SPECIAL ASPECTS OF NUTRITION OF PLANTS 125 



200. Sources of plant food. The different elements which 

 are necessary constituents of plant food are not taken up by the 

 plant as elements, except in rare cases, and possibly also with 

 the exception of the oxygen of respiration. Oxygen is taken into 

 the plant as an element in the process of respiration. If this 

 oxygen merely assisted in the combustion of plant material it 

 would not in this instance be a food constituent, but there are 

 reasons for believing that some of it at least is assimilated 

 into the living matter. Nitrogen is taken up as an element 

 in the nutrition of a few specialized bacteria (see fixation of 

 nitrogen, paragraphs 204-205). The mineral substances found 

 in the ash of plants when they are burned, containing among 

 other things calcium, magnesium, phosphorus, sulphur, iron, the 

 plant takes up from the soil in the form of nitrates, sulphates, 

 phosphates.* These are formed during the weathering and 

 disintegration of rocks, t The water (H 2 O) of the plant is 

 absorbed from the soil. This furnishes part of the hydrogen and 

 oxygen. The source of the carbon for green plants is the carbon 

 dioxide (CO 2 ) of the air, which the plant uses during photosyn- 

 thesis. Nitrogen is obtained by absorption of nitrates from the 

 soil and from some other compounds of nitrogen. For example, 

 in cultivated or waste fields, the nitrogen food is mostly in the 

 form of nitrates, while nitrates are scarce in the forest where there 

 is an abundance of decaying leaves and humus. In the forest 

 the nitrogen food is chiefly in the form of ammonium salts. 



201. Most of the substances used as plant food from 

 the soil exist there in quantities sufficient to last plants 

 for many years. But nitrogen compounds are rare,f and if 



* Examples, potassium nitrate, calcium sulphate, magnesium sulphate 

 calcium phosphate, etc. 



j" The small particles of rock make the basis of the soil, while dead plant 

 remains furnish the organic matter and humus which give it a darker color 

 and make it more retentive of moisture. 



J Phosphates are also comparatively rare in many soils, and during con- 

 tinued cropping when they are not returned to the soil, the soils become 

 deficient. Phosphorous occurs in the oldest rocks and these phosphates 

 appear in the soil from disintegration of rock. By the growth of plants they 



