Starter phosphorus not needed in some soils

Research conducted by North Carolina State University soil scientists has shown that by applying only nitrogen in their starter fertilizer, farmers who have fields already high in phosphorus can not only slowly decrease the amount of phosphorus that shows up in soil tests, but they also can reduce their phosphorus fertilizer application costs.

“Overall, using only starter nitrogen fertilizer would produce yields similar to those achieved with nitrogen and phosphorus starter fertilizer in soils that test very high for phosphorus,” says Deanna Osmond, professor and North Carolina Cooperative Extension leader in North Carolina State University’s Soil Science Department.

Osmond and Research Associate Sheri Cahill conducted the research. Cooperative

Extension agents and David Hardy, state Agriculture and Consumer Services Department (NCDA&CS) soil testing section chief, assisted them.

“Producers can reduce the cost of phosphorus fertilizer application and slowly decrease the amount of phosphorus in the soil as determined by the soil testing procedure by applying only nitrogen in their starter fertilizer. This will save money and help the environment at the same time,” says Osmond, a watershed, soil fertility and nutrient management specialist.

In 2003, more than 48 percent of soil samples submitted to NCDA&CS’s soil testing laboratory tested “very high” in soil P, she says (although 2003 data are the most recent available, soil test data tend to remain relatively stable over time).

“As soil test phosphorus increases, off-site phosphorus loss increases through erosion, soluble phosphorus runoff or leaching,” Osmond says.

Osmond and Cahill studied North Carolina Coastal Plain, Piedmont and mountain sites that NCDA&CS’s 2007 records indicated contained very high soil test phosphorus.

The researchers sought to determine if, when used on soils testing very high in P, starter-P fertilizer would affect the growth of corn (Zea mays L.) at 38 study locations and cotton (Gossypium spp.) at 13 study locations, 12 of those on the Coastal Plain.

They treated the test plot soils with starter-N and -P fertilizers, using 32 pounds of N and 13.4 pounds of P per acre (N plus P) or by using 32 pounds of N fertilizer per acre alone, both applied to the top of the soil in a band near the seed. They exactingly repeated the treatments four times.

On the day of planting, technicians or county Extension agents took soil samples from each plot and sent them to NCDA&CS’s Agronomic Division Soil Testing Laboratory for analysis.

The researchers measured N and P concentration in early-season plant tissue, how many days it took the corn to display silk and for the cotton’s earliest blooms to show, as well as yields for both crops during the growing season.

While results showed that corn yield was greater in the mountains than the Coastal Plain or Piedmont, and cotton yield was greater in the Coastal Plain than in the Piedmont, there were no differences between the N only and N plus P corn and cotton treatments within each region.

Moreover, the data indicated no yield differences resulted from the different treatments, Osmond and Cahill say.

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