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 mar (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.