18 PLANT LIFE ON THE FARM, 
the larger 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 importance 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—¢he 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 asa 
means of introducing phosphorus and other essential in- 
-gredients. At Rothamsted, in two of the plots upon 
