58 THE COMPOSITION OF FERTILE AND BARREN SOILS. 



root has time to penetrate the soil in search of food ; but we 

 cannot reason so with regard to turnips, which have a short 

 period from three to four months of vegetation. One might 

 say the soil contains as much as is sufficient for its growth ; very 

 possibly it contains six or twenty times the quantity required ; 

 but the practical question is can you get it into the plant in 

 the time ? You will find the food may be abundant in the soil, 

 yet it may not be present in that digested form to meet the pe- 

 culiar wants of the special crop you want to grow. You may 

 imagine it strange that the soil should retain sufficient phosphoric 

 acid to meet the wants of twenty crops of wheat, and yet not 

 supply a sufficient quantity for one crop of turnips, and the 

 analysis shows that a crop of turnips contains less phosphoric 

 acid than a crop of wheat. When these statements are made they 

 seem contradictory of theory, and they require much explanation 

 to the uninitiated. It must be remembered that the period 

 during which plants assimilate food from the soil should betaken 

 into account in considering the manure we should give to a plant. 

 There may be sufficient phosphate in a soil to grow wheat because 

 it is sufficient time in the soil to collect it, but the turnip, which 

 requires less phosphoric acid, has only half the time for assimi- 

 lating the food ; moreover, a turnip cannot throw its roots so far 

 in search of food as the wheat plant, which throws out its rootlets 

 through a great mass of soil ; therefore we must give it food in a 

 more prepared state than it is in the soil, and hence the great use 

 which superphosphates, or soluble phosphates of lime, are to the 

 growth of turnips and all root crops. Those facts show that it is 

 not merely by analysing soils that the chemist can be of much 

 advantage in agriculture. It requires, indeed, peculiar insight 

 into peculiar manures, and considerable acquaintance with physi- 

 ology as well as chemistry, in order to enable a man to give 

 correct opinions on any particular subject in agricultural che- 

 mistry that may be referred to him : and inasmuch as in many 

 matters, and with respect to many practices, we are totally igno- 

 rant of the true cause in operation, it is unwise to speak too 

 positively on any particular subject, if we have not the extended 

 experience of farmers with relation to it, not only in one district 

 but throughout the country at large. In the cases I have cited, 

 theoretically considered, the addition of superphosphate would 

 be of no service to the turnip ; but practical experience has shown 

 that it is of great use to the grower of turnips, and it remains for 

 the chemist to find out the cause, and not to say there is some- 

 thing wrong on the part of the fanner in using superphosphate 

 when he finds it answers his purpose. There must be some 

 cause for this divergence from theory, and I believe it to lie in 

 the shorter period of vegetation, which makes superphosphates 





