EVALUATION OF THE POTENTIAL OF BIO-FERTILIZERS AS A SOURCE OF NUTRIENTS AND HEAVY METALS BY MEANS OF THE EXHAUSTION LETTUCE TEST

It has been assumed that lettuce is an appropriate crop to test the potential supply of nutrients and heavy metals (HMs) by bio-fertilizers based on biomass ash (BA) and biogas digestate (D). This hypothesis was validated in an incubation test, in which six prototypes of bio-fertilizers (BAD) with different BA to D ratios (type A without urea: 75:25: 55:25; 25:75; type B with urea: 65:25; 50:25; 45:45) were pre-incubated for six weeks in light soil with neutral pH reaction. Then, lettuce was grown for another five weeks to assess its response to increasing doses of BAD (0, 2, 4, 8 g kg(-1) soil). The soil test showed a significant, BAD-dependent changes in the content of nutrients and HMs. The key factor impacting the potential N supply to lettuce was urea, whose presence in a given bio-fertilizer led to an extremely large release of N-NH4. The N concentration in lettuce reflected its soil supply. The response of lettuce to N supply was modified by the supply of micronutrients. As a rule, their supply in soils treated with urea-free bio-fertilizers was low, significantly limiting the yield of lettuce. The content of copper (Cu) in soil treated with urea-enriched bio-fertilizers was inversely related to the lettuce yield. Its high content, concomitant with a simultaneously elevated content of lead, was the major cause of reduced lettuce yields. The negative impact of these two elements was overcome by an excess of N-NH4. The decrease in the available Cu content led to a better supply of Zn, which in turn resulted in higher yields of lettuce. The antagonism observed between Mg and Zn versus Pb concentrations in lettuce evidently manifests the importance of the two former elements as necessary inorganic amendments in bio-fertilizer based on bio-ash. The experiment clearly demonstrates that the exhaustion test with lettuce provided data about limiting factors, like copper, and about the ameliorating role of N-NH4.