Given the worldwide effort to improve crop phosphorus (P) efficiency, it is important to understand the adaptive strategies of crops with different root types and properties under limited P availability. The present study examined root morphological and physiological responses of two graminoids (wheat and maize), and three legumes (soybean, white lupin and narrow-leafed lupin) to low P supply under two difference growth conditions. Two experiments using non-destructive root observation systems (Expt.1, semi-hydroponic system; Expt.2, rhizoboxes) were conducted under low P (Expt.1: 2 μM; Expt.2: 9 mg kg−1 soil) and optimal P (Expt.1: 200 μM; Expt. 2: 60 mg kg−1 soil). Plants were assessed 40 days after transplanting (Expt. 1) and 35 days after sowing (Expt. 2). Results showed that low P stress generally suppressed plant shoot and root growths of all crops and significantly increased root-shoot ratio in graminoid plants in both experiments. Root length vertical distribution in each 10 cm layer was relatively lower in low P than in optimal P treatments with the majority roots distributed in 10–40 cm layers in both experiments. Under low P, all plants accumulated significantly less P in plant tissues. Maize, soybean and white lupin plants had larger dry weights and P contents than wheat and narrow-leaf lupin under respective P treatment under both experiments except high P content in wheat cv Zhongyu 6 under optimal P in Expt. 2. Low P stress significantly increased plant P-use efficiency in both experiments with more profound increase in maize and white lupin in Expt. 1. Low P stress stimulated cluster roots formation in white lupin. Both lupin species had significantly higher rhizosheath acid phosphatase activity than other crops under low P supply. In conclusion, five crop species differed in their adaptive strategies to low P availability. Graminoid species had strong response by modifying their root morphology while legumes especially lupin species used the limited P more efficiently through enhancing rhizosheath acid phosphatase activity to cope with soil P deficit.