AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 2001, No. 6
• Availability of soil and fertilizer phosphorus is limited in acid and alkaline soils – pH 5.8 to 6.5 is usually optimum. Farmers in a short-term land tenure arrangement want to maximize the benefits of annual phosphorus applications. Those with longer-term rental arrangements or those who own the land want maximum benefits for the current crop and successive crops.
• Plant roots acquire phosphorus primarily by diffusion as phosphorus moves in the soil solution over short distances (less than about 0.13 centimeters or 0.05 inches), from areas of high concentration to areas of lower concentration. Maintenance of high soil test phosphorus through fertilization aids diffusion.
• Soil compaction, cool and wet conditions restrict root growth and may require higher phosphorus rates.
• Good surface and internal soil drainage enhances root growth and uptake of soil and applied phosphorus.
• In reduced tillage and no-till systems, surface broadcast applications result in phosphorus stratification near the surface. This surface stratification can limit phosphorus uptake as roots explore deeper layers for moisture. Before initiating reduced tillage systems, soil test phosphorus levels in the upper 6 to 8 inches should be raised to minimize potential constraints associated with phosphorus stratification from surface broadcast applications (e.g. no-till). Starter nitrogen and phosphorus fertilizers—in addition to broadcast phosphorus fertilization—frequently increase yields of corn and sorghum in conservation tillage systems.
• Band-applied phosphorus is generally superior to broadcast phosphorus in small grains, especially on low phosphorus soils with a high phosphorus “fixing” capacity. Dual banding of nitrogen and phosphorus in the fall or spring prior to seeding has increased efficiency and yields. Direct contact of diammonium phosphate (18-46-0) with the seed should be avoided to prevent ammonia injury.
• In wet soils, spring-applied broadcast phosphorus for soft red winter wheat has increased yields over fall applications. In dry soils, subsurface banding (2 to 6 inches) of phosphorus at planting has increased the yields of small grains.
• In soybeans planted on low phosphorus soils, narrower rows (less than 12 inches apart) have often increased yields and provided better incremental response to phosphorus fertilization than wider rows (more than 30 inches apart).
• Waterlogging and flooding release soluble forms of aluminum and iron in many soils. As these soils dry, phosphorus can be bound in insoluble forms and reduce plant uptake by upland crops. Higher phosphorus rates may be needed and applications near planting time may help.