It has been hypothesized that soil amendments based on biomass ash (BA) and biogas digestate (D) create favourable conditions for vegetable growth and yield on light, acid soil. This assumption was validated in an incubation test, in which six prototypes of bio-fertilizers (BAD) with different BA to D ratios were pre-incubated for six weeks. In the second step, an exhaustion experiment with lettuce was carried out. The incubation test clearly showed that the BA to D ratio is the key factor affecting the content of CaCl2 extractable soil nutrients such as K, Ca, Mg. Na, Fe, Cu, Zn. The enrichment of BAD with urea resulted in a significant increase in the soil content of N-NH4, which in turn affected the content of soil available nutrients, mainly Mn. The yield of lettuce significaritly depended on the supply of N-NH4. The shortage of N supply as recorded in the soil treated with urea-free bio-fertilizers resulted in some disturbance of the nutrient uptake by lettuce, leading to the shortage of Fe and the concomitant excess of Pb. The high contribution of digestate in the BAD fertilizer coupled with an addition of elemental sulphur resulted in an excessive release of N-NH4, which led to a decrease in lettuce yield. The concentration of Pb in lettuce corresponded to the amount of bio-ash incorporated into the soil. The threshold value of 0.2 mg Pb kg(-1) FW was exceeded in the soil treated with the highest doses of the urea-free bio-fertilizers. The content of Pb can be controlled, provided the tested bio-fertilizers are enriched with magnesium and zinc. The content of cadmium in lettuce was below the threshold value of 0.1 mg Cd kg(-1) FW. The main reason for its low concentration was a sharp decrease of its supply to lettuce on plots treated with BAD bio-fertilizers.
