18 PLANT LIFE Otf THE FAKM. 



the large? part of which are undoubtedly wasted under 

 existing circumstances. 



Potash salts are also essential, more so in some cases 

 than in others. At Rothamsted, potash, after having 

 been employed for a number of years as a manure-con- 

 stituent on a certain grass plot, was discontinued ; the 

 produce, in consequence, rapidly declined, and the 

 quantity of carbon fixed in the tissues of the plants pro- 

 portionately diminished, although the amount of nitro- 

 gen absorbed was the same in the two cases. The pres- 

 ence, therefore, of an adequate supply of potash, in the 

 soil, seems essential to the full assimilation of the car- 

 bon which is derived, as we shall presently see, from the 

 air, in the form of carbonic acid gas. It is believed from 

 recent experiments that without potash no starch can be 

 formed ; and starch, as we shall see hereafter, is of 

 primary impoctance in the nutrition of the plant. In 

 any case the value of potash manures for increasing the 

 yield of certain crops, particularly potatoes, is a fact be- 

 yond dispute. 



Sulphur and phosphorus are also derived from the 

 soil as sulphates and phosphates. Both occur in asso- 

 ciation with the albuminoid contents of the protoplasm ; 

 and phosphorus seems specially needful in the formation 

 of the pollen the fertilizing powder in the flowers and 

 in the ripening of seeds, while its effect on the growth 

 of turnips is familiar to all practical men. 



Iron is essential to the formation of leaf -green 

 " chlorophyll " and chlorophyll is essential to the pro- 

 duction of starch ; hence iron in some shape is essential 

 to plants, and it also is supplied from the soil in the 

 form of saline solutions. 



What precise function lime plays in the plant's economy 

 is not known ; but indirectly it is of importance as a 

 means of introducing phosphorus and other essential in- 

 gredients. At Bothamsted, in two of the plots upon 



