Integrating organic agro-wastes in potting mix can promote sustainable vegetable-crop system intensification. This study evaluated progressive partial substitution of coarse-textured topsoil (TS) in composted poultry manure (PM)-amended media with composted rice husk (RH), using carrot, during June-September of 2023 and 2024 in Nigeria’s derived savannah. Six potting mixes of varying TS-PM-RH volume ratios with PM fixed at 30% (TS70:PM30:RH0, TS60:PM30:RH10, TS50:PM30:RH20, TS40:PM30:RH30, TS30:PM30:RH40, and TS20:PM30:RH50) were characterised and used to grow Thema and Touchon carrot varieties. These growth media were blended and bagged afresh each year The partial substitution generally improved the growth media’s (particularly TS30:PM30:RH40) soil pH, organic matter, available phosphorus and overall fertility status. In 2023, TS70:PM30:RH0/TS20:PM30:RH50 hastened seedlings emergence; in 2024, TS20:PM30:RH50 tended to do so. In both years, TS30:PM30:RH40/TS20:PM30:RH50 produced the highest foliage weight (FW);TS60:PM30:RH10/TS30:PM30:RH40/TS20:PM30:RH50, together with TS50:PM30:RH20 (2024 only), produced the highest root weight (RW). The substitution generally increased percent marketable roots and RW-based harvest index over the control TS70:PM30:RH0. Thema gave higher FW but lower RW and RW-based harvest index than Touchon. Thema-×-TS30:PM30:RH40 and Thema-×-TS20:PM30:RH50 consistently produced higher FW than the rest, whereas Touchon with RH-containing media (excluding TS40:PM30:RH30) tended to enhance RW, with Touchon-×-TS30:PM30:RH40 showing the highest RW-based harvest index. The data support using RH-containing media (except TS40:PM30:RH30 whose PM-to-RH ratio is 1) and TS30:PM30: RH40/ TS20:PM30:RH50 to grow preferably Touchon and specifically Thema, respectively for increased root and foliage yields, respectively. Appropriate TS-PM-RH ratios enhance growth media’s fertility status and root vegetables’ below- and above-ground yields, the latter of which may also be influenced by crop variety.

Soil acidity and associated phosphorus (P) fixation in highly leached humid tropical soils are major constraints to their agronomic productivity. They thus require effective acidity-alleviating cum P-supplying fertility management. This study assessed the potential of co-application of wood ash (WA) and calcium oxide (CaO) as natural and synthetic limes with rock phosphate (RP) and single superphosphate (SSP) as natural and synthetic P-fertilizers, respectively for acid coarse-textured Ultisols. The WA and CaO were added at 157 and 5 kg ha(-1) equivalents, and RP and SSP at 50 and 333 kg ha(-1) equivalents, respectively to 5-kg potted soils. Over 9 weeks, treatment effects in the soil evaluated on soybean growth showed enhancements. Relative to the control, soil pH increased in WA/WA+RP by 107%-121%; the highest relative increases in total N were in WA+RP (2,350%), available P in WA+SSP (520%), and K+/Ca2+/Mg2+ and base saturation in WA (50%-391%), with highest N-P-K uptake from WA+RP (450%-600%). The highest relative increases in above-soil plant growth and dry matter were in WA+RP/WA+SSP/CaO+SSP (63%-102% and 86%, respectively). Treatment residual effects over also 9 weeks showed similar nutrient-uptake/growth trends. Agronomic responses to treatment largely reflected soil pH-regulated differences in soybean uptake of P found to be linearly related to soil available P. Unlike WA, CaO needs P-fertilizer to improve soil productivity. Overall, however, WA+RP/WA+SSP/CaO+SSP is suggested for the soils.

