362 THE FERTILITY OF SOILS IN THE TROPICS 



result of bacterial action; the investigation of the process 

 of nitrification in soils and also of the fixation of free 

 atmospheric nitrogen as the result of the action of bac- 

 teria of the Azotobacter type. 



In relation to soil temperature, a large number of 

 measurements have been performed at varying times and 

 seasons throughout the year, at elevations ranging be- 

 tween 100 ft. and 2,000 ft. above sea-level, and in soil 

 types varying between light sandy soils and heavy clays; 

 in all cases the measurements were made at depths of 

 6 in., 12 in., and 18 in. respectively. The results gener- 

 ally show a series of values ranging between 20 C. and 

 30 C. ; in the whole course of the measurements no 

 value below 20 C. was recorded, while this was only 

 attained at considerable elevations in the very early 

 morning during the cool season. The upper value of 

 30 C. was occasionally slightly exceeded in the case of 

 the upper 6 >in. of soils of light texture exposed to the 

 direct rays of the sun during the hottest portions of the 

 day. No doubt higher values than this are reached in 

 the uppermost layers, i.e., the top one or two inches, 

 but the temperature gradient from the topmost layer 

 down to those levels at which the root systems of plants 

 normally exist, appears to be very steep during the day. 

 As is to be expected, the actual daily range of tempera- 

 ture is least at the lowest point at which the measure- 

 ments were made, and more equable temperatures were 

 experienced throughout in the case of orchard soils in 

 which the surface is shaded from the direct rays of the 

 sun. It will be seen that the actual range O'f temperature 

 experienced is particularly favourable to the growth of 

 organisms and that, with the possible exception of the 

 top two inches of soil, at no time is a value attained 

 which is likely to check development to any marked 

 extent. 



A limiting factor which may occur in the case of arid 

 and semi-arid regions lies in the water supply, but under 

 the conditions governing our observations the soil mois- 

 ture content never falls so low as entirely to check the 

 growth of organisms, while in wetter localities it is usually 

 maintained in a condition highly conducive to biological 



