MANUAL FOR SUGAR GROWERS. 19 



The presence of humus or decayed vegetable mat- 

 ter is essential ; this humus plays many important 

 parts : in stiff clay soils it keeps the minute parti- 

 cles of the soil apart from each other, thus causing 

 the soil to remain both friable and porous ; in sandy 

 soils it holds the particles together, thus giving the 

 requisite coherence. Its presence assists the reten- 

 tion both of water and air, factors which have been 

 shown to be essential to fertility. The correct ap- 

 preciation of the part played by vegetable matter in 

 the soil is of vast importance to the sugar grower. 



The porosity of a soil which, as has been seen, de- 

 pends largely on two things, tillage and humus, has 

 a great influence on the retention of moisture dur- 

 ing a dry season and its escape during a wet one. 

 A compact, non -porous soil becomes much hotter 

 under the influence of the sun's rays than does a 

 porous one ; consequently during a drought moisture 

 is much more rapidly lost from a compact than 

 from a porous soil. Further, in a compact soil 

 moisture is drawn up from a considerable depth by 

 capillary attraction ; ^ in a porous soil, the air-spaces 

 being larger, capillary attraction is lessened : hence a 

 porous soil remains moist for a considerable time, 

 while a compact one becomes dry and cracked. The 

 importance of this fact can hardly be too strongly 

 insisted on, particularly in tropical agriculture. A 

 simple experiment clearly demonstrates the truth of 

 what has been said. Into two tins of similar size 

 and shape equal quantities of dry soil are placed, 



* Capillary attraction is the forco whicli causes fluids to ascend 

 tubes or cavities of very small bore. 



