Acta Agronomica Hungarica, 57(3), pp. 363–370 (2009) DOI: 10.1556/AAgr.57.2009.3.10 0238–0161/$ 20.00©2009 Akadémiai Kiadó, Budapest OPTIMUM TIME FOR PHOSPHORUS FERTILIZATION ON EGYPTIAN ALLUVIAL SOIL A. M. EL-GHAMRY 1 , A. A. MOSA 1 and E. M. EL-NAGGAR 2 1 SOILS DEPARTMENT, FACULTY OF AGRICULTURE, MANSOURA UNIVERSITY, MANSOURA, EGYPT; 2 FERTILIZERS DEVELOPMENT CENTER, EL-DELTA FERTILIZERS PLANT, AKAHLIA,. EGYPT Received: 29 July, 2008; accepted: 16 June, 2009 The rapid fixation of phosphorus fertilizers in soil with a high content of calcium carbonate requires efficient management methods for phosphorus fertilization, especially as regards the time of application. For this purpose, a field experiment was carried out in the summer season of 2006/2007 in the experimental farm of the Soils Department, Faculty of Agriculture, Mansoura University, Egypt to evaluate the best time for phosphorus fertilization to cowpea on Egyptian alluvial soil. The results showed that adding half the recommended phosphorus fertilization rate at sowing and the other half before the first irrigation was the best treatment to enhance phosphorus fertilizer use efficiency, contributing to an increased uptake of phosphorus by cowpea, reflected in the higher phosphorus concentration in the grains. Increased phosphorus uptake also caused an increase in the nitrogen and potassium concentrations in cowpea grains, whereas the zinc and iron concentrations decreased. Key words: Egyptian alluvial soil, phosphorus fertilization, application date Introduction Phosphorus is one of the 17 essential elements required for plant growth and development, and one of the three most important nutrients, with nitrogen and potassium. Phosphorus contributes to many vital functions in the plant, such as early root and seedling growth, improved winter hardiness, promotion of early heading and uniform maturity, seed formation and quality, and increased water use efficiency. While these effects are more visible, phosphorus also plays a number of unseen roles such as in photosynthesis, energy storage and transfer, respiration and cell division. Crops deficient in phosphorus tend to develop slower, exhibit limited growth potential, and yield less than expected (Johnston, 2001). This is attributed to the low availability of soil phosphorus, especially in arid and semi-arid regions, caused by intense calcium phosphate fixation (Diez