Availability of phosphorus and soybean yield in crop rotation with Congo grass

Abstract

Crop rotation with Urochloa ruziziensis during the off season of soybean has been widely adopted due to the appropriate size of this grass and the several benefits to soil fertility, such as phosphorus (P) cycling. However, has been observed reduction of soybean yield when cultivated after U. ruziziensis, which seems to compete with soybean by soil P, since the reduction is not observed in cases with phosphorus fertilization. There are reports that the same does not occur after U. brizantha cultivation. The main objective of this work is to determine whether the reduction of soybean yield when cultivated after U. ruziziensis occurs in function of P forms in the soil caused of the U. ruziziensis cultivation. It will be conducted 4 experiments. The first is a long-term field experiment, that half of the plots are cultivated with U. ruziziensis in the off season of soybean, whereas the other half remains with no crop. Beside the treatments with and without Congo grass, it will be evaluated three levels of P (0, 30, and 60 kg ha-1 of P2O5) applied in the seed furrow of soybean. The second experiment it will be conducted in pots, comparing the soybean development after three species of grasses (U. ruziziensis, U. brizantha, Panicum maximum), as well as the effect of these species in the soil according with the P level. In the third trial it will be evaluated the dynamic of P forms in the rhizosphere of soybean, maize, and white lupin after the cultivation of the three grasses. A fourth trial will be conducted in order to compare the exsudation of organic acids of U. ruziziensis and U. brizantha. (AU)