Evaluation of the nutritive value of phosphate fertilizer using an isotopic dilution technique

A new formulation of the Fried method is proposed in which the availability of the fertilizer phosphorus is directly compared with that of the Larsen phosphorus. A value G is defined representing the equivalent of the amount of P applied with the same availability as that of the Larsen phosphorus in the soil. During a pot experiment, the G values were determined for eight different Iranian soils. The fertilizer used was superphosphate, applied in the ratio of 11.52 mg P per kg of soil; rye-grass was chosen as test plant. The results obtained show that the G values vary between 11.94 and 45.16 mg P/kg. These values are 1.04-3.92 times greater than the quantity applied. The availability of the fertilizer employed is thus 1.04-3.92 times greater than that of the Larsen phosphorus. It can be seen from the L and G values that, in at least four or five soils, the amount applied is not enough to produce a significant increase in the size of the pool of phosphorus available to the plants, whereas in the less fertile soils the G values are equivalent to 61-82% of the quantity of Larsen phosphorus. A statistical calculation shows that there are significant correlations between the G value of a soil and the quantity of phosphorus in the plants which is derived from this soil, between the G value and the quantity of phosphorus in plants derived from the fertilizer applied, and finally between the L+G value of the soil and the total quantity of phosphorus in the plants. It follows from this that the G value can be regarded as a convenient criterion for determining fertilization procedures