The problem of persistence in pesticides can be highlighted by noting that this attribute exists in a range from moderately persistent (a lifetime of one to 18 months—2,4-D, atrazine); persistent (lifetime up to 20 years—DDT, aldrin, dieldrin, endrin, heptachlor, toxaphene); or permanent (lead, mercury, and arsenic). Presumably, the less persistent types should be more desirable, other things being equal; but those that degrade rapidly, such as the organophosphate insecticides, are extremely toxic and nonselective, which encourages rapid emergence of resistant insects and destroys their natural enemies. Thus, it is apparently not possible to adopt chemicals that function without some drawback or disadvantage.

Whether pesticides are applied by spraying or by surface application, air is the usual medium through which the chemicals move to their intended and unintended targets. Reliable data on how pesticides behave in air, such as distance travelled, are lacking, because adequate monitoring is unavailable. Their chemical and physical nature, method of application, and the atmospheric conditions will influence their concentration and ultimate fate. There is no doubt that pesticides may be transported long distances on dust particles. The rate of removal from air is difficult to predict, but in the long run the chemicals return to earth.

Odours, pollen, and dust

Odours from animal concentrations are recognized as being highly undesirable to air quality. Where these operations exist contiguous to urbanized areas, public reaction is usually unfavourable. Disposal of animal waste on the land may worsen the odour problem; in addition, high wind may move dry increments into the air. Smoke is emitted by operations designed to dispose of crop residues, or by controlled burning of weeds and brush. Air quality is also affected by transmission of allergenic pollen such as ragweed pollen, which can be blown for hundreds of miles.

Improper land use and treatment can cause considerable deterioration in air quality. Practices that strip the soil of plant growth or crop residues for long periods contribute to wind erosion, particularly in dry-farming areas. Fortunately, the technology of preventing wind erosion is well understood and widely used. Trash related to agriculture is moved freely by wind and distributed in unwanted fashion. Hulls of rice and wheat and cotton-gin trash are examples of this kind of airborne nuisance.

In contrast to most other technologies, however, the agricultural variety offers a major beneficial contribution to air quality. The photosynthesis of green plants removes carbon dioxide from the air and adds oxygen to it, thus helping to maintain the life-giving balance between these gases.

Sediment

Sediment is a resource out of place whose dual effect is to deplete the land from which it came and impair the quality of the water it enters. Aside from filling stream channels, irrigation canals, farm ponds, and irrigation reservoirs, sedimentation increases cost of water clarification. Suspended sediment impairs the dissolved-oxygen balance in water. The recreational value of farm ponds is diminished by sediment, while soil depleted farmland is reduced in value.

Plant nutrients

Nutrients of plants become resources out of place when they appear in groundwater and surface water; in fact, they become serious pollutants. Unwanted aquatic plants are nourished by plant nutrients derived from agricultural runoff, feedlots and barnyards, municipal and rural sewage, and industrial wastes. Aquatic plants clog irrigation and drainage structures, thus increasing maintenance cost and reducing capacity. Nitrates and nitrites in groundwater, which can poison human beings and livestock, result from both agricultural and industrial operations.

Inorganic salts and minerals

Inorganic salts and minerals that impair the quality of soil and water are derived from natural deposits, acid mine drainage, industrial processes, and drainage flow from irrigated areas. Salt accumulation on irrigated soils causes the most damage and loss in this category. A high proportion of sodium in irrigation water supply affects plant life adversely (see below Salinity). More than just a trace of boron is highly toxic; therefore, water used in municipal and industrial processes involving borax may not be usable for agriculture.

Organic wastes

Organic wastes emanating from municipal sewage, garbage, food-processing industries, pulp mills, and animal enterprises are attacked by aerobic bacteria. When this occurs in water, the oxygen content of the water is depleted or reduced to zero, at which point the anaerobic bacteria complete the process of reducing the wastes to inert material. This produces septic conditions that make the water unfit for recreational use, farmstead supply, or crop irrigation.

Infectious agents

Where not carried by wind, infectious agents are transmitted mainly by water and soil. Bacterial and virus diseases of crops are spread by machines that move contaminated soil. Insects are prime carriers of these diseases. Weed seeds are spread by irrigation water, as are nematodes. Animal diseases transmitted by water and soil include leptospirosis, salmonellosis, hog cholera, mastitis, foot and mouth disease, tuberculosis, brucellosis, histoplasmosis, Newcastle disease, anthrax, coccidiosis, and many others. Mosquitoes breeding in stagnant water can transmit encephalitis. Most crops and livestock in the world are susceptible to one or more highly infectious disease that may be transported by soil or water. The cost of losses from these diseases is staggering.