Phosphorus (P)

Phosphorus, another of the macro-nutrients is required in significant quantities by a cotton crop. It is taken up by the plant primarily as orthophosphate with the predominate form being highly dependent upon soil pH (PO43- at high pH; HPO42- at moderate pH; and H2PO4- at low pH). Utilization of the P nutrient in the plant is primarily associated with energy transfer with the molecules ATP and ADP. Phosphorus nutrition has also been shown to play an important role in the synthesis of cellular membranes (phospholipids) which help maintain cellular integrity. The amount that needs to be supplied through supplemental fertilization varies across the belt. However soil testing has proven to be an effective method of determining the need for supplemental P fertilization.

Soil phosphorus dynamics including potential tranformations and fates of applied fertilizer P.

In contrast to both N and potassium (K), soil P has low solubility and is therefore not readily available for crop uptake. Movement of soil P is negligible so placement of applied fertilizer P is critical in achieving adequate uptake efficiency. Banded applications of fertilizer P have proven the most effective thereby minimizing surface contact with the soil which tends to decrease the solubility of the applied P. Factors that influence uptake other than soil P levels include seedling vigor, absence of disease, and warm soils. Cool soil conditions decrease solubility of P leading to potential P deficiency symptoms early in the season. Soil testing for available P provides an indication of the need for supplemental P through fertilization. It is important that the proper soil analysis method is used for your particular soil in order to receive valid results. Available P in high pH, calcareous soils is best determined with the Olsen extraction method where sodium bicarbonate (NaHCO3-) is used as the extracting agent. In lower pH soils the Bray or Mehlich extraction methods should be used (NH4F + HCl). Testing in the western higher pH soils have resulted in recommendations of a soil P critical level at 5 ppm NaHCO3- extractable P. A positive crop yield response is expected with the application of supplemental P under these conditions. Individual state recommendations may be found in the “Resources from Individual States” section.

Images of P deficiency symptoms

Phosphorus deficiency symptoms in cotton.

Phosphorus deficiency symptoms in corn.