CULTIVATION OF CITRUS GROVES. 271 



fair admixture of humus. Now, if the ability of a soil to 

 retain moisture is increased by the presence of humus, 

 it follows that all plant food in solution will be held as 

 well and this is an important feature. 



Humus contains from three to twelve per cent of ni- 

 trogen and this in itself is a fact worth bearing in mind, 

 but it has been ascertained besides that soils rich in 

 humus are usually well supplied with phosphoric acid 

 and potash. 



Reference has already been made to the bacteria 

 which play such an important part in the formation of 

 plant food and which are so intimately associated with 

 the work of providing nitrogen for the use of the trees. 

 It may be said that but little nitrogen from those mate- 

 rials in which it is usually found in or supplied to the soil 

 would be available for the trees were it not for their 

 presence, and the bacterial content of the soil, other con- 

 ditions having been met, is dependent entirely upon the 

 presence of humus. 



Hence, it will be seen that humus is a very valuable 

 soil component and is intimately associated with the life 

 activities of a fertile soil. Growers of citrus fruits in 

 Florida use commercial fertilizer in large quantities and 

 in all districts the same practice will have to be followed 

 eventually. Many of the substances applied must under- 

 go certain changes before they can become available. 

 This change from an unavailable to an available state 

 is dependent either upon the action of soil bacteria or 

 chemical agents. Humus forms the media in which the 

 bacteria work and, furthermore, it holds the plant food 

 when in solution until the roots have a chance to use it. 

 The best and fullest effects cannot be secured from appli- 



