WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



37 



average turbidity of the Chattahoochee River in 

 Georgia was decreased to 82 percent by applying 

 improved forestry and conservation practices to 

 lands of the river basin (6) . 



Plant Nutrients 



High crop production to meet ever-increasing 

 food demands requires an adequate supply of plant 

 nutrients in the soil. Americans applied 32 million 

 tons of chemical fertilizers on their lawns, gardens, 

 fields, and pastures in 1966. Those who are con- 

 cerned with the adverse effects of plant nutrients 

 in surface water sometimes find this seemingly 

 large tonnage a convenient culprit. 



The following table reveals the rather low level 

 of chemical fertilizers being applied to arable soils 

 in the United States compared with the Nether- 

 lands — a nation that has responded to the economic 

 need of making highly efficient use of its available 

 cropland. 



Plant Nutrients Applied in 1965 



[In pounds per acre] 



Nutrient United States Netherlands 



Nitrogen 22 244 



P 2 5 17 93 



K 2 13 116 



Plant nutrients in water give rise to two prob- 

 lems of mounting concern: (1) eutrophication of 

 lakes and streams and (2) nitrate in well water 

 used for drinking. 



Eutrnahication 



Complaints abound that many rivers, estuaries, 

 ponds, lakes, and reservoirs are being ruined by a 

 tremendous growth of algae and other water 

 plants, forming what is known as "algal blooms/' 

 These blooms are not pretty, and on death and 

 deterioration of the plants unpleasant odors are 

 emitted. The decaying plantlife robs the water of 

 the little dissolved oxygen that may have been 

 present. The water becomes uninhabitable for fish 

 and unsavory for recreation. These excessive 

 growths of water plants constitute the eutrophica- 

 tion of surface waters. The process is an important 

 stage in the death of a lake. 



These algae, like all plants, must have mineral 

 nutrients in order to grow. In most waters the main 

 limiting factor for algal growth is phosphorus, 



since other elements, such as nitrogen, potassium, 

 calcium, and magnesium, are present, in much more 

 abundant supply. Accordingly, the phosphorus 

 level in river and lake waters becomes a matter of 

 prime concern. 



Available evidence indicates that these algae will 

 grow vigorously if the water contains only 0.1 

 p. p.m. of phosphorus. It is also evident that the 

 level of phosphorus in the water must be below 

 0.02 p.p.m., if the growth of algae is to be com- 

 pletely inhibited. The phosphorus content of the 

 water in some streams and lakes may reach 1 p.p.m. 

 Obviously, this water provides a good medium 

 for the growth of algae. 



About 1 million tons of elemental phosphorus is 

 applied to the land as fertilizer each year in the 

 United States. Average annual runoff of precipita- 

 tion into the streams of the conterminous United 

 States is 1,380 million acre-feet, or 1,875 billion 

 tons. If only 10 percent of the applied phosphorus 

 reached the streams as soluble phosphate, the 

 average phosphorus content of our streams would 

 be 0.05 p.p.m. On the basis of these figures, one 

 could erroneously conclude that the phosphorus 

 in sui-face waters is all coming from land runoff. 



Considerable phosphorus is present in domestic 

 sewage, largely arising from use of household 

 detergents. On the average, about 2 pounds of 

 phosphorus per person per year is delivered in 

 metropolitan sewage effluent, but the Public Health 

 Service reports that Lake Washington near 

 Seattle, Wash., receives 3.4 pounds of phosphorus 

 per capita, per year from the treated sewage of 

 76,300 people (10). Thus, effluent from a city com- 

 prising 1 million people will contain 1.000 tons of 

 phosphorus a year. If this amount of phosphorus 

 enters a stream with an average annual flow of 

 10,000 cubic feet per second, the average phos- 

 phorus content of the water will be 0.1 p.p.m. — an 

 adequate level for growth of al«ae. The Potomac 

 River has an average annual flow of nearly 10,000 

 c.f.s. 



In a situation like the one just cited, even if the 

 total contribution of phosphorus to the stream by 

 agriculture were eliminated, there would be little 

 effect on growth of algae since the 0.1 p.p.m. of 

 phosphorus in the sewage effluent would be 

 sufficient. 



Although some reports have implied that phos- 

 phorus in surface waters was a result of fertilizer 



