Metal concentrations of the compost were generally very low when compared to the permissible standards in compost [47]. The compost is therefore, safe for agricultural growing of arable crops, and also for bioremediation of metals. The major role of composting in bioremediation process is to degrade metals present in the soil matrix [7], [8] and [9]. However, the presence of metals in compost at high concentrations may result in soil metal pollution [31]. The contaminated soil has higher concentrations of metals than compost, except for Mn. The manganese concentration in compost (4.38 ± 0.07 mg kg−1) was slightly higher than the concentration measured in contaminated soil (4.23 ± 2.11 mg kg−1). Despite the slightly higher concentration of Mn in the compost, there was reduction in the concentrations of all the observed metals on introduction of compost in the two treatments. Further explanation on compost metal removal will be discussed in Section 3.3.
Generally, the contaminated soil sample was deficient in OC, TN, available phosphorus and other essential elements needed for plant’s growth. Mixing the soil samples with compost led to an increase in these essential macronutrients. The particle size distribution of contaminated soil showed domination of sand by over 80%. Having a clay content of less than 10%, the soil samples can be classified as sandy and hence, are not fertile [31] and [46]. We showed that introduction of composts to contaminated soils enriched the soil with essential nutrients and also removed certain percentage of metals.