PHOSPHORUS STATUS OF A SOIL CATENA UNDER LIBERIAN EVERGREEN RAIN-FOREST - RESULTS OF P-31 NMR-SPECTROSCOPY AND PHOSPHORUS ADSORPTION EXPERIMENTS

Phosphorus NMR spectra were recorded of the A horizons of a soil catena under vital stands of Liberian tropical high forest. Organic P compounds play a major part in the total P reserves (ca. 1/2 of total extractable P which is 1/5 of total soil P). Thereof, between 3.3 and 5.3 kg orthophosphate diester P per hectare participate in the plant available nutrient pool; similar amounts were calculated for the calcium acetate-lactate soluble fraction. Persistent orthophosphate monoester P makes up an even greater percentage of soil P (10 to 25 kg ha-1). Mineral fertilizer requirements are in the range of 25 to 50 kg per hectare for 10 cm depth (based on a 48 hrs. adsorption experiment) when a soil solution concentration of 0.02 mg L-1 is considered adequate for the P supply of plants. P sorption can be explained by the amorphous fractions of Al and Fe hydroxides and the percentage of clay. The apparent vitality and absence of P deficiency symptoms of forest trees suggest the existence of an intact, however delicate, natural P cycle.