Bio-fertilizers and Soil Health—An Approach Based on Balance of Elements in the Vegetable Cropping Sequence

It has been assumed that bio-fertilizers based on biomass ash and biogas leads to a depletion of soil macronutrients. This hypothesis was experimentally validated. Vegetables were grown in a cropping sequence of radish-green bean-radish grown on light soil and treated with two bio-fertilizers. They were both based on bio-ash and digestate (BAD) composed in contrasting ratio of 2.2:1 (FE1) and 1:2.2 (FE2) and phosphoric rock (15%). The BAD rates were as follows: 0, 20, 40, 80, 160, 320 g m⁻². The total yield of crops was limited by an uptake of K, Mg, and Cu. The decisive role of these three elements can be explained based on the course of their balance with respect to the type and rate of BAD. The absolute value of a particular element balance increased progressively with BAD rates. As a rule, low BAD rates led to depletion, while high rates resulted in the enrichment of soil resources for most elements, including heavy metals. The only exception was Fe and Mn, which soil resources increased along all of the applied BAD rates. The K balance pattern indicates that its supply within BAD, irrespective of the rate, was too low to prevent the exhaustion of its soil reserves. The strong depletion of soil resources at low BAD rate, but element specific, were recorded not only for Ca and Mg, but also for Zn, Cu, Pb and Cd. It can be concluded that soil amendments based on bio-ash and digestate applied in low rates should be enriched with the nutrients, which are crucial for an intensive and healthy production of vegetables.