**6. Discussion**

Exchangeable basic cations are significantly deficient in Nigerian soils particularly polluted soils. Application of composted cassava sludge will enrich these soils with basic cations thereby reducing predominance of acidic radicals. Composted cassava sludge has high content of these neutralizing cations plus appreciable values of total nitrogen which is easily leached in these tropical soils (Table 1).Again, the pH value of the compost (6.3) (Table1) is optimal for microbial activity in soils (Table 2) as most tropical crops perform optimally at ph range of 5.5 to 6.5 (Ahn,1979). Composted cassava sludge progressively removed heavy metals and total petroleum hydrocarbons in crude oil polluted soils of Nigeria (Table 3). In a similar study, Wen *et al. (2002)* reported a depression in the phytoavailability of copper using sludge compost but Sims and Kline (1991) observed variations in characteristics of different composts and extractants, implying compost type influences uptake and availability.in different media..It is possible sorption ability of organic materials my depend on the specific surface area of each type. However,the concentration of these toxicants vary with season as indicated in Table 4, implying that their application strategies will follow the temporal variability. It was observed that impact of composted sewage waste became significant after thirteen years in an experi‐ ment conducted by Nogueirol *et al*. (2013) using three rates of sludges with maize and sugar cane as test crops.

Knowledge on the fate of heavy metals in soil–plant system is of great importance as that helps in predicting food quality especially in popular crops. Highest concentrations of heavy metals were recorded in the topmost layer of soils (Yedilar *et al*.,1994) and they reported that concen‐ trations in rice leaves and grains were lower than values observed in rice roots in China.

Composted sludge has high content of organic carbon which has the capacity of absorb‐ ing organic pollutants in soil thereby reducing their solubility as demonstrated by several authors (Hernandez-Soriano et al.,2007; Stevens-Garmons et al.,2012). In addition, organic carbon inputs might enhance the soil buffer capacity. The application of slightly acidic sewage (6.3), will also affect the mobility and bioavailability of metals present in soils. The evolution of heavy metal distributions and bioavailability depended not only on total metal concentration but also on other properties such as pH, organic matter decomposition and dissolved organic carbon (Miaomiao *et al*.,2009) and they reported that composting systems decreased soil pH, soil organic matter and dissolved carbon. Their results varied between sewage sludge and swine waste. In some studies conducted in China, soil pH, soil organic matter and clay content were used to predict phytoavailability of Cadmium in paddy soils,stating that pH negatively correlated with cadmium cont of soils (Krebs et al.,1998). Martinez and Motto, (2000) reported that solubility availability, mobility and toxicity of heavy metals to plants increases as pH decreases. In a study at Suwon, Korea. Kim *et al*. (2009) remarked that translocation of heavy metals depended more on the variety of crop rather on soil physicochemical properties.
