteria do not 

 actually form the nitrate, but they form nitric acid, which acid 

 immediately combines with and is neutralized by some base, 

 provided such base be present, generally liine as being the most 

 active of bases, although it may combine with soda or potash, 

 and the result is a nitrate of one of these bases ; such nitrates, 

 being soluble in water, are therefore easily available for the 

 plant. 



.•\11 living organisms, if they are to live and carry on their 

 work, must have their proper food requirements and live under 

 a suitable environment, and in these requirements the nitrifying 

 organisms are no exception to the rule. 



Two conditions which are highly detrimental to these nitrifying 

 organisms are acidity of the soil and the lack of air. They also 

 require food, which they obtain from phosphates and other min- 

 erals already in the soil. While they cannot carr>' on their 

 work in acid soils, neither can they do so in those of extreme 

 alkalinity. Also, as is implied by a previous statement, nitrates 

 cannot be formed without the presence of some 'basic inatcrial. 

 .Another requirement is an adequate supply of oxygen. Thisis 

 obtained from the air in the soil. When soils are saturated with 

 water or in a condition known as water-logged, nitrification 

 cannot go on for want of oxy-gen. The formation of a hard, 

 dry crust on the surface of soils also checks nitrification. In a 

 very dry period the process is arrested for want of water. 

 Temperature is also a factor, the most favorable being from SO 

 to 93° F. The presence of organic matter in the soil is also 

 necessary and when thoroughly decomposed nitrifying bacteria 

 obtain part of their food from it. but the presence *of much 

 partly or undecomposed vegetable or animal matter, otherwise 

 known as organic, materially checks the work of these organisms. 

 In order to allow nitrification to proceed all the necessary con- 

 ditions must exist, and the process is frequently checked or 

 stopped entirely either because one or more of these conditions 

 are unfulfilled, or because of the existence of conditions which 

 are distinctly adverse to the life of the bacteria. 



F.'xcepting temperature out of doors, the cultivator has the 

 creating of the necessan,- conditions for nitrification practically 

 under his control. He can by cultivation cause his soil to be 

 well aerated so as to supply plenty of oxygen, which cultivation 

 also tends to keep the soil moist: he can correct acidity by liming, 

 which at the same time supplies a base for the nitric acid to 

 combine with ; organic matter can be supplied by the turning 

 under of cover crops and the phosphates can be' obtained from 

 those already in the soil. It will therefore be apparent that 

 along these lines available nitrogen can be supplied to the soil 

 at a merely nominal cost. 



Thus far we have considered nitrogen from the point of view 

 of rendering available the organic nitrogen contained in the 

 soil, but there are two other processes, which as they involve the 

 use of atmospheric nitrogen, are strictly speaking perhaps out- 

 side the range of our present discussion although as these 

 processes take place in the soil and are connected with the use 

 of nitrogen contained in the soil atmosphere, and have no con- 

 nection with the addition of nitrogen in the form of fertilizer 

 to the soil, it appears fitting to consider them. 



Outside the question of classifying the micro-organisms_ as 

 members of the vegetable kingdom, plants may be divided into 

 two classes, namely, nitrogen consutners and nitrogen gatherers. 

 The former obtain their nitrogen from the soil in the form of 

 nitrates, while the latter gather their nitrogen from the atmos- 



phere through the agency of parasitic bacteria which are found 

 in the nodules on their roots. Practically all plants belonging 

 to the family Lcgitminosa are nitrogen gathers, such as peas, 

 beans, clover, alfalfa, etc. Each separate genus, and perhaps 

 in some cases species as well, have their own special bacterium, 

 and this bacterium confines its attentions to its own plant. The 

 species ot bacterium found in the nodules on clover roots will 

 not work upon alfalfa roots, nor will those found upon peas 

 or beans work upon clover or alfalfa. The bacteria inhabiting 

 the root-nodules of the principal crop plants belonging to the 

 above family have been isolated and cultivated separately, and 

 can now be obtained through trade channels for the purpose of 

 inoculating the seed of any species or the soil upon which it is 

 to be grown so as to be sure that such plant's special bacterium 

 is present. 



