The simpler carbohydrates and starches are attacked and decomposed by a large variety of bacteria. The addition of such substances to soil causes a rapid increase in bacterial numbers. In nature the sugars are in all probability among the first plant constituents to be destroyed during the decay processes.
A large proportion of plant tissues consist of cellulose and its derivatives. These compounds are consequently of great importance in the soil. Unfortunately our knowledge of the processes by which cellulose is broken down in the soil is very inadequate. The early experimental study of cellulose decomposition, such as that of Tappeiner[60] and Hoppe-Seyler,[33] was mostly carried out under conditions of inadequate aeration, and the products of decomposition were found to include methane and CO2, and sometimes fatty acids and hydrogen. The bacteriology of this anaerobic decomposition was studied by Omelianski,[54] who described two spore-bearing organisms, one of which attacked cellulose with the production of hydrogen, and the other with the production of methane. Both species also produce fatty acids and CO2. It is probable that these organisms operate in the soil under conditions of inadequate aeration. In swamp soils, in which rice is grown, it has been shown that methane, hydrogen, and CO2 are evolved in the lower layers. In these soils, however, the methane and hydrogen are oxidised when they reach the surface layers. This oxidation is also effected by micro-organisms. Bacteria capable of deriving energy by the oxidation of hydrogen gas have been isolated and studied by Kaserer,[37] and by Nabokich and Lebedeff,[52] while Söhngen[57] has isolated an organism which he named Bacillus methanicus, that was capable of oxidising methane.
Under normal conditions in cultivated soils, however, the decomposition of cellulose takes place in the presence of an adequate air supply, and so follows a different course from that studied by Omelianski. Our knowledge of this aerobic decomposition is very scanty. A number of bacteria, capable of decomposing cellulose aerobically, are known. A remarkable organism was investigated by Hutchinson and Clayton,[30] who named it Spirochæta cytophaga. This organism, which they isolated from Rothamsted soil, though placed among the Spirochætoidea, is of doubtful affinities. During the active condition it exists for the most part as thin flexible rods tapered at the extremities. This form passes into a spherical cyst-like stage, at first thought to be a distinct organism ([Fig. 2]). Spirochæta cytophaga is very aerobic, working actively, only at the surface of the culture medium. It is very selective in its action. It appears unable to derive energy from any carbohydrate other than cellulose. Indeed, many of the simple carbohydrates, especially the reducing sugars, are toxic to the organism in pure culture. An extensive study of aerobic cellulose decomposition by bacteria was made by McBeth and Scales,[50] who isolated fifteen bacteria having this power. Five of these were spore-forming organisms. Unlike Spirochæta cytophaga, they are all able to develop on ordinary media such as beef agar or gelatine, and are thus not nearly so selective in their food requirements.
Fig. 2.—Spirochæta cytophaga. Changes occurring in culture. (After Hutchinson and Clayton.)
We are at present ignorant as to which organisms are most effective in decomposing cellulose in the soil under field conditions, or what are the conditions best suited to their activity. It is possible that fungi also help in the decomposition of cellulose to a great extent. This subject of the decomposition of cellulose offers one of the most promising fields of research in soil bacteriology. The difficulty of the subject is further increased by our present ignorance of the chemical aspect of cellulose decomposition. It has been supposed that the early decomposition products are simpler sugars, but these are not found under conditions in which cellulose is being decomposed by pure cultures of the bacteria mentioned above. Hutchinson and Clayton found that their organism produced volatile acids, mucilage, and a carotin-like pigment. The organisms isolated by McBeth and Scales also produce acids, and in some cases yellow pigments. It is known, however, that the decomposition products of cellulose can be utilised as energy supply for other organisms, such as nitrogen fixing bacteria.
When plant remains decompose in the soil there are ultimately produced brown colloidal bodies collectively known as humus. The processes by which this humus is produced are not yet properly understood. Humus is of great importance in the soil, in rendering the soil suitable for the growth of crops. It affects the physical properties of the soil to a great extent. In the first place, it improves the texture of the soil, making heavy clay soils more friable, and loose sandy soils more coherent. Secondly, it has great water-retaining powers, so that soils rich in organic matter suffer comparatively little during periods of drought. And lastly, it exerts a strong buffering effect against soil acids. Now, it is one of the problems of present-day farming that soil is becoming depleted of its humus. This is due to the increasing scarcity of farmyard manure in many districts, and the consequent use of mineral fertilisers to supply nitrogen, potash, and phosphate to the crop. A need has therefore arisen for a substitute for farmyard manure, by means of which the humus content of soils may be kept up in districts where natural manure is scarce.
Fig. 3.—Cellulose decomposed by S. cytophaga in media with increasing amounts of nitrogen. (After Hutchinson and Clayton.)
X-axis: Milligrams of nitrogen supplied as sodium-ammonium phosphate.