Y-axis: Milligrams of cellulose decomposed in 21 days.
It is well known that if fresh unrotted manure or straw be added to the soil, it often produces harmful effects on the succeeding crop. The problem, therefore, was to develop a method by which fresh straw, before application to the soil, could be made to rot down to a mixture of humus compounds such as occur in well-rotted farmyard manure. The solution of this problem came as a result of an investigation by Hutchinson and Richards,[30b] at Rothamsted, into food requirements of the cellulose decomposing bacteria. They realised that since more than 10 per cent. of the dry weight of bacteria consists of nitrogen, it would be necessary to supply the cellulose decomposing bacteria with a supply of nitrogen, in order that they should attain their greatest activity. Experiments with cultures of Spirochæta cytophaga showed that the amount of cellulose decomposed depended upon an adequate supply of nitrogen for the organism ([Fig. 3]). Similarly, materials such as straw will scarcely decompose at all if wetted with pure water. An adequate supply of nitrogen compounds is needed to enable decomposition to take place. Hutchinson and Richards tested the effect of ammonium sulphate, and discovered experimentally the proportion of ammonia to straw that produced the most rapid decomposition. They found that if a straw heap was treated with the correct proportion of ammonia, it decomposed into a brown substance having the appearance of well-rotted manure. This has resulted in the development of a commercial process for making synthetic farmyard manure from straw. The method of manufacture is as follows: A straw stack is made and thoroughly wetted with water. The correct amount of ammonium sulphate is then sprinkled on the top and wetted, so that the solution percolates through the straw. The cellulose bacteria attack the straw, breaking it down and assimilating the ammonia. This ammonia is not wasted, as it is converted into bacterial protoplasm that eventually decays in the soil. Field trials of this synthetic manure show that it produces an effect closely similar to that of natural farmyard manure.
While cellulose and related carbohydrates are by far the most important non-nitrogenous compounds left in the soil by plants, there are other compounds whose destruction by bacteria is of special interest. Such, for example, is the case of phenol. This compound is produced by bacterial action as a decomposition product of certain amino-acids. It occurs in appreciable amounts in cow urine. It is probable that it forms a common decomposition product in soil and also in farmyard manure. If this phenol were to persist in the soil, it would eventually reach a concentration harmful to plant growth. It does not, however, accumulate in the soil; indeed, if pure phenol or cresol be added to ordinary arable soil, a rapid disappearance occurs. This disappearance is of some practical importance, since it limits the commercial use of these compounds as soil sterilising agents. The cause of the disappearance has been to some extent elucidated at Rothamsted,[58] where it was found to be in part a purely chemical reaction with certain soil constituents, and partly due to the activity of bacteria capable of decomposing it. A large number of soil bacteria have now been isolated that can decompose phenol, meta-, para-, and ortho-cresol, and are able to use these substances as the sole sources of energy for their life processes. These organisms have a wide distribution, having been found in soil samples taken from all over Great Britain, from Norway, the Tyrol, Gough Island, Tristan da Cunha and South Georgia. Soil bacteria have also been isolated that are able to decompose and derive their energy from naphthalene and from toluene. The ability of the bacteria to break up the naphthalene is very remarkable, and all the more so since they can hardly have come across this compound in the state of nature. The naphthalene organisms have a distribution as world-wide as the phenol group.
(2) Ammonia Production.
The second main group of products left in the soil by higher plants are the nitrogen-containing compounds, such as the proteins and amino-acids. Plant remains are not the only source of organic nitrogen compounds available to soil bacteria. There are, in addition, the dead bodies of other soil organisms, such as protozoa and algæ. The relative importance of these sources of nitrogen is not known, but almost certainly varies greatly with the state of activity of the various groups of the soil population. Bacteria are able to utilise organic nitrogen compounds as energy sources, as can be exemplified in the oxidation of a simple amino-acid:—
It will be seen that, in the acquirement of energy from such a compound, ammonia is released as a by-product. It is not certainly known what is the exact course of the reactions brought about by bacteria in soil during the breaking-down of organic nitrogen compounds, but they result in the splitting off of most of the nitrogen as ammonia. Herein lies the great importance of the process, for the production of ammonia is an essential stage in the formation of nitrate in the soil, and on the supply of nitrate the growth of most crops largely depends.
Fig. 4.—Quantities of ammonia produced by pure cultures from 5 grams of casein in the presence of varying quantities of dextrose. (After Doryland.)
X-axis: Percentage of dextrose added.