Evaluation of residual and cumulative phosphorus effects in contrasted Moroccan calcareous soils

Consideration of factors affecting the availability of applied P in soils c ould improve P fertilization recommendations. Little information is availab le on the effects of continuous P fertilizer applications under cropping sy stems in Morocco. A greenhouse study was conducted to evaluate the residual and cumulative P effects on three succeeding crops, wheat ( Triticum aesti vum L.)-corn (Zea maize)-wheat, in contrasting calcareous soils from the ar id and semiarid zones of Morocco. The treatments were the amount (0, 3.4, 6 .7, or 13.4 mg P kg(-1) soil) and time of application of P. The residual P effects on grain yield, dry-matter production, and total P uptake were sign ificant. In this study, the increase in yields as a function of applied P w as explained by the model: Y = a + b*(P-ad)(0.5). The increase rate (consta nt b in the model) of dry-matter production of corn ranged from 0.56 (soil 10) to 2.89 (soil 11). At the same P fertilizer rate, single applications y ielded less grain production than repeated applications. These results show ed that if we want to take residual P into consideration in P fertilizer re commendations, the critical soil test P level should be lower than the one normally determined by soil test calibration method. Also, soils with low i nitial NaHCO3-P levels had the lowest residual value, inferring that a larg e portion of added P is fixed in these soils. This study showed that a sign ificant response of corn to residual P would occur in soils with initial Na HCO3-P test levels less than 6 mg P kg(-1). The response would be inconsist ent between 9 and 14 mg P kg(-1), and no response is expected above a soil test P level of 14 mg P kg(-1). In general, soils with more than 14 mg kg(- 1) NaHCO3-P could provide adequate P for maximum yield for three succeeding crops under greenhouse conditions.