VII.] GOOD AND BAD SOIL ORGANISMS 147 



group has been so increased by the excess of organic 

 matter that it maintains the balance by getting rid of 

 all the surplus of nitrogen. 



The " condition " of the soil in the farmer's sense of 

 the word — its capacity to keep the crop steadily and 

 fully nourished — is in the main determined by the 

 activity of the ammonia-makers, and this, temperature 

 and moisture conditions being equal, depends wholly 

 upon their numbers. The numbers are in the first place 

 conditioned by the food supply of the soil, and secondly 

 by another wholly different class of living creatures 

 which have recently been ascertained to play an 

 important part in the soil. These are various protozoa 

 — distinctly animal organisms a thousand times larger 

 than the bacteria upon which they prey, the numbers of 

 which they keep down to a certain limit. It is possible 

 on a small scale to kill off these protozoa and yet leave 

 the bacteria active, in which case the productivity of the 

 soil is raised, even to the extent of being doubled, with- 

 out any additions of manure : whether these small-scale 

 processes can be adapted to our fields yet remains 

 to be seen. 



Within the soil there are also numerous microfungi 

 and moulds of all kinds, many of which are breakers- 

 down of protein and carbohydrate matter like some of 

 the bacteria, while others are more harmful, because 

 they compete with the higher plants for food, or even 

 give rise to diseases like " finger-and-toe " or " club 

 root" in turnips and cabbages. In a general way it 

 may be said that bacteria predominate in soils which 

 are kept neutral by a sufficiency of carbonate of lime, 

 but that fungi become abundant and harmful as soon as 

 the soil is allowed to get acid, their activity being 

 promoted by the use of acid manures like superphos- 

 phates and sulphate of ammonia. While we are very 



