Factors Responsible for Poor Response of Corn and Grain Sorghum to Phosphorus Fertilization: II. Lime and P Placement Effects on P‐Zn Relations

Soil factors controlling Zn concentration in the corn plant are very complex, including among others a beneficial action of applied N and a detrimental action of P or lime inherently present or added to the soil. It is apparent that Zn deficiency is likely to be aggravated or created by row application at low rates or heavier mixed applications of readily available P fertilizer. The problem is greatest on soils that are calcareous and especially on those of high inherent P supply where plant Zn concentration already may be approaching a critical level. The more effectively the applied P is utilized by the crop, the more severe is the reduction in Zn utilization. Thus the calcareous soil with tendency for Zn deficiency perhaps should not be treated so as to afford maximum early uptake and utilization of applied P. It also seems evident that low P rates rather than heavy, infrequent applications are likely to give less damaging effects. Alternative to these procedures is to apply supplemental Zn whenever P is added to soil where the level of Zn availability is uncertain. The damaging effect of P on Zn utilization is considered to be largely physiological in nature, probably a plant root cell absorption phenomenon, and not an external Zn phosphate precipitation. It is recognized, however, that root proliferation in the band of row‐placed P may be a contributing factor due to inadequate exploration of a soil body where enough available Zn may exist.