HOW PLANT FOOD IS PRESERVED 67 



has no concern, for it contributes no food for the plant. 

 Perhaps it helps a little ; but so slowly, so niggardly, so 

 begrudgingly, its help needs not be included, as a rule, 

 in immediate results. 



An example of this form. Fine ground phosphatic 

 rock untreated with acid is a good example of this sec- 

 ond class this not-immediately-available plant food. 

 True, this rock has been ground and maybe as finely as 

 practical grinding machines are able to do it. Still, plants 



AT WORK IX THE CORX-FIELD 

 Cultivation is helpful in rendering plant food available 



t 



are unable to use it, fine as it is, for plant roots, you know. 

 never entice particles into their tiny cells ; they take only 

 dissolved materials. Hence this fine ground rock- 

 needed though it may be by the very plants that reject it 

 is still mere unavailable plant food. And so it will re- 

 main until air and water, or heat and cold, have tried 

 their pulverizing, disintegrating powers, until their 

 forces have crumbled it and humbled it into dust then, 



