AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Fall 1996, No. 3
PHOSPHORUS PLACEMENT FOR ALFALFA
Alfalfa has a big demand for phosphorus—almost 15 pounds of P2O5 per ton of production. Alfalfa needs phosphorus for photosynthesis, energy, cell division, carbohydrate production, protein synthesis, root development and early growth, winter hardiness and nitrogen fixation.
Phosphorus is the key element in alfalfa establishment and continued productivity. However, in some soils, applied phosphorus becomes fixed and relatively unavailable to the plant—especially fine textured, acid soils. In high pH soils, phosphorus is also fixed, but in forms that remain plant available much longer. Recovery of applied phosphorus ranges from about 10 to 30 percent in the year of application, but depending on the soil, alfalfa will continue to recover fertilizer phosphorus for years to come.
Because applied phosphorus moves very little in soil, its availability to the plant can be strongly influenced by the method of placement. Before seeding, phosphorus can be broadcast and worked into the soil or banded...either option “banks” phosphorus for later use. Banding on low phosphorus soils tends to be superior to broadcasting because concentrating phosphorus in a limited zone reduces the opportunity for reactions with the soil which lessen availability. Applying large amounts of phosphorus at seeding should suffice for several crops.
After seeding, surface application has been the most common and most effective method of placement. Although alfalfa has a taproot, root branching, density and activity are greatest near the soil surface, making alfalfa very efficient at utilizing surface-applied immobile nutrients. However, under dry conditions, topdressed phosphorus may become stranded on the soil surface and inaccessible to shallow feeding roots. In these conditions, placement below the soil surface into moist soil near more active roots may be the best placement option.
Banding phosphorus into established alfalfa can be advantageous if root disturbance and stand damage are minimized. An Alberta study is showing banding to be an effective way to apply phosphorus in existing alfalfa stands. Averaged over four years, spring band applications of phosphorus have produced about 750 pounds per acre more alfalfa than spring broadcast applications. The success of this banding operation is due to the disc-band coulter which cuts a narrow slot in the ground with minimal disturbance of the alfalfa roots. Other studies in established stands using shank openers have shown little advantage for banding over broadcasting because of injury to the stand during the banding operation.
Whether you are broadcasting or banding—alfalfa needs plenty of phosphorus. Build your soil tests to the high range before stand establishment and apply enough phosphorus annually to offset crop removal and maintain soil test levels.
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