PHOSPHORUS ASSIMILABILITY IN FRENCH BROWN PEAT
To appreciate the present fertility status of a substrate, the determination of water soluble mineral elements contents is used commonly (1/1.5 volume extract, SONNEVELD, 1974 ; DARTIGUES, 1981) ; in the case of substrates made with French brown peats, the water soluble P contents are small though a P fertilizer supply has been realized during the preparation of the substrate.
A study of the phosphorus fixation was conducted with an experiment without plant upon materials coming from three layers of a peat bog in the West of France and upon a German sphagnum peat. The variations of the water soluble phosphorus content were evaluated with the pH, time and the added phosphorus quantity.
The results show that, unlike the sphagnum peat, the different layers of brown peat retain the phosphorus with a variable intensity, which depends also on the pH and the time of contact.
The authors survey hypothesis about the organic and/or mineral origin of the phosphorus fixation in this type of peat.
They attempt to appreciate the degree of P fixation with the help of successive water extraction to know if this "fixed" phosphorus can become again available for the growing.
Some conclusions about the substrate analysis interpretation and the fertilization programme of growings in brown peat mixtures are given.