THE SOIL AND ITS EFFECT ON PLANT GROWTH. 9 



that exists within it ; (2) the search for organic substitutes for 

 farmyard manure. The two headings are, however, inter- 

 dependent, for if the search for organic substitutes is to be success- 

 ful it must be based on knowledge of the process by which plant 

 products dug or ploughed into the soil are made available for 

 plant food. 



It has for some time been recognised that the microscopical 

 living things known as bacteria which are found in the soil play 

 an essential part in the feeding of a crop by attacking the nitrogen 

 compounds supplied in the form of manure, and turning them 

 into a soluble form in which they can be absorbed by the plant 

 roots. More recent research has demonstrated that another 

 group of minute organisms, known as protozoa, equally invisible 

 to the naked eye, is present in the soil, taking an active part in 

 the progress of events, and though noiseless and unseen, helping 

 to determine the destiny of the crop on which the nation depends 

 for its food and the farmer for his livelihood. In fact, the soil is 

 no longer looked upon as an inert mass of mineral particles ; it is 

 a great living complex, teeming with countless millions of living 

 things each struggling for existence, and each having some influ- 

 ence on those complicated chemical changes on which the growth 

 of all plants depends, and which in the course of ages have turned 

 a stratum of bare rock into something approaching a vast chemical 

 laboratory. And just as the adventurer, in exploiting a new 

 country, must make himself acquainted with its woodlands, 

 rivers, soil and mineral wealth, no less than with the customs and 

 manner of life of its inhabitants, so must the scientist, in en- 

 deavouring to discover principles of a more intensive cultivation 

 of the earth than has yet been attempted, lay the foundation for 

 his work in a precise knowledge of all that is going on in the soil. 

 This fact has long been recognised at Rothamsted, but not until 

 comparatively recently have the necessary facilities been available 

 for making an adequate beginning in attacking the problem ; for 

 the living things in the soil vary in their relation towards each 

 other day by day, and a proper understanding of the part they 

 play in the economy of the soil can only be secured by the laborious 

 and difficult method of making daily counts of their relative 

 numbers over a long period of time. This has now been done. 

 Throughout the year ending on July 5th, 192 1, five trained 

 scientific workers were engaged, Sundays as well as weekdays, 

 in counting day by day the numbers of bacteria and of 19 different 

 kinds of protozoa contained in Rothamsted soil. The results 

 are now being worked up, and a considerable amount of investi- 

 gation is, of course, necessary, before any practical deductions 

 can be drawn. Not only must the relations between the various 



