By Robert H. Wells
Delta Research and Extension Center

STONEVILLE -- Waiting until spring to make phosphorus applications can mean a nearly 10 percent increase in rice yields, according to new research by Mississippi State University.

“We receive a lot of questions this time of year about applying phosphorus fertilizer in the fall when fields are dry and prepared for spring planting,” said Tim Walker, rice agronomist at MSU's Delta Research and Extension Center in Stoneville.

“After comparing phosphorus applications made in the fall and in the spring for two years at two on-farm locations in the Mississippi Delta, we found waiting until the spring to apply phosphorus increased rice yields by 8 percent to 9 percent over applications made in the fall,” Walker said.

The optimum timing for phosphorus fertilizer is the window between planting and just prior to flooding.

 “Many growers apply a starter application of nitrogen when rice reaches the one- to three-leaf growth stage. In situations where phosphorus is deficient, diammonium phosphate, or DAP, can be used to supply a small amount of nitrogen and cover phosphorus nutrition needs,” he said.

Walker said when phosphorus is applied as part of a larger fertilization program, DAP can be blended with ammonium sulfate to supply about 20 pounds of nitrogen, 23 pounds of phosphorus pentoxide and 12 pounds of sulfur per 100 pounds of product applied.

The rice agronomist said cool temperatures and wetting/drying cycles in Midsouth soils during the fall and winter months decrease phosphorus availability.

“Also, phosphorus is a relatively immobile nutrient in the soil,” Walker said. “It can be bound easily by other elements, making it unavailable to rice during the growing season if applied in the fall. With the high cost of phosphorus fertilizer, we want to maximize that investment and have it available for the plant at an opportune time.”

Steve Martin, Extension economist based in Stoneville, said phosphorus prices have risen in recent years with growing demand for the mineral.

“It started a few years ago as Brazil and other countries increased row-crop production,” Martin said. “This year, the increase in corn acres in the United States just added to the demand. Additionally, higher fuel prices have affected phosphorus prices since it is bulky and expensive to ship.”

DAP is selling for up to $443 per ton in October compared to about $275 per ton in October 2006, a more than $150 per ton increase in one year.

Phosphorus is important in rice production for root development and growth. It also promotes tillering, which is a major component of yields.

Nathan Buehring, Extension rice specialist, said rice producers usually need phosphorus on lighter, silt-loam soils that have been recently land-formed.

“Phosphorus is needed typically in the heavy-cut areas of the field because there is not much available phosphorus,” Buehring said. “We also have seen times when phosphorus is needed in a maintenance fertilizer program.”

The rice specialist said in high production settings, such as 60-bushel-an-acre soybeans rotated with 170-bushel-an-acre rice, phosphorus is mined from the soil at a faster rate than 10 to 20 years ago when those high yields were unachievable.

“In these situations, a maintenance program including phosphorus may be needed to keep producing high yields,” Buehring said. “Pulling and having soil samples analyzed is a good tool to monitor soil nutrient levels.”

Rice agronomist Walker recommended Mississippi rice producers have their soils tested once every three years to determine fertility requirements.