270 CITRUS FRUITS AND THEIR CULTURE. 



should be cultivated frequently during the first half of 

 the year, or rather from the conimencetnent of the grow- 

 ing season until the rainy season begins in districts where 

 the heaviest rainfall comes in summer. 



Humus and Its Value. Humus is a product of the 

 decay of organic substances. When these undergo de- 

 composition in the soil, humus is the intermediate product 

 formed; that is, just before the resolution of the organic 

 material into its component chemical parts. It is prob- 

 ably not too much to say that humus is the most impor- 

 tant substance found in any fertile soil and its presence 

 may generally be taken as the index of fertility. The 

 truth of this statement may be realized more fully when 

 we remember the fact that all barren soils are lacking in 

 this substance and the chief difference between a barren 

 and a fertile soil is usually a difference in humus content, 

 not in the mineral ingredients. 



The changes which are constantly taking place in 

 soil may be grouped as physical, chemical and biologi- 

 cal. The first relates to movement of water, the size of 

 the particles and the mechanical changes, such as those 

 produced by the displacement of the soil particles by ani- 

 mals or the roots of plants. The second includes all 

 changes wrought by the action of acids or other disinte- 

 grators of a chemical nature, while the third, in its use 

 here, refers to the changes brought about by soil bacteria. 

 All three of these changes, whether physical, chemical or 

 biological, are intimately associated with the presence of 

 humus. 



In regard to the first, it may be premised that soils 

 rich in humus are not solid and compact, but loose, open 

 and better aerated than those lacking humus and that 

 the water-holding capacity is materially increased by a 