The low fertility status of the highly weathered tropical soils offers the opportunity to study the potential and optimum application rate of biochar as an organic soil amendment, especially for the dominant coarse-textured Ultisols. Despite the relatively fast mineralisation of organics in these soils and the need to synchronise nutrient release crops critical stages of nutrient requirement, the time corresponding to peak effects of biochar remains unclear. The effects of rice-husk biochar (RHB) on the soil fertility of sandy-loam Ultisols at 0, 7.5, 15, 30, and 60 Mg ha-1 equivalents in 2-kg soils were assessed at 0, 2, 4, 8, and 12 weeks of incubation (WOI). Treatments were prepared in batches to enable concurrent sampling for all five incubation intervals. The RHB enhanced soil fertility across the incubation intervals, with optimal rates as 15 Mg ha-1 for soil pH and 30 - 60 Mg ha-1 for macronutrients availability. Relative to the its non-application, RHB increased soil pH-H2O, total N, available P, exchangeable bases, exchangeable acidity, apparent CEC and base saturation by 4-30%, 43-100%, 30-202%, 13-240%, 14-675%, 21-126% and 7-82%, respectively. Soil pH tended to decrease after, while available P progressively decreased before 8 WOI, when treatment effects were generally most pronounced. At an all-encompassing optimal rate range of 30-60 Mg ha-1, RHB could reduce soil acidity and enhance the macronutrient status of coarse-textured Ultisols over at least 12 weeks, soil fertility restoration effects of which are likely to be most pronounced around 8 weeks.

The persistent food shortage and associated hunger, and poverty crisis plaguing many tropical African countries is as a result of poor performance of the agricultural sector. This phenomenon is attributed mainly to the use of unsustainable agricultural production practices such as abuse of inorganic inputs and poor management of organic wastes. This has rekindled concerns with regard to land degradation as well as human and environmental health. Instructively, understanding of waste is shifting from being material requiring collection, handling and disposal to now embracing sustainable resource management in the circular economy. Microbial biotechnology is a promising option to facilitate the modification/transformation of huge amounts of assorted organic wastes generated by all categories of farms, agro-allied industries and municipalities to useful bio-organic resources in agricultural production systems, energy production and environmental management. The paper focuses mainly on the conversion of organic wastes to bio-organic fertilizers for solubilization/mineralization of plant nutrients in agronomic production systems as well as use of these wastes in production of microbial secretions against biotic/abiotic stresses. Some microbes also find beneficial use as probiotics and recombinant DNAs in the livestock industry and bioremediation of polluted ecosystems. As a case study, it discusses the potential of microbial biotechnology in tropical Africa vis-a-vis the prevailing relatively rich biodiversity and degraded agroecologies.

A major limitation to the use of some biowaste as manure is their high C/N ratio and hence slow mineralisation rate. Combined use of such slow-mineralising animal manures with N-rich fertilizers and/or synthetic lime may valorize them. The objective of this study was to assess the synergistic effects of cattle dung (CD), Urea and lime on crop productivity and physicochemical properties of sandy tropical soils. Treatments of CD, Urea, Lime, CD+Urea, CD+Lime, Urea+Lime, CD+Urea+Lime, standardized CD+Urea and Control were used to grow okra on sandy soils from a long-term fallow land. The CD and Lime were added at 20 and 0.15-t/ha equivalents three weeks before okra sowing, and Urea at 5-t/ha equivalent at sowing except for standardized CD+Urea where this mix (with reduced C/N of ca.10) was added three weeks before sowing. Agronomic and soil data were collected during and after nine-week crop growth phase, respectively. Treatment affected okra growth and soil variables. Plant height and leaf area generally showed CD+Urea+Lime > CD+Urea/CD+Lime/CD > Urea+Lime/Control, with 56-81% and 124-125% increases, respectively in CD+Urea+Lime relative to Urea+Lime/Control. However, CD+Urea and Urea+Lime/Urea/Lime/Control produced the most and fewest leaves per plant, respectively. Treatment CD+Urea+Lime gave the highest okra dry matter, lowest soil bulk density and highest available P and cations exchange. The CD+Lime improved macro-aggregation the most but, together with standardized CD+Urea, also caused soil colloidal dispersion. Okra dry matter reflected soil available P (R2 = 0.63). Fortifying CD-manure with lime and, afterward, Urea could produce synergistic agronomic effects mostly via boosted P-bioavailability, and is hereby proposed for physicochemical fertility management of coarse-textured acid tropical soils.

Biochar and nitrogen (N) fertilizer application have been shown to increase rice yield in conventional rice production system. However, studies evaluating the responses of rice yield and farmers' incomes to biochar and N fertilizer application under no-tillage are rare. A two-year field study was conducted to assess the agronomic and economic effects of integrating biochar and N fertilizer to no-till rainfed lowland rice production system in Cote d'Ivoire. The treatments were factorial combination of two biochar rates (3 and 6 t ha(-)(1)) and three N fertilizer rates (0, 60 and 120 kg N ha(-)(1)) under no-tillage (T0), and a control (i.e. the farmers' practice of manual tillage with hoe and N fertilizer at 120 kg N ha(-)(1)). The results indicated that biochar and N fertilizer application under no-tillage significantly (p < 0.05) increased rice yield and labour productivity by 6 - 9% and 2 - 45%, respectively, over the control. The highest partial productivity of N (113%) was observed under no-tillage with biochar at 6 t ha(-)(1) and N at 60 kg N ha(-)(1), which reduced the production cost by 22% and increased the benefit by 224% relative to the control. Additionally, the soil properties improved significantly (p < 0.05) under biochar and N fertilizer application without tillage.

