70 THE PHYSIOLOGY OF PLANTS 



of the soil available to plants as a sulphate, while lime is de- 

 posited by this change. In practice it is usual to strew the 

 gypsum on plants freshly covered with dew or moistened by 

 rain. This method is justified by the fact that a solution of 

 gypsum is rapidly formed on the wet plants, and dropping 

 down from these into the soil, it becomes almost immediately 

 active in proximity of the roots. 



§ 11. How is the nutrition of pot-plants effected ? 



The points which are of greatest importance in dealing with 

 the manuring of crops in the open are of secondary im- 

 portance in speaking of pot-cultures, unless in some excep- 

 tional case there should be, by some gross neglect, an actual 

 deficiency of some food substance. The various soils which 

 are used for potting purposes all contain at least sufficient 

 food matter for the growth of the plants ; but even the 

 richest can only last for a certain time in the very limited 

 space of a pot. In all cases a renewal of the soil has to take 

 place by re-potting, and it is the gardener's object to select 

 the proper time for this proceeding. 



If, therefore, the root of a pot-plant finds in every soil the 

 necessary quantity of food substances, the usual choice of 

 heather soil, leaf or peat mould, loam or turfy soil, for different 

 plants, is not made because one kind of mould contains food 

 substances which are absent from another, or because some 

 substance is present in greater quantity in one soil than in 

 another. Such differences exist of course in the different 

 soils,'' but are of little importance in the case of pot-plants, 

 because usually the earth of a pot is renewed before the roots 



' The following table of Loges gives a comparison of the amount of the various 

 nutritive substances contained in a leaf-mould from various sources. The dry 



