According to a widely cited article in the Agronomy Journal (1999, v. 91, p. 357-363) “Worldwide, nitrogen (N) use efficiency (NUE) for cereal production is approximately 33%.” (42% in the developed world). “... affordability ($0.22/pound) combined with the convenience of not having to apply N again during the growing season is attractive to farmers. In this regard excess N is applied as insurance, and because farmers are often overly optimistic concerning expected yields and yield goals. Because of this, the affordability of N in the developed world has led to its misuse and over application.” “Alternative N application strategies, specifically split applications (e.g., part pre-plant, part in-season) of N that are known to increase NUE, have not been widely adopted, largely because of the ease and affordability of applying more N than needed at or before planting. Agriculture's focus in developed countries has been on maximizing yields per unit area, and not until recently have we considered the environmental consequences of over application of nutrients.”

According to these authors, “There is no published research today where scientists have designed a package of practices specifically for high NUE.” “Excess nitrogen flowing down the Mississippi each year is estimated to be worth $750,000,000. At an average value of $490 per ton of N, $750,000,000 would comprise over 13% of the total value of N fertilizer ($5,480,356,000) applied in 1996 in the entire United States.” No wonder the huge dead zone exists in the Gulf of Mexico! Agronomists must change their mind-set and focus on NUE and the environmental consequences of both nitrogen and phosphorus fertilization practices much more seriously than has been done in the past. Agronomists are concentrated in land-grant colleges, originally established by the government to provide education aimed at crop productivity, and where funding from companies that profit from chemically intensive agricultural practices perpetuate the policy to “produce, profit, and damn the consequences.”

An easy way to think about the problem of conventional agricultural fertilization is that in Northumberland County, the average yield for corn is 130 bushels per acre. Conveniently, about 130 pounds of chemical nitrogen fertilizer (usually ammonium nitrate) is applied per acre. A bushel of corn weighs 64 pounds and contains about 15% moisture and about 1.2% nitrogen on a dry-weight basis. This means that about 85 pounds (130 * 64 * 0.85 * 0.012) of nitrogen is removed from each acre of the field with the crop. What happens to the other 45 pounds of nitrogen (130 - 85)? Most of the nitrogen not used by the crop ultimately ends up as nitrate in the groundwater, either directly or by oxidation of other nitrogen compounds like ammonia or NOx in the atmosphere, soil or groundwater. A small percentage of the nitrogen is converted to nitrogen gas by denitrification, but unlike salt marshes and rice paddies, denitrification is not a major process in oxidized soils.

When animal waste (poultry litter, manure or municipal sewage sludge) is used as fertilizer, pollution is greatly increased because about half the nitrogen is not “crop available.” Approximately twice as much nitrogen is land-applied using animal waste as would be applied using conventional chemical fertilizer, to grow exactly the same crop. In the example above, 236 pounds of nitrogen (130/0.55 - see below) would be land-applied in Virginia to grow exactly the same amount corn, using sewage sludge as fertilizer. Using sewage sludge results in 136 (.9 (assuming, very charitably, 10% denitrification) * (236 - 85)) pounds of nitrogen pollution per acre. Every time you see a farmer using sewage sludge (or poultry litter or manure), nitrogen pollution will be generated on each acre at a rate equivalent to dumping about 25 50-pound bags of 10-10-10 fertilizer into the Bay (ignoring possible uptake by riparian buffers.)

I observed the land-application of sewage sludge in Northumberland County in March of 2004, and believe that the property owner and spreader adhered strictly to current policies of the Virginia Department of Health, albeit in violation of Virginia Statute. Based on the submitted Nutrient Management Plan, 24,770 pounds of nitrogen were spread on 72.4 acres in accordance with Table 9-1 in the Virginia Department of Conservation and Recreation’s “2005 Nutrient Management Standards and Criteria” (“Standards”.) If chemical fertilizer had been used, 7,431 pounds of nitrogen would have been applied. Lime-stabilized sewage sludge is applied on the basis that 30% of the nitrogen is crop-available the first year, 10% the second and third years, and 5% the fourth year. The N application rate is determined by dividing the chemical fertilization rate by 0.3, or the amount of N available to the crop the first year (24,770 = 7431 / 0.3). 55% of the nitrogen is presumed to be crop-available over four years and the remaining 45% is pollution. If the nitrogen is not removed from the field with the crop, it is released to the environment, mostly as nitrate. This specific application caused at least 11,000 pounds of nitrogen pollution (2200 50-pound bags of 10-10-10, or 30 bags per acre.)

In the case of phosphorus, the soil from 5 fields encompassing 55.9 acres tested “Very High” in phosphorus. This acreage should have received no phosphorus (and no sewage sludge) according to Virginia Statute 12VAC5-585-550.A “The applied nitrogen and phosphorous content of biosolids shall be limited to amounts established to support crop growth” and DCR policy as set forth in “Standards.” According to “Standards” the remaining 16.5 acres, based on soil tests, should have received no more than 1151 pounds of phosphorus. In fact, 10,912 pounds of phosphorus were disposed (again, nearly 2200 50-pound bags of 10-10-10.)

It is well known that “...much of the crop land in the Chesapeake Bay watershed is now considered ‘optimum’ or ‘excessive’ in phosphorus from an agricultural perspective and hence needs little additional phosphorus, from any source, to ensure that economically optimum crop yields are attained.” (A. N. Sharpley, Ed., Agriculture and Phosphorus Management: The Chesapeake Bay, 1999, CRC Press, p. 66.) Despite this fact, the State, EPA and CBF all continue to advocate the land-application of phosphorus-rich animal waste. Massive immediate nitrate pollution is the result, and the resulting high-phosphorus soils will “bleed” phosphorus into waterways for decades.

Agronomists should concentrate on developing reliable, inexpensive timed-release fertilizers, so as to maximize fertilization efficiency, and assure that as much of the applied fertilizer as possible is consumed by the crop. Consider a corn crop fertilized with an efficient timed-release fertilizer. When they crop is mature and drying in the field at the end of summer, the fertilizer would have been consumed, and very little nitrogen and phosphorus would be released to the environment. Animal waste, in contrast, and unavoidably, continues to be actively decomposed by microbes at the high soil temperatures, and all the nitrogen and phosphorus released constitutes pollution because there is no uptake by the mature crop. Efforts by agronomists to improve the uptake efficiency of nutrients in animal waste, or adjust animal diets, are futile and can never significantly reduce pollution from such inefficient sources of plant nutrition. Funding from municipal wastewater and poultry operators is the only reason agronomists “research” animal waste, when they should be concentrating their efforts in a productive direction, namely to develop efficient timed release fertilizers.

/ Home / Water Quality / Groundwater Nitrate / Nitrogen Use Efficiency