Article

Fertilizer research

, Volume 45, Issue 2, pp 91-100

First online:

Dynamics of phosphate in soils. An isotopic outlook

  • J. C. FardeauAffiliated withDépartement de Ecophysiologie Végétale et Microbiologie, Commissariat á l'Energie Atomique - Cadarache

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Abstract

The quantities of phosphorus leached or carried away by erosion are always low in comparison with the quantity of P taken by plants. Therefore, without P application, dynamics of P in soil-plant systems depends mainly on plant P uptake from available soil P. This paper concerns mainly the description and the analysis of available soil phosphate. Available soil P is generally determined using extraction methods. It was shown, using isotopic exchange, method developed in soil-solution mixtures in steady-state, that available soil P could be described as a system with many pools. For agronomic purposes, available soil P was described with five pools of phosphate ions which have increasing rates of transfer from soil particles to soil solution. The most important is a pool which contains only directly available phosphate ions. These phosphate ions leave the solid phase of the soil to enter in the soil solution in a time shorter than 15 seconds: this time is the minimum time required to realize its determination. The intensity, quantity and capacity factors recommended to describe the available soil P are derived from experimental data characterizing this pool. The four other pools all are directly branched on this central pool. They are characterized by the quantities of P they contain and by the time required for the phosphate ions they contain to enter into the soil-solution. Effects of continuous P uptakes and of various P applications on these compartments were described. It was shown that: (i) continuous cropping without P return decreases ineluctably the intensity and quantity factors, and consequently soil P fertility (ii) applications of P quantities higher than P outputs with crops do not always increase the intensity and quantity factors, and consequently available soil P and its availability. The absolute values of the increase depends on: the type of P fertilizer or crop residues applied on the soil, on the level of soil P fertility before P application and on the fixing capacity of the soil for phosphate ions.

Key words

bioavailable P compartments isotopic exchange manure P fertilizers P fixation P release residual wastes