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:-- H O | // H--C--C + 3O = 2CO₂ + H₂O + NH₃ + 152 Cal. | \ NH₂ OH