The quantity/intensity relation is affected by chemical and organic P fertilization in calcareous soils.

The use of organic fertilization increases the availability of phosphorus (P) in calcareous soils by affecting the colloidal properties of soils. Accordingly, it was hypothesized that chemical and organic fertilizers affect P availability in calcareous soils by influencing P sorption and buffering capacity. The objective was to investigate the quantity/intensity (Q/I) relation in calcareous soils as affected by chemical and organic P fertilization. Three different soil types with different Olsen-P values including Qazvin1 (very low P, VLP), Qazvin2 (low P, LP) and Dizan (medium P, MP) were fertilized with 50 mg P kg soil using triple superphosphate (TSP), sheep manure (SM), and municipal solid waste compost (MSWC). The treated experimental soils were incubated for 90 days, and P sorption and buffering capacity indexes were determined using calcium chloride solutions in a range of 0-100 mg P L. A greenhouse experiment was conducted to determine wheat (Triticum aestivum L.) response to the experimental treatments. Wheat P content at tillering (60 days after planting) was determined. The SM and TSP treatments were the most efficient sources of P for plant use in the greenhouse, as they resulted in the highest wheat growth and P content. The incubation data were fitted to Langmuir, Freundlich, Temkin and surface sorption isotherm models. Langmuir model, as the best fitted one, indicated the highest P sorption (A) was resulted by the SM treatment for VLP and LP soils, compared to the other treatments. According to the model, the SM and MSWC treatments resulted in the least (0.04) and the highest (1.11) sorption energy (K) by the VLP soil, respectively. In the VLP soil the SM and MSWC treatments, and in the LP soil the MSWC treatment decreased P sorption, at the final concentration of P (100 mg L), compared to the control treatment. Organic fertilizers decreased buffering index, phosphorous buffering capacity, and K1 indexes in the VLP soil, compared to the control treatment. The corresponding reductions for SM were equal to 35.99, 2.7, 1.19 mL P g and for MSWC were equal to 12.33, 36.2 and 1.19 mL P g. In the VLP and MP soils, (compared with control), the SM treatment decreased the rates of maximum buffering capacity at 0.38 and 0.52 mL P g, respectively. There were high and significant correlations among the soil P buffering indexes with soil and wheat P content. Fertilization affected soil P availability by affecting the Q/I relation and the buffering capacity indexes. It is possible to predict plant response to available P using the tested fitting models.