PLANT NUTRITION. 21 
nevertheless, not proportionately benefited by the appli- 
cation of potash manures, though they are so to some 
extent. These cases show that, by virtue of the varying 
osmotic and digestive powers already mentioned, the 
plants in question take what they want, and when they 
want it, and are not induced to take more by the addi- 
tion of larger supplies. They further show the errors 
that may arise from the farmer acting too implicitly on 
the results obtained by the chemist in the laboratory. If 
he followed the indications of the chemist unchecked by 
other experience, he would apply to his land what was 
really not required by the crop. Thus Messrs. Lawes and 
Gilbert tell us that the exact composition of the crops is 
no direct guide to the description and amount of manurial 
constituents that will be most effective, thus although 
wheat removes more phosphoric acid from the soil than 
does barley, yet the application of the phosphate is more 
beneficial to the barley than to the wheat. They con- 
clude, then, that it is not necessary to supply to the land 
all the constituents that have been removed from it, or 
that would be contained in the crops it is wished to grow, 
but that we should supply all or some, more or less, ac- 
cording to circumstances. 
