Source: Mycogen Seeds Agronomy Update

All crops require adequate levels of macronutrients and other trace elements to one degree or another. Sunflowers are no exception. Building a good soil fertility program begins with a soil test as the foundation. A soil test provides a snapshot of what nutrients are in the soil profile. Because soil is always changing, a soil test loses accuracy when there is a longer time span between the test and planting the crop.

"One unique aspect of sunflowers is the ability to use deep penetrating roots to reach nitrogen that was leached down beyond the reach of many other crop roots," says Bruce Due, District Agronomist, Mycogen Seeds. "Growers need to evaluate if any potential nitrogen exists lower than the soil-testing zone."

Sunflowers also need acceptable levels of nitrogen in the upper soil profile. Other factors to review are the prior nitrogen application program, the field crop history and precipitation records. The presence of more precipitation means more nitrogen has escaped to deeper soil depths. Sunflowers planted continuously on one field will not benefit as much from deep nitrogen as farmland without sunflowers in the crop rotation over the past four years, according to Due.

Sunflowers grow best in a well-drained soil type with good aeration properties. Sunflowers also make good use of their massive tap root system when no barrier to deep root penetration exists. In lighter soils, if a water table is within root reach, sunflowers can thrive in quite dry environments.

"Planned sunflower fields should be evaluated for potential weed problems, disease history, (especially for sclerotinia or white mold) and any potential herbicide interactions from previous years," Due explains.

Nitrogen Application Recommendations

Sunflowers require five pounds of nitrogen for every 100 pounds of sunflower seeds produced. Many growers will plan on a nitrogen program where they add enough nitrogen to bring soil nitrogen up to 70 to 80 total pounds, and let the sunflowers go deeper to secure the remaining amounts.

"However, excessive nitrogen can also harm sunflowers by causing other serious problems," Due says.

First, sunflowers use excess nitrogen to grow larger, taller plants, which may cause greater root and stalk lodging problems. These plants also exhibit larger leaves, which results in poorer air circulation among the plants. Good air circulation reduces plant wetness and allows for better disease tolerance by the sunflowers.

Other Soil Nutrients

Phosphorus, unlike nitrogen, does not readily move in the soil and no significant amounts exist in the deeper soil profiles. Sunflowers require two pounds of phosphorus for every expected 100 pounds of sunflower yield. It is very important to add the total amount of expected phosphorus needed for the crop.

Other nutrients such as potassium and boron should be closely monitored, especially if they are suspected to be at low levels in the field. This is often the case in sandy soil types with lower organic matter, Due concludes.

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