It is acidic, contains primarily aluminum compounds and has limited minerals for plant growth. The oxisols are found under some areas and are comprised mainly of kaolinite clay. This is thick, sticky clay that has almost no nutrients available for plants. Under the higher lands that do not flood, a soil type known as spoldsols is found. This is fairly sandy and acidic.
Nutrients wash out of it easily, leaving little for plants to use.
Thus the people of the Amazon have multiple problems with farming the soil. The clays are relatively acidic, which stunts root development in plants. The clay contains a lot of aluminum that is toxic to plants. Last, the clay is thick, which makes it hard for plant roots and even water to penetrate. Putting a 1 mm layer of decayed leaves across the top will represent the O and A horizons.