The assumption underlying this study was that bio-fertilizer amendments applied to acid soil would affect the yield of edible parts of vegetables, but would not increase their content of heavy metals. This concept was validated in an artificially prepared micro-plot experiment, carried out in a cropping sequence of radish-green bean-radish grown in poor loamy sand. Three series of a two-factorial experiment were based on two bio-fertilizers, as the first factor, with different ratios of biomass ash (BA) and solids of biogas digestate (D) (BAD: FE1, 2.2:1; FE2, 1:2.2). The second factor was a dose of the applied BAD: 0.0, 20, 40, 80, 160, and 320 g m-2. The share of digestate in the tested bio-fertilizer was the key factor affecting both the yield and elemental concentration of edible parts of the tested vegetables. FE2, rich in digestate, resulted in an enormous yielding response of plants. The yield of radish was significantly limited by the supply of macronutrients, both directly, such as by Mg affecting the first crop, and indirectly, due to the inefficient action of N towards the third crop in the studied cropping sequence. The yield of green bean grown on the FE1 treated soil followed the model detected for the 1st radish crop. The model of yields from for plants grown on the FE2 treated soil showed an increase reverse to the dose of FE2. Low, single doses of both bio-fertilizers resulted in low concentrations of certain nutrients, including Mg, Ca, Cu (all crops), and K (only green bean). High doses of bio-fertilizers, especially the ones rich in digestate, created a potential threat of excessive accumulation of heavy metals, as demonstrated in roots of the first radish, and pods of green bean. A single dose of any type of bio-fertilizer composed of biomass ash and digestate should be therefore adjusted based on i) grown crop requirements for a particular nutrient, ii) content of key nutrients in the applied fertilizer, iii) the level of respective nutrient content in cropped soil.

23.1.2018