Mixed culture of wheat (Triticum aestivum L.) with white lupin (Lupinus albus L.) improves the growth and phosphorus nutrition of the wheat

The aim of this study was to investigate how mixed culture of wheat (Triticum aestivum L.) with white lupin (Lupinus albusL.) improves the growth and phosphorus (P) nutrition of the wheat. Wheat and white lupin plants were grown either in mixed culture or in monoculture in soil columns containing an adequate supply of nitrogen but a limited supply of P. Mixed-culture of the wheat with the lupins increased the shoot growth and shoot P uptake of the wheat by 33 and 45%, respectively, without significantly affecting the growth or P uptake of the lupins. After 6 weeks of plant growth, the soil columns were leached first with distilled water and then with 10 mM citric acid monohydrate (pH 4.0). In the initial fraction of the water leachate, the phosphate concentration for the lupin monocultures was very high, as high as for the uncultivated soil and approximately 10-fold higher than for the wheat monocultures. In contrast, in the initial fraction of the citric acid leachate, the phosphate concentration for the lupin monocultures was very low, approximately only 10% of the concentration for the wheat monocultures. For the mixed cultures, the phosphate concentrations for all fractions for both the water and the citric acid leachates were consistently low. Thus, the lupin monocultures preferentially depleted the citric acid-leachable soil P pool whereas the wheat monocultures preferentially depleted the water-leachable soil P pool and the mixed cultures depleted both pools. The lupin monocultures lowered the soil pH by 0.3 pH units whereas the wheat monocultures raised it by 0.8 pH units and the mixed cultures gave a soil pH intermediate between the two monocultures. Thus, the lupins in the mixed cultures partially offset the alkalinisation of the soil caused by the wheat and vice versa. This will be discussed in relation to the impact of soil pH on the plant availability of soil P.