Greenhouse study of P uptake by crops at low levels of residual P reserves in a calcareous soil

Two series of crops were grown in the greenhouse to study the decline of P availability and the response of plants to P fertilization. The soil used was a calcareous Udifluventic Ustochrept collected from an experimental field where the crops taken in the last five years had lowered Olsen P from 23.1 to 12.0 mg kg-1 of soil. Five treatments based on increasing amounts (16  80 mg P kg-1) of added P were compared with a control; pots were arranged in randomized blocks with seven replications. One cropping series started with Apium graveolens L. and went on with three successive sowings of Lolium multiflorum Lam. (celery series). The second series started with Medicago sativa L., and continued with Lolium multiflorum Lam. (lucerne series). Celery and lucerne showed a different yield response to P availability; the former gave place to outstanding yield increments, the latter exhibited only some feeble effects, if any. The subsequent grass crops offered recurrent yields which were significantly affected by P dressings in most cuts, but on the whole the extra dry matter produced was only 7 to 12% above that of the control. P concentration of shoots was only moderately influenced by the rates of added P; while the first two crops displayed figures which are to be considered in the normal range, ryegrass soon reached very low values. P uptake was almost always affected by the experimental treatments, and increased with the amounts of supplied P. At the end, the control soil had halved its initial Olsen P value, while the treated soils had lost all trace of the added amounts of P. In the fifth year, a new P enrichment was carried out, with 24  120 mg P kg-1, and Lycopersicon lycopersicum (L.) Karst. ex Farw. and Beta vulgaris L. were cropped. Only the latter gave yield responses significantly related to the amounts of added P. Both crops, however, showed clear differences in P uptake. As for Olsen P, relevant residual availability was detected in the soils of P-enriched treatments. These findings are in agreement with the theory of slow reactions (Barrow, 1980,1983). They are discussed in the light of the trend of P availability decline observed under open field conditions.