Three Residual Benefits of Applying Phosphate Fertilizer

Core Ideas Phosphate continues to react with soil thereby decreasing its effectiveness. However, the continuing reaction conveys negative charge to the reacting particles. This reaction decreases both the buffering capacity and the rate of further reactions. Both of these changes increase the effectiveness of subsequent applications. Fertilizer P provides three benefits to subsequent crops. The first is that P not lost in produce or leachate remains in soil. However, its effectiveness usually decreases with time because the P slowly diffuses into the adsorbing particle. The second benefit stems from the resulting increased negative charge and decreased buffering capacity. We test whether this decreases the P required for a given yield. The third benefit occurs because, given enough P, the penetration reaction stops. We test whether it is then sufficient to only supply P needed by the plants. We incubated for 35 d at 60°C samples of a P–deficient soil to which we had added five levels of P. At the end of this incubation period, we measured P sorption and desorption on one set of samples of the soil. On another set, we added several different levels of P and measured plant response. On a third set, we similarly added several different levels of P but incubated soil plus the extra P for a further 3 d at 60°C before measuring plant response. Incubating the soil with P decreased the P buffering capacity and increased the effectiveness of P applications. The effects were nonlinear with greatest effect occurring at low levels of application. The incubation with P also virtually eliminated sorption–desorption hysteresis. With increasing levels of incubated P, the difference in plant response between the once–incubated and the twice–incubated treatments decreased linearly. All of the observations are consistent with slow penetration of the adsorbed P.