The relationship between nutrient concentration and yield of plant forms the basis of using plant analysis to assess nutrient status of plants. This study determined critical levels of phosphorus (P) in diagnostic plant parts of four grain crops. The crops were grown in greenhouse using a P‐deficient Typic Ustochrept fertilized with 0, 10, 30, 90, and 270 mg P/kg soil. Crop sensitivity to P deficiency was: sorghum > maize > sunflower > mungbean. Fertilizer requirements (mg P/kg soil) for near‐maximum grain yield were: sunflower, 89; and maize and mungbean, 74. Fertilizer requirement of sorghum was much greater than the other species. Critical P concentrations in whole shoots (≤30 cm tall) were: mungbean, 0.30%; sunflower, 0.29%; and maize, 0. 24%. Equivalent values for diagnostic leaves were: mungbean, 0.33%; sunflower, 0.31%; and maize, 0.26%. Critical P concentrations in mature seeds were: mungbean, 0.34%; maize, 0.29%; and sunflower, 0.20%.