nitrates. It was held that as plants (other than the legumes) practically take in all their nitrogen as nitrates, then the rate of nitrate-making or the nitrifying power of a soil would be on one side at least a measure of its fertility. In the course of the experiments on the partial sterilization of soil by heat or anti- septics it has become apparent that the nitrification organisms are very susceptible and may be killed off while the ammonia- making organisms are still active. Again acid soils have been found in which nitrates are not produced. Yet in such soils plants grow freely, taking in their nitrogen as ammonia, not as nitrate. It becomes clear that nitrification is only the end process, and the rate at which it will proceed is determined in a normal soil by the rate at which the other organisms supply ammonia. This is seen from the fact that nitrates will heap up in the soil, whereas the ammonia remains comparatively con- stant at a very low level provided that the soil is normal and nitrification is going on.
For a long time the only organisms capable of " fixing " nitrogen, i.e. bringing the free gas from trie atmosphere into combination, were the so-called " nodule " organisms (Pseit- domonas radiclcola) discovered by Hellriegel and Wilfarth, which live in symbiosis with the leguminous plants. More recent in- vestigations have discovered methods whereby these organisms can be grown and made to fix nitrogen independently of a host plant, and have also cleared up the forms in which they exist in the soil and find their way into the roots of the leguminous plant. The attempts to improve the growth of leguminous crops by inoculation with strains of the particular organism have not been attended with any practical success, though soils, generally of the new or reclaimed order, destitute of the nodule organism, can now be effectively inoculated and thereby made to grow good crops of legumes, provided always that the soil is first made a fit medium for the organism by a supply of lime and appropriate mineral manures. Without this preliminary acid heath or peat soils would neither support the nodule organisms nor the legu- minous crops and inoculation would be of no avail. But as " fixers " of nitrogen apart from the leguminous plants Pscu- domonas radicicola is ineffective compared with a widespread group of organisms isolated by Beijerinck, to which he has given the name of Azotobacler.
Azolobacler. These organisms, found in both virgin and cultivated soils from all parts of the world, are comparatively large oval bodies 4 to 5 /t in length and 3 /i in width, which differ from normal bacteria in containing glycogen and act as powerful agents for the oxidation of the sugars and other carbo- hydrates. From the carbohydrates they produce in the main carbon dioxide and water, but also small quantities of organic acids and of a characteristic deep brown pigment. It is by means of the energy derived from the oxidation that they are able to bring nitrogen into combination and the nitrogen fixed under favourable laboratory conditions may amount to i% of the carbohydrate oxidized. To be effective Azotobacler requires certain conditions a neutral medium with calcium carbonate present to neutralize the acids produced, for which reason the organism is generally absent from acid soils, also the presence of such nutrients as phosphoric acid and potash, and finally a favourable temperature. It has been found at Rothamsted that a soil will accumulate nitrogen, as evidenced by an increased crop, after the application of starch or sugar, carbohydrates containing no nitrogen, if these materials are mixed with the soil in the early autumn when the land is still warm and Azolo- bacler is active. On the other hand spring applications of carbo- hydrates are followed by a diminished crop, because at a low temperature other organisms in the soil which are consumers of combined nitrogen, attack the carbohydrate and by their multiplication withdraw some of the soil nitrogen from circula- tion and so reduce the supply for the crop.
