Partial sterilisation of soil by antiseptics

Abstract

1. Quantitative determinations have been made of the effect on soil protozoa and bacteria of various antiseptic substances, including benzene and its homologues and derivatives, carbon disulphide, ammonia, formaldehyde and chlorpicrin. Ammonia and nitrates were determined at the same time in many cases. The effect on fungi, eelworm, etc., was also determined roughly.2. It was found that nearly all the substances disappeared from the soil fairly quickly and at the same time the numbers of bacteria fluctuated. The usual march of events was that the bacteria were reduced in number for the first few days, then rose to a maximum and finally fell slowly towards normal. The whole fall was sometimes very slow and the whole process was much slower in field soil than in the richer, lighter and better aerated greenhouse soils.3. Aeration was found to have great influence on the rapidity of the changes.4. The increase of the bacteria during the early days of an experiment varied in the same direction as the molecular weights and heats of combustion of the antiseptics and is attributed to the latter property. Naphthalene, for instance, which has a large heat of combustion, caused enormous rises, while benzene with its lower heat caused smaller rises.5. This rise was independent of the effect of the substance on the protozoa. Both naphthalene and toluene in large doses cause high rises; the first has no effect on the protozoa, while the latter kills all amoebae and ciliates.6. Similar results were obtained when the experiments were made on soils already free from protozoa, such as a field soil which had been in bottle for 76 years, soil in which they had been killed by steaming, and soil in which they had been killed by antiseptics. If the protozoa were killed by the use of a strong dose of a suitable antiseptic and the soil were then set aside for a long period, a second dose caused an even greater rise than the first.7. It is therefore concluded that the rise in the number of the bacteria is largely due to the feeding effect of the antiseptic on the bacteria and not only to the destruction of the protozoa, and that the increased fertility observed by Russell and Hutchinson is to be attributed in large measure to the activity of the greater bacterial population in breaking down the organic matter of the soil. Bacterial rises following treatment with lime or steam are similarly caused in part by the preparation of the plant residues.8. Aliphatic compounds cause quicker but smaller rises than those of aromatic series.9. The introduction of a CH3 group into the benzene ring lessens toxicity to soil organisms, while a single Cl or nitro-group increases both toxicity and stability in the soil.