A replicated pot experiment was conducted to study the effect of five levels of fluorine (0, 25, 50, 100, and 200 ppm F) and four levels of phosphorus (0, 25, 50, and 100 ppm P), applied to a rice crop grown in soils of two sodicities, exchangeable sodium percentage (ESP) 30 and 70, on their transformations in soil and uptake by a subsequent wheat crop. Application of fluorine resulted in increased water-extractable fluorine in the soil, the relative increase being greater in soil of high ESP, which was associated with high pH. Fluorine uptake by plants increased linearly (r = 0.89) with increased water-extractable fluorine in the soil, and this resulted in a significant reduction in the wheat yield. A fluorine content of 35 ppm in mature wheat straw, which was associated with 22 ppm of water-extractable fluorine in the soil, was observed to be the critical concentration for grain yield. Higher uptake of fluorine by the plant resulted in increased uptake of sodium although composition in respect of other constituents remained unaffected.
Application of both phosphorus and fluorine resulted in higher extractability of each other in soil, the relative increase being more at higher fluorine levels. The effect of phosphorus on soil fluorine was more marked at low than at high ESP. From the results it seems that while there is a positive effect of phosphorus on soil fluorine, it has a negative effect on its uptake by the plant. Increasing phosphorus in the soil and in the plant resulted in marked reduction in the chlorine content of the plant (r = 0.79).