Illinois
Fertilizer Conference Proceedings
January 23-24, 1990
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R.G. Hoeft1
Biblical sources tell us that "bread" is the staff of life, but it is in large part protein, a nitrogen (N) containing compound, within that bread that provides the sustenance we need. Protein requirement varies by age from 23 grams per day for very young children to 56 grams per day for adult men. Assuming an average of 40 grams per day for the entire populace will mean a yearly consumption in excess of 110 million tons of protein. A readily available supply of N will be required to meet this protein need plus produce the fiber for housing, clothing, and other necessities of life.
Neither plants nor humans are very efficient in harvesting N. This inefficiency results from biological and environmental factors in the soil-plant system, multiplication of inefficiencies as one converts from plant to animal protein, and the inefficiencies of the human race in harvesting, storing, distributing, and processing the food. While it is difficult to quantify, the ultimate harvest of N by humans is probably less than 20 percent efficient. As a result, the amount of N required by society is several magnitudes greater than that indicated for protein needs.
While the previous discussion has centered on the importance of N for protein production, it must be remembered that the primary benefit of N is the increase in yield per unit land area associated with increased N availability. For most grain crops, increased N availability is associated with minimal increases in grain protein, unless one is comparing crops being grown under severe deficiency with those with adequate N availability. This lack of change is likely a protective mechanism to insure that the seed has adequate components including protein to reproduce satisfactorily even when the seed is produced under adverse conditions such as N deficiency. Increased N availability often results in increased protein content of the forage which is of importance for crops where the forage is consumed by humans or animals. Adequate N availability frequently results in improved color, enhanced maturity, and reduced incidence of disease. Although N is but one of 16 elements essential for plant growth, it is one of the most critical, as it is the one most frequently deficient in agricultural soils. Recent estimates by U.S. specialists indicate that corn yields would decrease 40-50 percent if fertilizer N were eliminated.
It is estimated that the world N need may exceed 275 million ton by the year 2000 (Hauck, 1984). A significant portion (90 to 130 million tons of this) will come from grain legumes with another 55 to 80 million tons coming from biological N fixation in permanent pastures and meadows. The remaining 120 to 175 million tons will be derived from fertilizer. This is significantly higher than the current world usage of about 70 million tons of fertilizer N because of the need for significantly more food and fiber and because of the decrease in supply of N from organic matter in soils. Many soils of the world were inherently productive when they were first tilled as that process resulted in increased oxidation of organic matter and thus release of N. With continued farming, these soils have reached, or are approaching, an equilibrium level of organic matter. At such time as that equilibrium is established, there will be no net release of available N.
In recent years, questions have been raised about the effect of N fertilizers on the environment and consequently on human health. The environmental concern centers on the movement of nitrates through soil into water systems, both surface and groundwater supplies, and on the evolution of nitrogen oxides into the atmosphere.
Over the last several years, analysis of samples from domestic and community water supplies have shown nitrate levels in excess of the public health standard of 10 ppm nitrate-N at some time during the sampling period. However, until recently there has been no organized sampling program designed to identify the magnitude of the problem. The U.S. EPA is currently conducting a national survey of domestic and community wells. As of Sept. 30, 1989, it had completed the analysis of 295 wells, consisting of 180 community and 115 domestic wells. Of those, eight (less than 3 percent) had levels in excess of the 10 ppm standard. Only 145 (49 percent) tested positive for nitrates. Since most well water would have at some time passed through soil that contained nitrates, it is surprising that more did not contain a detectable level of nitrates. As further results are obtained from this survey, it will be interesting to utilize the information to determine the likely source for specific situations.
In at least some isolated instances, high nitrate levels have apparently resulted from natural causes. Analysis conducted in the mid-1800s in Washington County, Ill., indicated levels five times the public health standard. Since agriculture was still in its infancy at that time, it is doubtful that it was the cause of the high nitrate levels.
Even in modern times, there are good indications that agriculture is not the sole source of increased nitrate levels in water supplies. Recent results indicate that urbanization, rather than agriculture, was a major factor controlling the soluble reactive phosphorus in stream concentrations throughout the entire year, and was important in explaining the majority of the variance associated with nitrate-N during roughly 50 percent of the year in one Illinois watershed. Agriculture had its greatest influence on nitrate-N concentration in streams during late winter and early spring when crop growth and development was low (Osborne 1988). This low crop N use coincides with the time period of increasing biological activity and thus the release of organic N and application of fertilizer N for the following crop. It is difficult to determine the relative contribution of each of those two sources. However, results from Minnesota indicate that the amount of N lost into tile lines following soybeans was slightly higher than that lost following corn fertilized in the spring at a rate adequate to provide optimum yield. Fall application of that same N rate resulted in a slight increase in N loss through the tile lines.
Some have suggested that continued use of fertilizer N, particularity anhydrous ammonia, will destroy biological organisms in the soil. Research in Florida several years ago, clearly showed that not to be the case. Others have suggested that continued use of fertilizer N will destroy the physical and chemical properties of the soil. Results from both Kansas and Nebraska have disproven that theory.
