189 



and only small ciliate infusoria were found in the toluened soil, 

 while all these organisms are found in the untreated soil. 



The authors therefore conclude that these large organisms 

 (protozoa, etc.) constitute the factor or one of the factors limiting 

 the bacterial activity and therefore the fertility of the untreated 

 soil. Direct evidence of this fact is furnished by inoculating toluened 

 soil, or soil extract, with cultures of large organisms, thus determin- 

 ing the consequent depression in the rate of ammonia formation. 



Not only does partial sterilisation kill these destructive and com- 

 peting organisms and thus make the conditions more favourable 

 for the new bacterial flora, but indirectly increases the food supply, 

 as dissolution of killed protozoa by the bacteria could be observed 

 under the microscope. 



So far as plant growth is concerned, in partially sterilised soils 

 this growth is greater and plants contain an increased percentage 

 of nitrogen and phosphoric acid; organic matter in the soil being 

 mode rapidly decomposed with production of a greater amount of 

 ammonia, but no nitrate. 



In order to ascertain in what form plants take up their nitrogen 

 from partially sterilised soils two series of experiments were made 

 in special conditions, so that no infection could take place. The 

 percentage of nitrogen in the dry matter of the rye cultivated was 

 higher where no nitrate was formed in the soil, i. e. in the two 

 heated soils and the inoculated toluened soil. Nitrification is there- 

 fore not essential to plants, but it may be economical. 



B. DYER. Fertilising effect of Soil Sterilisation. Nature, 

 vol. 83. March 24, 1910, p. 96. 



Some of the large growers of cucumbers, tomatoes, etc. under 

 glass for the London market have for some little time adopted the 

 plan of injecting jets of steam into their soil before planting, not 

 with any view of increasing its fertility, but with the view of de- 

 stroying slugs, insects, etc. In the experience of some growers, the 

 productivity of the soil after steaming has become so greatly in- 

 creased that, if anything like the usual quantity of stable manure 

 is mixed with the soil, the plant grow with such rank luxuriance 

 as to spoil their bearing capacity, exhibiting all the symptoms cor- 

 responding to an overdose with nitrogen. The growers were 

 unaware of the Rothamsted experiments on heat-sterilisation of 

 soils. The observation appears to afford striking independent 