The great significance of these observations of the mode of action of Azolobacler is that they afford a solution of the problem of how the great stocks of combined nitrogen came to be accumulated in virgin soils, especially in certain black soils such as occur on the prairies and in the Canadian North-West. Of itself the mere growth and dying down of vegetation for however many years repeated, could
not add to the stock of combined nitrogen in the soil. The plant itself fixes no nitrogen, but only draws upon the capital in the soil, restoring whatever it took out when the vegetation is allowed to die back to the soil without loss. But the falling vegetation contains carbohydrates derived from the air and if they are added to a soil containing A zotobacter under conditions favourable to its growth, the carbohydrate supplies the energy whereby the Azolobacler can fix some more nitrogen from the air and add to the stock in the soil. In this way the annual cycle of vegetation when the leaves fall back to the soil can result in a yearly accretion of nitrogen which in time may amount even to the remarkable accumulation found in the deep black soils of Manitoba and similar " steppe " lands, soils that are invariably found to be well supplied with carbonate of lime and also to contain the Azotobacter organism. The clue to this interpreta- tion of the accumulation of nitrogen in virgin steppe and forest soils was derived from the examination of the soils of the wheat field at Rothamsted. The soil of the unmanured plots which has been in arable cultivation for over half a century shows a steady decline in the amount of nitrogen it contains, a decline which is approximately equivalent to the nitrogen which is known to have been removed in the crops harvested year by year. Doubtless the soil has suffered other losses of nitrogen by drainage, removal of weeds, etc., that cannot be estimated, but the analysis of the soil shows that any recuperative processes which may have been at work restoring nitrogen to the soil have only been able to repair these minor losses butvnot to restore any of the nitrogen removed in the crops. A portion, however, of the same plot was allowed to go to waste, i.e. it was allowed to cover itself with a natural vegetation of weeds and grasses, which were neither cut nor grazed but allowed to die back to the soil. After 30 years an examination of the soil of this wilderness showed it had been accumulating nitrogen at the rate of nearly 100 Ib. per ac. per annum, the greater part of which must have been due to the action of Azotobacter working upon the carbonaceous matter supplied by the decaying vegetation reaching the soil in the autumn and winter.
On the arable land where the vegetable matter reaching the soil is minimal, only the roots and stubble of the crop, there is a- steady loss of nitrogen ; on the wilderness which may be compared to a natural prairie, the return of the vegetation to the soil causes nitro- gen to accumulate not because of the nitrogen contained in its material, but because its carbonaceous matter supplies the energy whereby the Azotobacter fixes nitrogen. The Azotobacter group of organisms, though not the only ones capable of bringing free nitrogen gas into combination, constitute the group which has played the fundamental part in building up not merely the vegetable soil but the whole substratum of organic life in the world.
Soil Protozoa. The outlook on the organisms in the soil has been entirely changed since Russell and Hutchinson showed the part played by the protozoa in limiting the development of bacteria in the soil. The soil protozoa, which are large, definitely animal organisms of varied character amoebae, ciliates and flagellates exist in large numbers in all cultivated soils, and as they feed upon bacteria, any conditions which encourage the development of bacteria by increasing their food -supply stimulate the multiplication of the protozoa which thereby put a check to the increase of the bacteria. Thus normally the number of bacteria in a soil, however rich and favourable to bacterial development the conditions may be, does not pass a certain limit because it is kept in check by the increasing number of the protozoa. As the fertility of the soil among other things depends on the rate of production by bacteria of ammonia and nitrates from the nitrogenous residues in the soil, the fertility of the soil is also limited by the presence of the protozoa. Certain processes of partial sterilization of the soil, such as heating to the tempera- ture of boiling water or even to 1 70 F. or again treatment for a time with some antiseptic, e.g. chloroform or toluene vapour, effects a selective destruction of the soil organisms. The protozoa are almost entirely killed off, but many groups of bacteria, notably the ammonia-makers, resist destruction though they may be reduced in numbers. But if after treatment the treated soil is placed under normal conditions for growth, the bacteria that remain multiply with great rapidity and rise to a level of numbers and activity they were unable to attain before, because now the protozoal check to their multiplication has been removed. In consequence the fertility of the soil is greatly increased, in fact the yield from a given soil may be doubled. This discovery suggests immense potentialities of increased production from the land but as yet it has not been found possible to apply the method of partial sterilization to ordinary field soils in the open. Heating would be inordinately expensive and the difficulty is to find an antiseptic that combines cheapness with the right degree of
