PHOSPHORUS AND LIFE — HOPKINS 241 



Both the loss by soil erosion and the loss by sewage outflow — man- 

 created losses — are conserved by the marine cycle ; whether conducted 

 through sewers, washed away into rivers, or blown away in dust 

 storms, most of the phosphorus thus removed eventually reaches 

 the sea, where a great part of it will be assimilated by plankton. 



A very minor but interesting link between the land and marine 

 cycles has not yet been mentioned. Certain oceanic islands, rocky and 

 uninhabited by man, and in almost rainless areas, are regularly used 

 by sea birds (pelicans, albatrosses, etc.) during their breeding season. 

 Their excreta and the bodies of young birds that die accumulate. 

 Rain would wash out the phosphate but instead the sun dries and 

 concentrates this material. As these birds feed entirely upon fish, 

 their organic wastage is directly derived from the marine cycle's 

 "phosphorus currency." This naturally dried material has long been 

 known as guano. Guano islands were exploited as a source of manure 

 by the Incas. In more recent times the most famous guano islands, 

 off the Peruvian coast, have yielded 10,000 tons of guano a year. 

 Guano collection is illegal during the four-months' closed season when 

 the birds flock to the islands ; similar protective regulations are said to 

 have been imposed by the Incas. Today the total contribution of 

 guano is insignificant compared with the millions of tons of mineral 

 phosphate brought into the land cycle. Nevertheless, it represents 

 a unique transference of phosphate and other nutrients from the sea 

 to the land. 



Is modern man to be indicted for his agricultural acceleration of the 

 land cycle ? An adult requires just over one pound of phosphorus per 

 year as a maintenance standard. Many of the world's soils are ex- 

 ceedingly deficient in available phosphorus. Since 1800 the world's 

 human population has more than doubled. The marine cycle's de- 

 posits of concentrated mineral phosphate took many millions of years 

 to accumulate and they are being consumed at an exhaustion rate to 

 be measured in centuries. Yet it is difficult to see how man, faced 

 with so vast an increase in his numbers, could have done otherwise. 

 If there is any indictment, it rests upon the charges of wastage, upon 

 the huge losses in sewage and the even greater losses through un- 

 checked soil erosion. 



