When it comes to cotton fertility, focus on the basics, says the head of the soil testing lab at the Agronomic Division in the North Carolina Department of Agriculture and Consumer Services.
Focus on soil pH and liming, phosphorous, potassium, nitrogen and sulfur in the context of a budget, says David Hardy, NCDA&CS section chief for soil testing.
Typical fertilizer costs for the season might include costs of $56 per acre; $17 for a half ton of lime; $13 for starter fertilizer; $11 for potash; and $15 for 30 percent nitrogen at layby.
“Soil pH is especially critical in cotton production because cotton does not tolerate acidity well,” Hardy told a group of farmers at the recent Southeast Cotton Conference.
In 2004, where growth problems occurred with cotton on Coastal Plain soils, almost 70 percent of the samples had pH levels below the target pH of 6.2.
In routine pH samples taken from Coastal Plain's soils, 35 percent had the most ideal pH range of 6.0-6.4. Similar results were found in the Piedmont, however, 30 percent there had a soil pH of 6.5 or higher. “Some growers may be over-liming in the Piedmont. This is usually not a great fertility concern, but they are liming and probably spending money unnecessarily,” Hardy said.
In regard to phosphorous, less than 2 percent of the cotton tissue samples received by NCDA in 2004 had low phosphorous. “This tells us that P deficiency is not a major limitation in growing cotton,” Hardy says.
“Some growers are over-applying phosphorous.” Almost 60 percent of routine soil samples related to cotton from the Piedmont had very high soil test P levels, whereas about 50 percent from the Coastal Plain had very high soil test P levels.
Hardy says that only about 14 pounds of phosphate is removed from the soil for each bale produced.
If a soil test recommends P, it should be incorporated into the plow layer for best results. There is no need to apply phosphorous if a soil test does not indicate a need, unless it is used as a starter at planting.
Phosphorous as a starter may be beneficial, even if adequate levels are found in the soil. Rates of 20 to 30 pounds of phosphate (P2O5) per acre should be enough. Response to P is more likely when cotton is planted early on cool, dark soils or under no-till conditions.
Since the starter effect is sometimes due to nitrogen, equal rates of nitrogen and phosphate are used often in starter. This can be achieved by applying 5 gallons of 10-34-0 or 11-37-0 plus 5 gallons per acre of 30 percent nitrogen solution (UAN), resulting in about 22 pounds per acre of nitrogen and 22 pounds of phosphate per acre.
Potash application can be prioritized based on the soil test as well. K is more of a concern on deep sands where clay is 16 inches or more below the surface. In those cases, consider split applications, half at planting and half at first square.
Begin a tissue sampling program just before first bloom and sample weekly for three to four weeks into bloom. If deficiency is seen early in bloom period, it is best to apply potash to the soil.
As for nitrogen, “be careful not to over-fertilize,” Hardy says. “Rate is based on soil production potential.” The biggest concern is applying too much, too early. To see if your cotton needs N, petiole sampling is very important.
On sandy soils, apply 20 pounds to 30 pounds of sulfur per acre based on the soil test
For more information, visit the Soil Testing Section of the Agronomics Division at http://www.ncagr.com/agronomi/sthome.