Long-term impacts of municipal solid waste compost, sewage sludge and farmyard manure application on organic carbon, bulk density and consistency limits of a calcareous soil in central Iran

Increasing soil organic carbon (SOC) is vital in terms of improving the soil physical and mechanical properties related to conditions for tillage and traffic, and to crop development in arid and semiarid regions. This study was conducted to characterize the SOC, bulk density (BD) and consistency limits (shrinkage limit, SL; plastic limit, PL; liquid limit, LL) of a calcareous soil (Typic Haplargids) in relation to the seven-year application of manures (municipal solid waste compost, MSWC; sewage sludge, SS; farmyard manure, FYM) at three rates (25, 50 and 100Mgha⁻¹) and one inorganic fertilizer (NP) management in irrigated wheat-corn rotation. There was also a control treatment (UNF), i.e. without any organic and/or inorganic fertilizer addition. The SOC, BD, consistency limits, plasticity index (PI=LL−PL) and friability index (FI=PL−SL) were measured for the soil taken from the 0-20cm layer. The SOC in SS, FYM and MSWC treated soils increased on average by 2.5, 2.2 and 2 times of the amount in the UNF, respectively. The BD varied from 1.03 to 1.37Mgm⁻³. The lowest BD was recorded in 100Mgha⁻¹ application rate of manures, whereas it was the highest in NP treatment. The PL and LL varied from 19% to 30% and 30.1% to 40.9%, respectively. The PL for the SS treated soil was significantly higher than the FYM and MSWC treated soils. However, the LL and PI were not significantly affected by manure type. The SL and PL were increased significantly but in small percentages by the application of NP as compared with the UNF. The optimum water content (WC) for tillage (i.e. 0.9PL) as well as the workable WC range (i.e. FI) was the largest in the soil amended with 100Mgha⁻¹ of SS and the smallest in the UNF. There was a significant relationship between the application rate of the manures and the SOC. The positive relationship between SOC and SL, PL, LL or FI was found to be significantly linear whereas the relationship with the soil BD was significantly negative. For soils amended with SS which the data on cation exchange capacity (CEC) were available, the relationships of PL and LL with CEC were significantly linear as with SOC. This study showed that the manures improved soil WC ranges at which the optimum conditions for tillage and traffic can be obtained. Moreover, the improvement depended on the application rate of the manures.