Effects of nitrogen sources oi phosphorus sensitivity in soybeans were investigated using three nitrogen source ($NH_4-N,;NO_3-N$ and urea) and six cultivars (Lee, Hill, R56-49, Harosoy, Clark-63 and Chippewa), differing in phosphorus sensitivity. Growth inhibition with ammonium or urea medium increased with phosphorus sensitivity indicating that the phosphorus sensitive cultivars are also sentitive to ammonium. The sensitive Clark-63 was inhibited two folds more than the tolerant Lee in ammonium medium. These results strongly suggest that phosphorus sensitivity is closely linked to nitrogen metabolism and ammonium toxicity. Nitrogen sources gave special morphorogical growth patterns (short internode-many branchs in $NO_3-N$, long internode-no branches in $NH_4-N$ and a low top to root ratio in urea) by affecting differently the growth of plant organs.