Agriculture, including the use of nitrogen fertilizers is an energy-demanding process. However, it is one of the few processes of either nature or man that results in a net gain in energy. When applied at proper rates, there is a net energy harvest in the grain of about four units per unit expended to produce, transport, and apply nitrogen fertilizer.
Consumption of excess levels of nitrates have been shown to cause a condition called methemoglobinemia in infants. At birth, the gastrointestinal tract of infants in not sufficiently acid to prevent the growth of bacteria that convert nitrate to nitrite. Hence, a heavy intake of nitrates can lead to toxic levels of nitrites being formed and absorbed into the blood stream. While the primary concern is with nitrate levels in water, it is important to remember that it is the total nitrate intake including that from foods that is of concern. Some of the leafy vegetables fed to young infants, namely spinach, are known to contain high nitrate levels. Where nitrate levels of water are known to exceed the standard, acceptable water (water with a nitrate concentration of less than 10 ppm nitrate-N) should be obtained from other sources for infants under one year of age. Do not boil high nitrate water as that serves to increase rather than decrease the concentration.
The relationship of nitrates to other human health problems has not been extensively evaluated. What work has been done has not shown any clear adverse effect of nitrates on either heart disease or cancer. In fact, an Illinois study of a population of nearly 600,000 indicated no change in the rate of cancer deaths for the male population associated with a difference in nitrate level of the water consumed. On the other hand, the lowest nitrate levels had the highest death rate for total and food tract cancers for females. Similarly, a recent Pennsylvania study concluded that fertilizer use was largely unrelated to cancer mortality. In the one case where it was statistically significant (digestive cancer), it was negatively related (Stokes and Brace, 1988).
The effect of nitrates on livestock health depends on the species. Both swine and poultry have relatively high tolerance to nitrates. Ruminant animals -- cattle, sheep, goats, etc. -- are more susceptible to nitrate because it is only slightly acid in the rumen where fibrous materials are digested, and thus it allows bacteria that convert nitrate to nitrite to thrive.
The literature on nitrate effects on livestock is filled with apparent contradictions. In Washington County, Ill., there was an outbreak of illness and death among baby pigs apparently related to high nitrate levels in the water supply. On the other hand, researchers at the University of Wisconsin reported that mature dairy cows were not adversely affected by concentrations of 100 ppm nitrate-N in drinking water.
Is society now at an impasse between the use of fertilizer N to produce the quantity and quality of food and fiber needed to feed, cloth, and house the people of the world and the purported risk to the environment and human health? The answer to that is NO.
While there are areas where nitrogen fertilization practices have adversely affected water quality, these are for the most part related to specific environmental conditions or to point sources of contamination usually caused by inadequate management. At this point, with the limited data base available, it appears that the major concern exists in areas of intense crop production on sandy soils. The relatively low water holding capacity of these soils allows water to move greater distances in shorter periods of time. As water moves through soil, it will carry soluble nutrients below the rooting zone and thus the higher potential for nitrates to reach groundwater supplies. Additional research is needed to identify new nitrogen fertilizer management techniques that will reduce the potential for nitrate movement into groundwater in these situations.
Point sources of contamination are usually associated with human error, such as fertilizer spills or inadequate management of waste materials in and around wells. It is not unusual to find high levels of bacterial contamination in some wells having high nitrate levels. This is often due to poor well construction or to the topographic position of livestock-holding facilities that results in run-off of livestock wastes into the well. Better use of currently known technology will minimize these potential problems.
On the heavier soils, movement of nitrogen from agricultural land into water supplies has not been shown to be a major problem. As pointed out earlier, less than 3% of all wells sampled had nitrate levels in excess of the public health standard. Unfortunately, a base level from years preceding intensive agriculture is not available to determine whether or not that has changed. In other words, it is not possible to determine the amount of contamination that might be naturally occurring. It is unlikely that much could be done to correct those situations where nitrate levels in water supplies are naturally high. Movement of some nitrogen into surface and, groundwater supplies will continue as long as soils are biologically active. However, man has the knowledge to minimize the rate of such movement to levels that will be acceptable for the preservation of the environment.
Technology based on sound research will continue to be developed to reduce the potential for movement of nitrates into the environment.
Farmers and their advisors have done a remarkably good job of designing fertilizer programs to minimize the potential for nitrogen loss to the environment. They have more incentive for such programs as the loss of N translates into an economic loss. In addition, the farm family is at greater risk of having contaminated water as most farm wells are shallow and close to the land area receiving fertilizer N.
Agriculturalists must continue to fine-tune fertilizer programs. They must eliminate those situations, although limited in scope, where excess levels of fertility relative to the management ability of the farmer or the productive ability of the soil type are being used
1Robert G. Hoeft is Professor of Soil Fertility, Department of Agronomy, University of Illinois at Urbana-Champaign.
