The experts say excessive applications of nitrogen, phosphorus and potassium in the Mississippi River Basin are leading to more organic matter in the northern Gulf of Mexico, a situation that contributes to hypoxia in the Gulf's so-called dead zone.

The latter is an area covering more than 7,000 square miles along the coast of Louisiana. Each summer, scientists say, increased levels of nutrients result in decomposing organic matter depleting the oxygen, creating an area that cannot support marine life.

The main problem with this hypothesis is that a recent study shows farmers are harvesting more phosphorus and potassium in their crops in the 20-state area included in the Mississippi River Basin than they are applying to their fields. Economics and environmental regulations are also forcing growers to reduce nitrogen applications.

“An evaluation of the recent balance of P inputs as fertilizer and manure and the removal of P in harvested crops in the 20 major states shows that overall, the crop harvest removal of P exceeds fertilizer plus recoverable manure P inputs,” says Cliff Snyder, southeast director of the Potash and Phosphate Institute.

“The crop harvest of removal of P exceeds fertilizer plus recoverable P inputs in 11 of the 20 states in the basin.”

Snyder was referring to data from a 2004-05 study of soil test levels for P, K and pH in the United States and Canada. The study, conducted by PPI with the cooperation of about 70 public and private soil test laboratories, looked at 3.4 million soil samples.

The summary indicates that 40 percent of the 20 major states in the Mississippi River Basin have experienced a decline in soil test P (based on median values) since 2001. The average decline in the median soil test P levels exceeds the average increase in soil test P since the 2001 PPI soil test summary.

“The potassium budget in the 20 major states in the basin and the 2005 soil test summary results both show that significantly less K is also being applied than is being removed in crop harvests,” said Snyder, a soil scientist based in Conway, Ark.

While the soil test summary might provide a glimmer of hope for farm groups arguing against even more restrictions on fertilizer applications in the Midwest and other MRB states, it does not bode well for farmers trying to increase their yields.

“Contrary to popular belief, there are more soil samples with soil P testing in the agronomically responsive range than in the non-responsive range,” Snyder said. “More than 78 percent of the soil samples tested below 50 parts per million in Bray 1 equivalent-extractable P and 94 percent tested 100 parts per million or below.

“Clearly, elevated soil test P is a relatively minor issue in most of these MRB states.”

The frequency distribution of soil test K levels for the 20 states also reflects the K balance, “and it shows the strong need for continued and increased K fertilization,” says Snyder.

“The bulk of the soil samples test below 200 parts per million, levels at which an agronomic response to potassium may be expected by corn and soybeans in Iowa and other states.”

P and K deficits could be contributing to the hypoxia problem in the Gulf, but not in the way the experts think,” he says.

“When soil P and K levels are below optimum, fertilizer nitrogen use efficiency can decline,” he said.” So, some of the P and K soil test declines, and the P and K deficits may indirectly be contributing to reduced nitrogen use efficiency, and result in a potential for greater loss of nitrogen from farm fields via leaching and runoff.”

Nationwide, the PPI study found that phosphorus and potassium nutrient levels differ markedly from state to state, according to Paul Fixen, PPI's senior vice president, Americas Program coordinator and director of research. He cautioned against using the numbers as a guide to management decisions on a single farm.

“This summary can be viewed as an indicator of the overall nutrient supplying capacity of soils in North America,” he said. “It offers a snapshot view of soil test levels in 2005, but also provides a comparison to the previous summary which was completed in 2001.”

Fixen said some of the key findings of the summary — the ninth completed by the Institute since the 1960s — include:

  • The median P level for North America is 31 parts per million, with 42 percent of the samples testing less than 25 ppm, a “middle of the road” critical level.

  • The median K level for North America is 154 ppm, with 33 percent of the samples testing less than 120 ppm and 53 percent testing less than 160 ppm.

  • The median pH level for the United States and Canada is 6.3, with 31 percent of the samples testing less than 6.0.

“Critical Bray P-1 equivalent levels for the soils and cropping systems of the Great Plains and western Corn Belt are usually assumed to be around 20 parts per million and to increase to 25 or 30 ppm for the eastern United States,” he said.

“Recent research indicates that critical levels may be higher for high yield management crops. Certain crops, such as potatoes on some soils, will require much higher P levels — research shows response in the 100-ppm range. Bottom line…critical levels are site-specific.”

For the relatively high cation exchange capacity soils of western and central North America, calibration research usually indicates critical levels of K in the 140 to 200 ppm range, Fixen noted. Critical levels are usually lower in eastern North America and on low CEC soils may drop to 80 ppm.

“As with P, specific crops and management systems have different critical K levels than those indicated in the preceding paragraph,” he said.

Snyder said the data for fertilizer and recoverable manure use, nutrient balance estimates and soil test P and K results in the 20 Mississippi River Basin states indicate the following:

  • There is a strong need for P and K fertilization to sustain soil productivity.

  • Almost 80 percent of the sampled soils have extractable P in the agronomic range.

  • Few soil test P levels have been raised to levels that would appear to present a direct threat to water quality.

  • P and K removal from farm fields in crop harvests is out-stripping fertilizer plus recoverable manure in 40 to 50 percent of the 20-major MRB states.

  • Farmers, crop advisers and fertilizer dealers should pay close attention to individual field and farm nutrient budgets to ensure that P and K are not limiting N use efficiency and farm profitability.

  • Continued soil testing, evaluation of trends and estimates of nutrient balance can contribute to a better understanding of agronomic opportunities and potential environmental challenges.


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