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Is the isotopically exchangeable phosphate of a loamy soil the plant-available P?

C. MOREL and C. PLENCHETTE
Plant and Soil
Vol. 158, No. 2 (January (I) 1994), pp. 287-297
Published by: Springer
Stable URL: http://www.jstor.org/stable/42939395
Page Count: 11
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Is the isotopically exchangeable phosphate of a loamy soil the plant-available P?
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Abstract

This study compared the validity of using the isotopically exchangeable phosphorus (P) as an accurate measurement of plant available P by comparing the specific activity of P, i.e. the ³²p/³¹p ratio, in soil solution (Ss) against the specific activity of P in plants (Sp) growing in a loamy soil after applying a ³²P-labelled fertilizer (NaH₂PO₄2H₂O) at different rates (F) and specific activities (Sf). Non-mycorrhizal and mycorrhizal (Glomus intraradices) plants of two species (soybean and barley) were grown in greenhouse experiments. Ss values were determined on 1:10 soil suspension after periods of incubation ranged from 1 min to 35 d. At a given rate of P application, the Sp values of both non-mycorrhizal and mycorrhizal soybean and barley did not show significant difference although the plant P uptake varied 18 fold for all the (crop species x mycorrhizal infection) treatments over soil solution P values ranging from 0.02 to 5.46 mg P L⁻¹ (0.6-176 µM). Ss values decreased with time and reached a steady state after 35 d of equilibration period. Both Sp/Sf and Ss/Sf increased with applied P and there is a 1:1 correspondence between Sp/Sf and Ss/Sf values. The identity between the isotopie composition of both P in soil solution and in plant indicates that the isotopically exchangeable P (E = F(Sf/Ss-1)) is the only source of phosphate in solid soil phase which replenishes P of the soil solution after P has been removed by the plant, i.e. the only source of P which participates in plant nutrition. The isotopically exchangeable P of a loamy soil is the P available to growing plants and mycorrhizal fungi increases the P uptake giving plants wider access to isotopically exchangeable P in soil, and not making previously non-exchangeable P available. An immediate application of the 1:1 correspondence between a soil parameter (Ss/Sf) and a plant parameter (Sp/Sf) concerns the agronomic evaluation of P fertilizers.

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