Clay has completely special and unique features that make it predestined for use as substrate for aquarium plants. It has similar features to an ion-exchanger, and because of this it guarantees an even supply of nutrient substances, in that it holds back when excess is there, but gives-up when a deficit is present.
The black peat part contains up to 35% humic-acid. It provides for a desired low pH-value in the soil environment that together with the features of humic-acid, effectively works as a weak chelator, in this way providing, that the added nutrient substances remain in plant absorbable form.
Together with the clay this guarantees that the nutrient substances are released, but only very slowly