FOOD-MATERIALS AND THEIR ABSORPTION 207 



nitrogen or potassium are present in great abundance avails 

 notliing, for these cannot be utilised until the necessary phos- 

 phates are available. 



The food-materials from which plants obtain the sulphur, iron, 

 magnesium, calcium, carbon, hydrogen and oxygen are almost 

 always present in the soil and air in sufficient abundance for the 

 needs of all crops, but the compounds which yield nitrogen, 

 phosphates and potassium are generally removed in such 

 quantities that the supply is soon reduced to such a point that 

 for full crops manure containing one or all of these elements 

 must be added to the soil. 



