AGRI-BRIEFS   AGRONOMIC NEWS ITEMS

From Agronomists of the Potash & Phosphate Institute 655 Engineering Drive, Suite 110 Norcross, Georgia 30092-2837 Phone (770) 447-0335

Summer 2001, No. 7

Several factors control wheat response to phosphorus fertilization. Soil test level is probably the most obvious of these factors. While this is certainly one of the most important, there are numerous others. Soil characteristics such as temperature, moisture, pH, salinity, and compaction are important, as is placement of phosphorus fertilizer. Stratification of fertilizer phosphorus and how soils are sampled may also be important variables. These and other factors act individually and in combination to affect response.

The influence of phosphorus fertilization and placement on winter wheat was evaluated over 17 years and 48 locations in the Texas Blackland prairie. This is a region characterized by heavy textured soils and annual average rainfall from about 30 to 40 inches. Phosphorus was applied in-furrow with the seed, drilled half way between seed furrows, and broadcast incorporated. Fertilization with up to 40 pounds P₂O₅ per acre, regardless of method of application, increased grain yields by an average of 12 bushels per acre. Not surprisingly, the largest yield increases were where soil test phosphorus was low. Although phosphorus significantly increased yields in general, in-furrow placement was the most efficient application method. The more massive root system and enhanced tillering associated with the in-furrow application resulted in greater yield potential. In addition, phosphorus placed with the seed produced significantly more winter forage than did other methods of application.

Wheat forage and grain production in drier climates may benefit from deeper placement of phosphorus fertilizer. Research has been conducted in the Rolling Plains of Texas for several years on the effect of deep (six to eight inches) banding phosphorus on forage and grain yield. Yields were generally higher where phosphorus was deep banded than where broadcast on the surface and incorporated. This can be explained in terms of the position of available soil moisture, fertilizer phosphorus, and root activity. Where fertilizer phosphorus is surface applied and shallow incorporated, stratification of available phosphorus in the upper few inches of soil tends to develop. When the upper two to three inches of soil are moist, the crop can use this phosphorus since roots are active in this zone. However, in drier climates, as the crop reaches deficit moisture conditions in the fall and winter, the shallow phosphorus-enriched zone is too dry for active root uptake of phosphorus. By placing phosphorus fertilizer deeper (six to eight inches) the crop is able to maximize phosphorus uptake since it is at a depth in the profile where soil moisture and root activity are more abundant. Hence, soil moisture is an important consideration in maximizing positional availability of phosphorus fertilizer.

As you look ahead to the fall season, don’t overlook phosphorus fertility. It is a vital and fundamental component of profitable wheat forage and grain production. Furthermore, to ensure maximum phosphorus fertilizer use efficiency, consider placement options and the many factors that affect wheat response to phosphorus.