Abstract Phosphorus (P) uptake by plant roots depends on P intensity (I) and P quantity (Q) in the soil. The relative importance of Q and I on P uptake is unknown for soils with large P sorption capacities because of difficulties in determining trace levels of P in the soil solution. We applied a new isotope based method to detect low P concentrations ( < 20 lgPl )1 ). The Q factor was determined by assessment of the isotopically exchangeable P in the soil (E-value) and the I factor was determined by measurement of the P concentration in the pore water. A pot trial was set up using four soils with similar labile P quantities but contrasting P buffering capacities. Soils were amended with KH 2 PO 4 at various rates and pigeon pea (Cajanus cajan L.) was grown for 25 days. The P intensity ranged between 0.0008 and 50 mg P l )1 and the P quantity ranged between 10 and 500 mg P kg )1 . Shoot dry matter (DM) yield and P uptake significantly increased with increasing P application rates in all soils. Shoot DM yield and P uptake, relative to the maximal yield or P uptake, were better correlated with the P concentra- tion in the pore water (R 2 =0.83–0.90) than with the E-value (R 2 =0.40–0.53). The observed P uptakes were strongly correlated to values simulated using a mechanistic rhizosphere model (NST 3.0). A sensi- tivity analysis reveals that the effect of P intensity on the short-term P uptake by pigeon pea exceeded the effect of P quantity both at low and high P levels. However, DM yield and P uptake at a given P intensity consistently increased with increasing P buffering capacity (PBC). The experimental data showed that the intensity yielding 80% of the maxi- mal P uptake was 4 times larger in the soil with the smallest PBC compared to the soil with the largest PBC. This study confirms that short-term P uptake by legumes is principally controlled by the P intensity in the soil, but is to a large extent also affected by the PBC of the soil. Keywords P buffering capacity Æ P intensity Æ P quantity Æ P uptake Æ Pigeon pea Introduction Highly weathered tropical soils are often character- ized by large phosphorus (P) sorption capacities and small P concentrations in the soil solution, and the subsequent low P availability is an important limita- tion for crop production on these soils. The incor- poration of legumes such as Mucuna pruriens or Lablab purpureus in a rotation system can lead to an Section Editor: N. J. Barrow P. Pypers (&) Æ J. Delrue Æ J. Diels Æ E. Smolders Æ R. Merckx Division of Soil and Water Management, Department of Land Management and Economics, K.U.Leuven, Kasteelpark Arenberg 20, 3001 Heverlee, Belgium e-mail: pieter.pypers@biw.kuleuven.be Plant Soil (2006) 284:217–227 DOI 10.1007/s11104-006-0051-y 123 RESEARCH ARTICLE Phosphorus intensity determines short-term P uptake by pigeon pea (Cajanus cajan L.) grown in soils with differing P buffering capacity Pieter Pypers Æ Josefien Delrue Æ Jan Diels Æ Erik Smolders Æ Roel Merckx Received: 14 November 2005 / Accepted: 1 March 2006 Ó Springer Science+Business Media B.V. 2006