WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



27 



once-used waters have great significance in their 

 reuse potential for agriculture. 



Although all crops tend to exhibit decreased 

 growth and yield with increasing salinity of the 

 water supply, the differential salt tolerance of 

 crops and the specific sensitivity of some crops to 

 some ions make a single-scale set of values for 

 irrigation water quality quite unrealistic. Soil, 

 climatic, and management factors also influence 

 how damaging an increase in salinity of the irri- 

 gation water supply will be. When all conditions 

 are favorable, an increase in total salt content of 

 the water supply from a few hundred parts per 

 million to even a few thousand parts per million 

 may not affect agriculture adversely. Under less 

 favorable conditions, such an increase of salinity 

 may render the water totally unfit for further use. 



A high proportion of sodium in the irrigation 

 water supply will affect soils and plants adversely 

 because sodium causes soil aggregates to disperse 

 and renders soil highly impermeable. Thus, water 

 softened for metropolitan use is usually unsatis- 

 factory for irrigation. The sodium may also be 

 directly toxic to woody plants (fruit crops) at 

 levels well below those that cause deterioration of 

 soil properties. 



Other specific salts or ions may be highly in- 

 jurious to plants. Boron, although essential for 

 plant growth in concentrations of a fraction of a 

 part per million, rapidly becomes toxic as concen- 

 trations in the water increase above 1 part per 

 million. Municipal use and some industrial proc- 

 esses involving borax or other boron compounds 

 may add enough boron to the water to preclude 

 its further use for agriculture or for supporting 

 growth of native or ornamental plants. 



It has been estimated that about 25 percent of 

 the irrigated lands of the United States are now 

 affected to some degree by salinity. As available 

 water resources are used more intensively, the 

 incidence of salinity problems will increase because 

 saline return flows usually mix with the river or 

 ground water supplies, thereby salinizing the water 

 available for further use. 



The use of salt on highways to melt snow and 

 ice from winter storms is creating very serious 

 problems to rural areas in many States. Salt run- 

 ning off the highways has killed trees and shrub- 

 beries along the frontage of farmsteads. Worse yet, 

 the salt has penetrated into the ground water, ami 



rural wells have been condemned because the water 

 was polluted with excessive levels of chloride. 



Salinity from return flow in the lower Colorado 

 River has caused an international problem with 

 Mexico. This was solved by building a bypass con- 

 crete-lined conveyance which returns the drainage 

 water to the Colorado River below the point at 

 which Mexico diverts its water for irrigation. 



Agricultural and forestry endeavors using trib- 

 utary streams for recreational benefits are con- 

 cerned with acid mine drainage. In Pennsylvania 

 alone, about 2,000 miles of streams are ruined by 

 acid mine drainage. The acidity of mine drainage 

 is usually due to the oxidation of iron pyrites to 

 ferrous sulfate and sulfuric acid. Estimates of the 

 amount, of sulfuric acid reaching streams in the 

 United States run into about 3 million tons an- 

 nually. At present there is no satisfactory way of 

 reducing this acid load. The problem is serious in 

 the coal mining areas of the Ohio River Basin, 

 but the adverse effects on agriculture and forestry 

 are minor in relation to the effects of other wastes. 



Organic Wastes 



Sir Albert Howard (56) had an answer for the 

 problem of coping with vast accumulations of 

 vegetable, animal, and human wastes: Make 

 humus out of the stuff by his Indore Process. The 

 humus so produced would benefit many soils. For 

 the product to have economic feasibility, someone 

 would have to provide large masses of very cheap 

 labor for all the collection, piling, stirring, repil- 

 ing, and distribution activities. 



We lack economical procedures for effectively 

 handling all the mountainous quantities of putres- 

 cible organic substances coming from domestic 

 sewage, garbage, food processing industries, lum- 

 bering operations, pulp mills, crop residues, and 

 animal and poultry enterprises. This group of 

 wastes is rather readily attacked by aerobic bac- 

 teria to reduce them to more stable compounds. 

 Thus, when these wastes are dispersed in water. 

 the oxygen content of the water may be seriously 

 or completely depleted by the action of decompo- 

 sition processes. When the oxygen in the water i> 

 reduced to zero by the demands of aerobic decom- 

 position, a different type o( bacterial action oc- 

 curs — nonoxygen-demanding, or anaerobic, bac- 

 teria take over the processes of reducing these 



