Ratio of Regenerated Materials 307 



dance of the elements in the organic matter that has decomposed. 

 Only those nutrients will be provided by decomposition that were 

 present in the dead organisms. The fact that chemical fertilizers 

 added to the soil do not replace all the nutritive substances, nor fur- 

 nish them in the same proportions as those removed by the growth 

 of the farm crops, has already been mentioned. Procedures recom- 

 mended by "organic farmers" in which the unused portions of the 

 crops are returned to the soil as a green manure and in which animal 

 manure is used extensively go far toward maintaining a proper bal- 

 ance of organic matter and the various mineral nutrients, but further 

 correction of proportions or the addition of trace elements may also 

 be necessary. 



The interdependence of the ratios of materials regenerated and 

 those of nutrients assimilated is also demonstrated in the aquatic en- 

 vironment. Analysis of samples of sea water from many localities in 

 the Atlantic, Indian, and Pacific Oceans reveal a striking parallelism 

 in the ratio of phosphate and nitrate in spite of the great fluctuations 

 in the concentration of each (Fig. 8.9). The ratio of carbon to the 

 other two elements also tends to remain constant, giving the follow- 

 ing average values: 



C:N:P = 41:7:1 grams = 106:15:1 atoms 



The ratios of these three elements in the plankton are found to be 

 very closely the same. Plankton is thus known to take up nutrients 

 in the ratio in which they are provided in the water; when the plank- 

 ters die, these materials are restored to the water in the same ratio 

 (Redfield, 1934). It is not easy to decide how this unique situation 

 arose. Do these ratios represent the composition of the primitive 

 ocean in which species of phytoplankton evolved that could assimilate 

 nutrients in just these proportions? Or do the ratios represent an 

 equilibrium reached and maintained by the activity of nitrogen-fixing 

 and denitrifying bacteria in the sea? The answer to these questions 

 must await further investigation. 



Let us cast a backward glance over the web of interdependencies 

 involved in the nutritive relations between the organism and the en- 

 vironment. We have seen that the lack of nutrients in quantity or 

 quality, or in chemical or mechanical availability, limits the growth 

 and distribution of both plants and animals. As living things grow, 

 they reduce the supply of nutrient materials in their environment; 

 but, when they die and decompose, their substance adds to this sup- 

 ply directly or indirectly. In situations in which either the organ- 

 isms or the nutrients are mobile the shortage of food affects the whole 



