Sensitivity of Phosphorus Uptake to Changes in Root Length and Soil Volume

Root length is generally considered important in phosphorus uptake, but it is not known whether a crop with longer roots is always efficient in absorbing P from soils low in available P. Consequently, we investigated the relationships between P uptake and root length, as affected by soil volume, and soil P status. Field‐grown buckwheat ( Fagopyrum esculentum Moench), castor ( Ricinus communis L.), peanut ( Arachis hypogaea L.), pigeonpea [ Cajanus cajan (L.) Millsp.], sorghum [ Sorghum bicolor (L.) Moench], and soybean [ Glycine max (L.) Merr.] were compared in their ability to absorb P from soils with low and high P status. The relationships between root length, soil volume, and P uptake were then examined in pot‐grown buckwheat, peanut, and sorghum using varying pot sizes. Under field conditions, maximum P uptake was by sorghum, which produced the longest roots. A strong correlation ( r = 0.85*) between root length and P uptake among crops was observed in high‐P soil, but not in low‐P soil. In pot culture, differences in P uptake were correlated to changes in soil volume rather than root density. Phosphorus uptake per unit soil weight was higher in peanut than in sorghum or buckwheat, especially in low‐P soil. Our results indicate that P uptake by crops is strongly correlated with root length in soils where P availability is high, but not in soils with low P availability, or where volume is limited. Results also suggest that peanut uses additional mechanisms beyond root length to increase P uptake.