This inoculation is, however, frequently barren of results from 

 want of recognition of the fact that, in common with those 

 previously mentioned, these bacteria must have right soil con- 

 ditions for the carrying on of their work. As they obtain the 

 nitrogen upon which they work from the soil atmosphere, aera- 

 tion of the soil is, of course, absolutely necessary ; there must 

 also be a supply of phosphates and potash, with sufficient lime 

 to prevent acidity. If any one of these conditions are absent, 

 or any distinctly adverse conditions, such as want of drainage, 

 present, then all the inoculation in the world will be useless. 



Crops of this family of plants always enrich the soil in nitro- 

 gen, even when entirely removed from it, and a crop of clover 

 turned under will add some two hundred pounds of nitrogen to 

 the acre, which nitrogen would cost today in the form of nitrate 

 of soda, about eighty dollars. We see at once w-hat great op- 

 portunities the gardener and the farmer have to obtain their 

 nitrogen almost for nothing, if they make leguminous crops 

 serve as the source of nitrogen for crop production. It must 

 be borne in mind that the nitrogen in the clover or other simi- 

 lar crop turned under docs not become available until acted 

 upon by the nitrifj'ing organisms first mentioned. 



.\nother source of available nitrogen is to be found as the 

 result of the activities of certain free-living bacteria which have 

 the power of fixing atmospheric nitrogen on their own account 

 and do not require the co-operation of a living plant. The work 

 of this class of micro-organisms has not yet been followed so 

 closely and so finally as with the others previously mentioned. 

 Their existence and initial work upon free nitrogen is known, 

 and their activities require the same environment as do other 

 nitrifying organisms. 



Unfortunately available nitrogen as such does not remain long 

 in the soil, that which is not used by the plant being easily 

 washed out by rain. Also the reverse of nitrification, namely, 

 denitrification, may take place and is the result of the workings 

 of a class of organisms which act upon the nitrates, forming free 

 nitrogen which is liberated as gas. One of the conditions for 

 denitrification is the absence of oxygen. Denitrification occurs 

 in soils saturated with water and where the soil is so com- 

 pacted that air is practically excluded. 



It is not possible to store our soils with available nitrogen as 

 it does not. as is the case with other plant foods, undergo fixa- 

 tion. Fixation is a chemical change taking place in the soil 

 whereby a plant food in an easily soluble form, like nitrates, 

 undergoes a chemical change and becomes less soluble, but which 

 is at the same time, as a rule, in an available condition or may 

 readily become so by cultivation, and in this state the plant by 

 the acid secretion from its roots is free to render soluble in 

 quantities and at times desired. 



This non-fixation causes a great loss of nitrogen to take place 

 when the ground is bare, as in the cases of a bare fallow, and 

 when a cover crop is not used in the .\utumn after the crops for 

 use have been removed. 



In addition to the micro-organisms which carry on the work 

 above mentioned there are a great many others, some of which 

 are benehcial while others are in various ways harmful, to plant 

 life. There are some of the latter which are injurious by reason 

 of the fact that they prey upon those which carr\' on the work 

 of nitrification. In a general way conditions which are harmful 

 to the beneficial bacteria are such in w'hich the harmful bacteria 

 thrive. .\n accumulation of partly or entirely undecomposed 

 vei'elablc matter in the soil, and the soil condition known as 

 "sick" due to continuous heavy manuring, all encourage the 

 growth of the harmful bacteria, as do also undrained and badly 

 cultivated soils, the want of lime and any other soil condition 

 harmful to higher plant life. 



These harmful bacteria preying upon the nitrifying organisms 

 are. while microscopic, of a considerably larger size, which size 

 causes them to be more susceptible to conditions which are 

 harmful to bacteria as a class, such as extremes of heat and cold. 

 Tliis enables us to adopt measures of partial sterilization of the 

 soil by which the harmful species are killed. Exposing the soil 

 to severe frost is destructive to them; the same effect is brought 

 about when "sick" soils are treated with quick-lime ; they are 



