SOILS AND THEIR PROPERTIES 81 



also good substances for occluding gas. Gases occluded on 

 the surface are more active than ordinary gases, but little 

 work has been done to follow up exactly what effect this has 

 upon soil life. The action of occluded gas is probably 

 generally overwhelmed by bacterial actions, to which much 

 more attention has been paid. Russell and Hutchinson have 

 shown that, in addition to the bacteria in the soil, there 

 are considerable numbers of bacterial enemies, which reduce 

 the numbers of the bacteria. Whether the idea that soil 

 amoebae and paramecia play the part of microscopic 

 beasts of prey is a true or only a fancy picture has never 

 been determined, but the ultimate results have been the 

 subject of careful investigation. Certain organisms living 

 in the soil are able to fix nitrogen, provided they can obtain 

 organic matter in some way, and provided they can obtain 

 a proper supply of phosphates and potash (see p. 29). 



At Cockle Park, in Northumberland, the amount of 

 nitrogen in the soil has been steadily increased by the 

 application of phosphatic manures. The plot which received 

 no manure has steadily decreased in its nitrogen content from 

 0*197 per cent, nitrogen in 1899 to 0*174 P er cent, in 1916, 

 whilst the plot that was treated with basic slag reached 

 0*227 P er cent, nitrogen in 1908 and 0*244 per cent, in 1916. 

 All these figures refer to the top six inches of soil, and have 

 for the most part been done in duplicate or triplicate, show- 

 ing probable errors varying from nothing to 0*008 per cent. 

 Other plots with other treatments have shown somewhat 

 similar results. That this fixation of nitrogen is by no 

 means purely superficial is also shown in these Cockle Park 

 experiments by taking the soil to each three inches depth. 

 In 1916 the unmanured plot gave at each three inches step 

 the following figures : 0*217, 0*131, 0*100, 0*070, and the 

 corresponding figures for the manured plot were, 0*304, 0*184, 

 0*137, 0*100. It will be noted that the improvement is very 

 marked in the top three inches, slightly less marked in the 

 next three inches, while in the depths from six to nine inches, 

 and from nine to twelve inches, there is still a steady increase. 

 The gain in nitrogen is clearly still proceeding at all layers 

 D. 6 