The inherent fairly high structure stability of tropical soils notwithstanding, poor aggregation still contributes appreciably to their water erosion. To understand the role of soil and water management in this regard, a field trial was conducted on runoff plots to assess the effects of cattle-dung (CD) manure and dry-grass mulch (DGM) on aggregate stability and permeability of well-drained Ultisols in southeastern Nigeria. The trial involved incorporating CD manure into topsoil at 10 tha-1 and/or surface-applying DGM at 20 t/ha. Water erosion monitored during 1-7 months after manuring was related to soil aggregation and hydraulic behaviour at 3 and 7 months. Treatment had no effects on runoff and soil loss but affected few soil properties; degree of aggregation (3 months), proportion of water-stable aggregates, clay dispersion ratio and cation exchange (7 months). Runoff and soil loss were lower in DGM than no-mulch plots. Proportion of water-stable aggregates increased by 18% due to CD and 27% due to DGM at 7 months, when DGM similarly increased degree of aggregation (from 56.40% to 75.80%) and state of aggregation (from 49.50% to 66.40%) including aggregates mean-weight diameter (from 1.60 to 2.25 mm) and percolation stability (from 236 to 298 cm3). Neither CD nor DGM affected soil organic matter, steady-state infiltration rate and internal hydraulic properties of soil bulk density, total porosity and saturated hydraulic conductivity. Infiltration capacity seemingly increased with reduced leaching and hence soil acidity and enhanced soil fertility, macro-aggregation and macro-favoured porosity. Soil loss mirrored its runoff and mid-season aggregates mean-weight diameter and microporosity (R2 = 0.94). For soil conservation from a broader perspective, CD+DGM is suggested for well-drained humid tropical soils; however, for only soil stability against erosion, CD or preferably DGM alone is suggested. Runoff-based soil loss predictions could benefit from macro-aggregation and water retention data.

The soil remains the largest carbon sink, the capacity of which varies spatiotemporally and with anthropogenic activities, with implications for carbon dioxide (CO2) emissions and associated global warming. Despite the increasing popularity of organic amendments in the ecologically and anthropoculturally diverse farming systems of tropical Africa, manure-induced CO2 emissions in this region with high soil-carbon-sequestration potential are poorly documented. This study assessed seasonal CO2 emissions from poultry manure-amended sandy-clay-loam and sandy-loam soils in the Rainforest-Savannah and Southern Guinea Savannah zones, respectively, of Nigeria. Following manure application at 3 rates (0, 10, and 15 t ha-1), soil samples were collected during 5 periods (onset, peak and cessation of the rainy season and onset and peak of the dry season). Soil CO2 emissions were monitored after 7-, 14- and 21-day incubations. The emissions varied across manure rates, sampling periods and locations. Compared with 0 t ha-1 treatment, 15 and 10 t ha-1 treatments consistently resulted in higher emissions. These emissions were generally highest at the peaks of the rainy/dry season and lowest at the end of the rainy season, while higher at the southern Guinea Savannah than the Rainforest-Savannah zone. Overall, all three factors studied together described the emissions, which were lower for 7-day than for 14/21-day incubation. To reduce CO2 emissions, poultry manure application at rates ≥ 10 t ha-1 should be discouraged in the humid tropics, particularly locations with fairly warm climates and sandy soils. This study highlights the global warming implications of the increasing adoption of animal manures as organofertilizers and, by extension, the nonconfined rearing of poultry/livestock in tropical African agro-ecosystems. Studies involving different organic soil amendments at varying application rates and agronomic management practices are suggested, especially for coarse-textured soils of the widespread savannah, to deepen the understanding of the mechanisms regulating CO2 emissions.

