AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Summer 2003, No. 7
Adequate phosphorus nutrition is essential to profitable winter wheat grain and forage production. A 60-bushel wheat crop will take up about 41 pounds of P2O5 and will remove about 30 pounds of P2O5 from the field. Phosphorus affects wheat growth throughout the season in several ways. Early in the season the young plant has limited root surface for absorbing phosphorus, so adequate available phosphorus in the root zone aids in uptake and early development. Sufficient phosphorus enhances root proliferation and results in increased tiller production, which in turn increases forage and grain yield potential. Adequate phosphorus fertility is also associated with reduced winterkill, maximum water use efficiency, hastened maturity, and lower grain moisture at harvest.
Several factors influence wheat response to phosphorus fertilizer. While soil test level is certainly an important factor, there are numerous others that may affect response. Soil characteristics such as temperature, moisture, pH, salinity, and compaction are important, as is placement of phosphorus fertilizer. Stratification of phosphorus and how samples for soil tests are taken may also be important variables. These and other factors act individually and in combination to affect grain and forage response.
Generally speaking, if soil phosphorus level is high or very high there is a low probability of response to applied phosphorus fertilizer. However, other factors such as soil temperature and pH can “overshadow” the effect of soil test level such that response can occur in soils that test high in available phosphorus. Consider, for example, a study conducted in the Texas Blacklands on a soil testing high in available phosphorus. Three planting dates were evaluated…one in September, another in November, and the latest in December. Phosphorus fertilizer placement methods were also evaluated…preplant incorporated and in-seed furrow at planting. With the early planting date there was no response to phosphorus fertilizer, but as planting was delayed (cooler soils) response to phosphorus was observed. Where wheat was planted in November there was about a 10 percent grain yield increase from phosphorus fertilizer, regardless of placement method. At the latest planting date (December) the yield increase from in-furrow phosphorus was substantially higher than broadcast-incorporated phosphorus, 80 percent vs. 13 percent.
A similar effect was observed with forage production. At the latest planting date, response to in-furrow phosphorus was double what it was with broadcast-incorporated application. The results of this study demonstrate how some factors have the potential to overshadow the effect of soil phosphorus level, and how phosphorus fertilizer placement can enhance this effect.
As we look ahead and begin planning for the fall season, don’t overlook the importance of phosphorus fertility. It’s a vital and fundamental component of profitable winter wheat forage and grain production. Use soil testing to help determine optimal rates of phosphorus application; also consider local data and experience. Furthermore, to insure maximum phosphorus fertilizer use efficiency, consider placement options and the many factors that affect wheat response to phosphorus.
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