280 Nutrients 



ing the saprophytes involved in the "return" processes of decomposi- 

 tion and regeneration. 



Influence of Nutrients on Green Plants 



Nutrients Required. The nutritional requirements of green plants 

 embrace several major materials and a longer list of minor substances. 

 The principal elements that go into the construction of plant tissues 

 are: carbon, oxygen, hydrogen, and nitrogen. We have already dis- 

 cussed the sources of carbon from CO2, of hydrogen from H2O, and 

 of oxygen from 0„ COo, and H.O. An enormous supply of nitrogen 

 exists in the air but atmospheric nitrogen is unavailable to most 

 plants. In this respect nitrogen presents a complete contrast to CO2. 

 We have seen that the quantity of CO2 in the atmosphere is extremely 

 small but that it is directly and readily available to plants. Since 

 nitrogen makes up 79 per cent of the atmosphere, terrestrial plants 

 would seem to be practically surrounded by this gas— yet only a iew 

 special types of bacteria can use free nitrogen. Aquatic plants are 

 similarly unable to utihze directly the large amount of nitrogen gas 

 dissolved in natural waters. Green plants must obtain their nitrogen 

 from nitrogen compounds; these are not generally plentiful and are 

 derived primarily from organic decomposition. Most plants grow 

 best when supplied with nitrate— many can obtain their nitrogen only 

 in that form— but others such as some green algae can assimilate 

 nitrite, ammonia, and amino acids (Algeus, 1951). 



Two other elements necessary in moderate amounts for the growth 

 of holophytic plants are sulphur, which is abundant in the soil and in 

 the sea as sulphate, and phosphorus, which occurs as phosphate but 

 is not at all plentiful. Although some phosphate is derived from the 

 disintegration of parent rock, the decomposition of organic matter is 

 the immediate source of this material for most green plants. Essen- 

 tial in smaller quantities are potassium, calcium, and magnesium. 

 Potassium originates from potassiferous sihcates found in almost all 

 rocks; the occurrence of calcium has been discussed in the previous 

 chapter. Magnesium, the keystone in the structure of chlorophyll, 

 is usually sufficiently available in the soil and in natural waters. 



Other elements known to be necessary for the growth of plants at 

 least in trace quantities include: iron, manganese, copper, zinc, 

 molybdenum, and boron (Stiles, 1946). The fact that plant growth 

 can be affected by zinc in the amount of one part in 200,000,000 

 illustrates the minuteness of the quantities of these elements that may 

 be effective. Sihcon is used by diatoms in the construction of their 