Mulching is an effective soil-water conservation technique in high-evaporative-demand tropical climates. Because of the drawbacks in bulk application of organic mulches, we introduce the concept of bio-economic mulching (BEM), a one-time low-rate application of organic mulch to improve soil productivity while sustaining economic viability. The study evaluated the effects of BEM (dry-grass mulching at 0, 2, 4, and 6 t ha–1) on soil hydrothermal properties of sandy-loam Ultisols using okra growth during 4–9 weeks after sowing in successive rainy-to-dry/partially rainfed season (PRS) and rainy/completely rainfed season (CRS). During the PRS, soil volumetric moisture content (q) increased (10.02%–25.50%), but soil temperature decreased (37.67–26.67°C) as BEM rate increased. A similar q trend (8.71%–18.37%) occurred during the CRS. Soil thermal conductivity (0.78to 4.88 W m–1 K–1), thermal diffusivity (3.95 × 10–7 to 35.97 × 10–7 m2 s–1), and heat flux (15.00 to 85.56 W m–2) generally decreased as q increased with BEM application rate particularly during the PRS; the reverse prevailed for volumetric heat capacity (1.33 × 106 to 2.25 × 106 J m–3 K–1). Okra plant height differed (BEM-6 > BEM-4 > BEM-2/BEM-0) in the PRS, but BEM-6 and BEM-4 gave the tallest and shortest plants, respectively in the CRS. Fruit yield was 1.8- and 9.5-fold higher in BEM-6 than BEM-4 in PRS and CRS, respectively. Mulch treatment-induced temporal variations in soil q influenced okra performance indices of plant height (r2 = 0.85) and total fresh fruit yield (r2 = 0.69). In droughty tropical environments, BEM implementation at 6 t ha−1 could engender soil hydrothermal regime favoring vegetable production beyond the ‘drier’ first season and even more pronouncedly in the second season.

The authors have requested that this preprint be removed from Research Square.

Soil erosion is one of the greatest environmental degradation challenges the tropical region and the world in general are facing. Tropical soils are less erodible than those of the temperate region, but climatic erosivity is higher in the tropical region due to high erosive power of rain than those of the temperate region. In the past and present, physical processes of soil erosion and soil erosion control have been assessed using the revised universal soil loss equation (RUSLE) model. This review aims at revisiting the questioned reliability of the RUSLE for soil erosion prediction in the tropics. The RUSLE model estimates the long-term average annual soil loss (A) by multiplying out factors of rainfall erosivity (R) and soil erodibility (K), slope length and slope steepness factors (LS), land cover and management factor (C) and the soil conservation or prevention practices factor (P). The RUSLE model is not as data demanding and so more often used than any other tool available. It is also characterized by high degree of flexibility, data accessibility and suitability in computer program. As a drawback, the development of the model is site-specific and often criticized as being a mere temperate-based empirical erosion model which cannot be applied in tropical soil erosion predictions. However, amongst all other soil erosion prediction tools, the model is dependable and reliable in the tropics especially now that it combines with remote sensing (RS), digital elevation model (DEM) and geographical information system (GIS) to estimate annual soil loss (on a pixel-bypixel basis) and spatial distribution of the soil erosion.

Liming and manuring to ameliorate soil acidity and enhance mineralization in acid tropical soils could promote their agronomic productivity.Synthetic lime and poultry-droppings manure’seffects in sandy-loam Ultisols were evaluated on soybean growth, exploring its relationships with soil pH, soil available P and plant P content. Lime was applied at 0, 2.5 and 5 t ha-1 equivalents and manure at 0, 25 and 50 t ha-1 equivalents to 2.5-kg potted soils. Crop growth data were collected during andsoil-plant analysis after sixweeks of growth. Treatment had more pronounced effects on plant height than leafiness, beinggenerallyhighest in lime 2.5 + manure 50 where the plants were about 3 x taller comparedtounamended control. Soybean grew better due to 2.5 than 0 and 5 t ha-1 lime, and to 50 than 0 and 25 t ha-1 manure. Plant heightand leaf area depended on soil pH’s influence on soil available P (R2 = 0.69**) and plant P content (R2= 0.85**), respectively, whileabove-soil biomass on soil pH or plant height (R2 = 0.74**). Moderate liming (2.5 t ha-1) with ample manuring (50 t/ha) could enhance crop early-stage vegetative growthin low-fertility tropical soils, due largely to amelioration of soil acidityto enhance uptake of bioavailable P.

