One of the principles of the circular economy requires the effective recycling of organic waste, especially since its volume is increasing from year to year. Therefore, one of the best solutions to solve this problem is the composting of organic waste. This paper presents the results of a comparative study of 10 different composts in terms of their quality. The composts varied with regard to waste materials used for composting, the adopted composting method, and the degree of compost maturity. Both biological (seed germination rate and intensity of plant root growth) and chemical methods (contents of macro- and micronutrients, presence of heavy metals, as well as the quality and quantity of humus compounds) were applied to evaluate the fertilisers, providing comprehensive characteristics of individual composts. It was found that composts prepared on the basis of sewage sludge had the highest contents of macro- and micronutrients as well as heavy metals. In addition, these composts contained the greatest amounts of humic substances, although these were of low quality. These composts also strongly inhibited seed germination and plant root growth. The least desirable chemical composition was found for the composts based on yard trimmings and household wastes prepared in home composters. These composts contained good quality humus compounds and had a positive effect on seed germination and plant root growth.

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Sewage sludge derived from municipal sewage treatment plants is an important source of macronutrients, micronutrients and organic matter. For this reason composting of sewage sludge, along with combustion and co-combustion, is a new management priority in Poland. In this study six composts of different origin and composition were evaluated in terms of their abundance in phosphorus, because it is an essential nutrient for all living organisms. Analyses were conducted on the samples at the initial and at the maturation phase of composting. The bioavailability of phosphorus was estimated on the basis of amounts of the nutrient in isolated fractions using the sequential extraction method. First of all quantitative changes of the total nutrient content and its amounts in separated fractions were dependent on the mixture composition. Irrespective of compost type, 34.5-75.0% of the total amounts of phosphorus were found in hardly available combinations (Fr. III), while available phosphorus forms (Fr. I) accounted for only 6.6-21.6%. As a result of composting together different organic wastes an increase was observed both in the total content and the amounts of this nutrient in separated fractions. This phenomenon was observed particularly in composts with smaller levels of sewage sludge (30-40%), characterised by rapid organic matter decomposition, which was indicated by higher bioavailable amounts of phosphorus. Under such conditions the content of P ranged between 3.68 and 7.4 g kg . In comparison to the labile pool of P obtained for matured composts C5 and C6 (65 and 75% of sewage sludge in their composition) amounting to 2.45-3.0 g kg −1 the above values were considerable. Bioavailable phosphorus contents potentially introduced to soil with composts doses calculated at 170 kg total N/ha/yr ranged from 69.8 to 80.2 kg for compost with the lowest share of sewage sludge and from 11.2 to 20.7 kg for compost with the highest share of sewage sludge.

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The study discusses changes in lead contents in soil and crops after application of compost and fly ash. A three-year experiment was conducted on narrow-leaved lupine (Lupinus angustifolius L.), camelina (Camelina sativa L.) and oat (Avena sativa L.), which were grown on medium soil fertilised with compost or fly ash (FA). The lead amounts in various combinations were evaluated using sequential (BCR analysis) and single (DTPA solution) methods. The total contents of lead and amounts of selected macronutrients were assessed in the biomass of plants. The Pb contents in the soil and plants were used to calculate the risk assessment code (RAC), individual contamination factor (ICF), bioconcentration factor (BCF) and contamination coefficient level (CCL). Lower amounts of bioavailable Pb in the soil were observed as a result of compost and FA application. Fly ash immobilised Pb more effectively, as evidenced by the BCF and CCL values. The increase in the Pb contents in the sequentially separated fractions and in plant biomass were caused by applied compost or FA. The Pb contents in exchangeable forms decreased, whereas its contents in residual forms increased.

This work presents quantitative changes of nickel in soil and plants under the influence of compost and fly ash. The research was carried out in a 3-year experiment on medium soil fertilised with compost or fly ash. The plants: narrow leaf lupine (Lupinus angustifolius L.), camelina (Camelina sativa L.), and oat (Avena sativa L.) were planted in consecutive years. The soil from the experiment was subjected to extraction by sequential analysis with the Community Bureau of Reference (BCR) method, and single extractions using 1 mol·dm−3 HCl and DTPA solutions, obtaining the amount of nickel in various combinations with the soil solid phase. Total contents of the metal in soil and cultivated plants were determined. On the basis of Ni contents in the soil and cultivated plants, the bioconcentration factors and the risk assessment code were calculated. The type of amendments had a significant impact on the nickel content in lupine, for camelina and oat was the greatest in the control conditions. The differences between the amounts of Ni determined for bioconcentration factors were significant and depended on the amendments and nickel obtained by different methods. Regardless of the experimental conditions, the amount of Ni in the exchangeable bonds (Fr. I) had the greatest impact on the content of Ni in lupine and oat, whereas NiDTPA in the case of camelina.

This research deals with the effects of soil amendment with sewage sludge compost (SSC) on the accumulation of microelements (Cu, Zn, Mn, Ni, Fe) in plants—white mustard (Sinapis alba), triticale (Triticale) and white lupine (Lupinus albus)—cultivated on two contrasting soils (light vs. medium agronomic class). Additionally, the influence of experimental factors on variability of the harvest index (HI) was determined for individual plants and micronutrients. A 3-year pot experiment was conducted and SSC at the rate of 6 t·ha−1 was applied into the soils. These changes were determined by ANOVA and subject to regression analysis and subsequently visualized. The study clearly demonstrated that SSC as an organic fertilizer had a significant, but weak effect on the microelements level variability in the shoots and grains of plants cultivated in crop rotation. Regardless of the experimental factors, on average, the plants accumulated higher amounts of Cu, Zn and Mn in the grains, and Fe and Ni in the shoots. Harvest index values confirmed the significance and variable translocation of microelements within plant organs. The influence of SSC on micronutrient contents in plant organs was more significant in the case of medium soil.

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