The most important work done by these soil bacteria is to bring about the production of ammonium salts and convert them into nitrates. These bacteria are most active at a temperature of 86° F. (30° C.) according to some authorities, and at 98° F. (37° C.) according to others, and become less active as the temperature rises above or falls below these points. It follows from this that nitrate-producing bacteria manufacture more plant food during the summer months, when the temperature of the soil is about 20 degrees warmer than in winter. This is just as it should be, for plants require far more nourishment when in an actively growing state in summer than when in a comparatively inactive condition in winter.

This wonderful nitrifying process is perhaps brought to the highest state of activity under glass at almost any season of the year. The temperature is raised by artificial means, and plants are "forced" into growth. This simply means that the bacteria respond to the higher temperature and the moisture, and proceed to attack the substances in the soil and place them more readily at the disposal of the crop. If the temperature is suddenly reduced the plants are said to "catch a chill" and cease to grow. The sudden change from say 80° to 50° F. would injure the active bacteria and put them completely out of action, with the result that food supplies are immediately cut off from the plants. One of the great problems the cultivator has constantly before him therefore is to maintain just the temperature to promote the greatest activity amongst the soil bacteria. The grower under glass succeeds in doing this often at a great cost; but the open-air grower must make use of hot beds and plenty of good manure to achieve favourable results.

There are several kinds of nitrate-forming bacteria in properly cultivated soils, amongst the best known to scientific research being Bacillus mycoides, B. mes-entericus vulgatus, B. subtilis, and Proteus vulgaris. With a proper supply of organic manure, a certain amount of lime to check acidity, a genial temperature, and a deeply worked soil these bacteria render valuable services.

In The Standard Cyclopaedia of Modern Agriculture we are told that "bacteria have a very simple structure - a speck of living protoplasm surrounded by a capsule or cell wall. They are unicellular, and are the smallest living organisms known, some being less than 1/25000 in. in diameter. Although there are hundreds of different species, there are only three general forms - the spherical (termed a coccus), the rod-shaped (termed a bacillus), and the spiral (termed a spirillum). A curved rod is termed a vibrio." Professor W. B. Bottomley once humorously classified the three kinds - billiard balls, cigarettes, and corkscrews. The diagrams (fig. 90) illustrate the various forms and the methods of vegetative reproduction.

A single bacterium dividing in two, and taking one hour for the completion of the process, will in twenty - four hours produce 16,000,000 under suitable conditions. In ordinary cultivated soil the number of bacteria varies from 300,000 to 10,000,000 per gramme of soil (100 gm. = 3527 oz. avoir.). They occur in the greatest number in the first 6 in. of the soil; below this they rapidly diminish until at a depth of about 3 ft. very few are to be found. Professor Hilgard, of the Californian University, states that in a black loam with a considerable amount of humus there were 33,931,747 bacteria to 1 cub. cm., and as many as 53,596,060 in 1 cub. cm. of similar soil containing more humus. As there are over 16 cub. cm. to 1 cub. in., and 1728 cub. in. to 1 cub. ft., one can scarcely realize the teeming millions of bacteria there must be in a fertile soil. Most cultivators will accept Professor Hilgard's figures to save the trouble of counting them themselves. Speaking generally, the bacteria may be classed into three groups: (1) The "decomposition" bacteria, that attack and bring about the decay of manure and other organic matter; (2) the "nitrifying" bacteria, consisting of two distinct organisms: the one (a) Nitrosomonas, which converts ammonia into nitrous acid and nitrite; the other (b) Nitrobacter, which changes nitrites into nitrates; and (3) the "nitrogen-fixing" bacteria (Azotobacter), which absorb free nitrogen from the air and fix it in the nodules of the roots of leguminous plants.

Diagrammatic Representation of the Methods of Vegetative Reproduction among Common Bacteria.

Fig. 90. - Diagrammatic Representation of the Methods of Vegetative Reproduction among Common Bacteria.

a, A bacillus successively dividing at a1, a11, and a111. 6, A coccus giving rise to chains, b111 (Streptococci); pairs, biv (Diplococci); and irregular groups, bv (Staphylococci), c, A coccus giving rise by division in two directions to c111 (Micrococci), and by division in three directions to cIV (Sar-cinae).