Phosphorus Utilization by Flax, Wheat, Rape, and Buckwheat from a Band or Pellet-like Application. II. Influence of Reaction Zone Phosphorus Concentration and Soil Phosphorus Supply

Two pot experiments have been conducted to study separately the influence of two factors on applied P utilization by four crops: flax (Linum usitatissimitm, L., cv. ‘Redwood’), wheat (Triticum vulgare L., cv. ‘Manitou’), rape (Brassica napus L., cv. ‘Tanka’), and buckwheat (Fagopyrum esculentum Moench). These crops vary greatly in their ability to utilize P from band or pelletlike phosphate applications. To Simulate a pellet-like phosphate application, ³²P labelled calcium phosphate was mixed with, and confined to a small fraction of the soil mass. In one experiment the quantity of P recovered from fertilizer reaction zones containing one of three calcium phosphates of widely different solubility products [hydroxyapatite (HA), octacalcium phosphate (OCP), or dicalcium phosphate dihydrate (DCPD)] demonstrated the existence of a direct relationship between reaction zone P concentration and crop recovery of applied P. In the other experiment the amount of P recovered from a reaction zone containing ³²P-labelled DCPD was reduced dramatically when the availability of P in soil external to the reaction zone was increased by mixing calcium phosphate throughout. This finding indicates the existence of an inverse relationship between the availability of soil P and the utilization of P from a band or pellet-like phosphate application. Utilization of P from phosphate fertilizer reaction zones by these crops can be explained on the basis of the performance of that portion of the root system which feeds on the applied phosphate, i.e. the proportion of the root system which develops within the fertilizer reaction zone, and the rate at which it absorbs the applied phosphate. The influence of the two factors, reaction zone P concentration-and soil P availability, appear to affect the utilization of applied P by their control over the performance of the roots within the fertilizer reaction zone.