The low soil fertility status of South African marginal soils threatens sustainable production of biofuel feedstock in smallholder farmers. It is therefore imperative to development sustainable and optimal management practices that improve soil fertility. The objective of this study was to determine the effect of tillage, rotation and crop residue management on nutrient availability in a bioenergy sweet sorghum-based cropping system in marginal soils. Two tillage levels, no-till (NT) and conventional tillage (CT); two crop rotations, sweet sorghum–grazing vetch–sweet sorghum (SVS) and sweet sorghum–fallow–sweet sorghum (SFS); and three crop residue retention levels, 0%, 15% and 30%, were tested. No-till enhanced total nitrogen, total organic nitrogen (TON), magnesium (Mg) and sodium (Na) by 3.19% to 45% compared to CT. SVS rotation increased ammonium (NH₄⁺-N) and nitrate (NO₃⁻-N) by 3.42% to 5.98% compared to SFS. A 30% crop residue retention increased NH₄⁺-N, NO₃⁻-N, available phosphorus (Available P), cation exchange capacity (CEC), calcium (Ca), Mg and potassium (K) by 3.58% to 31.94% compared to crop residue removal. In the short term, a 30% crop residue retention was the main treatment that enhanced soil fertility. The application of NT−30% was a better practice to enhance soil fertility. However, research on inclusion of crop diversity/intercropping can add more value to the NT–30% practice in enhancing soil fertility.

Questions on sustainable and appropriate cropping systems for bioenergy sweet sorghum in the smallholder farming sector still exist. Therefore, a short-term experiment was carried out to study the influence of management on microbial biomass carbon (MBC), β-glucosidase, acid phosphatase, and urease activities in a sweet sorghum cropping system in South Africa. Tillage [no-till (NT) and conventional tillage (CT)], rotation [sorghum-vetch-sorghum (S-V-S) and sorghum-fallow-sorghum (S-F-S)] and residue retention [0%, 15% and 30%] were evaluated. Tillage× rotation× residue management interaction influenced (P < 0.05) MBC whilst crop rotation residue influenced (P < 0.05) β-glucosidase. Tillage affected β-glucosidase (P < 0.05), acid phosphatase (P < 0.001), and urease enzyme (P < 0.01) while crop rotation only influenced acid phosphatase (P < 0.01). Residue retention affected acid phosphatase (P < 0.001) and urease enzyme (P < 0.001). NT + S-V-S+30% interaction resulted in the highest MBC content compared to CT + S-F-S+0%. NT+30% enhanced β-glucosidase activity, S-V-S enhanced acid phosphatase compared to S-F-S. MBC and enzyme activities were positively correlated with each other. Tillage and residue management were the main factors influencing soil biological indicators under bioenergy sweet sorghum in South African marginal soils in the short-term. Soil biological indicators were higher under NT and 30% residue retention respectively. NT + S-V-S+30% was a better treatment combination to enhance soil quality under bioenergy sweet sorghum in South African marginal soils.

The increase in greenhouse gases (GHG) emissions in the world has significantly contributed to climate change, prompting an active search for renewable and sustainable biofuels. Sweet sorghum (Sorghum bicolor(L.) Moench) is a leading biofuel feedstock that is produced with minimum inputs and does well even in semi‐arid areas with soils of low fertility. However, a sustainable production system for sweet sorghum is not yet established in South Africa. Lately, conservation agriculture (CA) has gained research focus because of its benefits as a sustainable crop production system. Therefore,CAmay offset the negative impacts of intensive agronomic practices during biofuel crop production. This paper reviewedCAas a possible sustainable crop production system for sweet sorghum as a biofuel feedstock.CAenhanced soil quality, reduced carbon dioxide emissions, and increased yield of sorghum and related cereals. It was concluded thatCAhas potential to enhance sweet sorghum production as a biofuel feedstock under semi‐arid conditions in South Africa. Therefore, local field experiments on sweet sorghum production underCAare desirable in South Africa.

Labile organic carbon (LOC) fractions are considered as sensitive indicators of change in soil quality and can serve as proxies for soil organic carbon (SOC). Although the impact of tillage, crop rotation and crop residue management on soil quality is well known, less is known about LOC and SOC dynamics in the sweet sorghum production systems in South Africa. This short-term study tested two tillage levels: no-till and conventional-tillage, two crop rotations: sweet-sorghum/winter grazing vetch/sweet sorghum and sweet-sorghum/winter fallow/sweet sorghum rotations and three crop residue retention levels: 30%, 15% and 0%. Tillage was the main factor to influence SOC and LOC fractions under the sweet sorghum cropping system in South Africa. NT increased SOC and all LOC fractions compared to CT, which concurs with previous findings. Cold water extractable organic carbon (CWEOC) and hot water extractable organic carbon (HWEOC) were found to be more sensitive to tillage and strongly positively correlated to SOC. An increase in residue retention led to an increase in microbial biomass carbon (MBC). This study concludes that CWEOC and HWEOC can serve as sensitive early indicators of change in soil quality and are an ideal proxy for SOC in the sweet-sorghum cropping system in South Africa.