Increasing bioavailable forms of phosphorus (P) in acid tropical soils could promote their productivity. Liming and manuring ameliorate soil acidity and enhance P mineralization and hence bioavailability. The study evaluated the effects of synthetic lime (0, 2.5 and 5 t ha-1) and poultry-droppings manure (0, 25 and 50 t ha-1) in acid sandy-loam Ultisols on soybean growth, and explored the relationships among its growth indices, plant P content, soil available P and soil pH. The experiment was executed under glasshouse conditions with crop growth data collected weekly during 2-6 weeks after sowing, while plant P content, soil available P and soil pH were determined at the end of the six-week growth phase. Treatment had more pronounced effects on soybean plant height than leaf area and number of leaves, with the highest values generally in 2.5 t ha-1 lime plus 50 t ha-1 manure. Soybean plants were about three times taller in this treatment than the control. Main effects showed tendency for better growth in potted soils limed at 2.5 t ha-1 over 0 and 5 t ha-1, and consistently highest values in those manured at 50 t ha-1 compared to 0 and 25 t ha-1. Plant height and leaf area depended on soil available P (R2 = 0.76**) and plant P (R2 = 0.88**) contents, respectively, both of which depended on soil pH (r = 0.80* and 0.74*, respectively). Moderate liming (2.5 t ha-1) and ample manuring (50 t ha-1) could enhance soybean early-stage morphological growth in readily leached, low-fertility tropical soils. These increases are largely a reflection of acidity-ameliorating effect of such soil amendments on Pbioavailability and hence uptake.

Mulch regulates soil hydrothermal regime while suppressing weed growth, effects of which may depend on mulch material and/or similar to that of closed crop canopy. Combined use of organic or inorganic material as surface mulch and appropriate plant spacing may produce synergistic agronomic effects in water-stressed environments. This study evaluated the sensitivity of rainfed okra to mulch-spacing variants in droughty derived-savannah soils. With dry-grass mulch (DGM) and sawdust mulch (SDM) representing organic mulch material and black polythene (BPM) as inorganic/plastic, treatments were factorial combinations of surface-applied DGM, SDM and BPM with close (60 cm × 30 cm), intermediate (60 cm × 45 cm) and wide (60 cm × 60 cm) spacings. Crop growth/performance and fruit yield data were collected. Mulch material affected okra growth (with overall better results in BPM than SDM), but not flowering/fruiting and fruit yield. For plant spacing, sum of weights of pods and fruit yield were higher in close than intermediate and wide spacings, with fruit yields as 2.42, 1.21 and 0.76 t ha-1, respectively. Mulch-spacing interaction effects showed that BPM with close spacing gave the highest values for stem girth 8 weeks after sowing (SDM treatments gave the lowest), sum of weights of pods and fruit yield (2.93 t ha-1). Fruit yield, however, depended not on stem girth but on certain okra yield attributes. The BPM or, where not feasible, either of DGM and SDM, could be effectively combined with close spacing to improve okra productivity in droughty soils of the derived savannah.

Manure effects on soil organic matter (SOM) and related physicochemical fertility indices wane rather fast in the tropics due mainly to the prevailing high temperatures. In texturally similar soils, SOM-mediated aggregation controls hydraulic properties including rainfall-to-field-capacity time (FCtime) and field capacity water content (FCwater) that relate to water/nutrients availability to crops. This study assessed the residual effects of poultry-droppings manure at 25, 50 and 75 t/ha on SOM and aggregation, P-fertility and hydraulic properties of sandy-loam Ultisols in southeastern Nigeria. Mulch-protected treatment plots were water-saturated weekly during the dry season. Sampling for immediate effects was done one month after treatment; that for residual effects in the subsequent rainy season, 7-8 months after treatment, when FCtime/FCwater was monitored at 3-24 h intervals after three rainfall events each ≥ 30 mm. A given monitoring time was designated FCtime if the corresponding FCwater was similar to that of the succeeding one. Immediate effects showed higher soil pH, SOM, aggregates’ mean-weight diameter and available P in 50 and 75 t/ha than unamended control, but similar sand-corrected water-stable aggregates and permeability indices among treatments. Residual effects toed similar trends except that aggregates’ mean-weight diameter was unaffected, while soil bulk density was lower and microporosity higher in 75 t/ha than the rest. Across the three sampling periods, FCtime averaged 42 and 26 h, respectively at ≤ 25 and ≥ 50 t/ha, while FCwater increased steadily (0.08-0.22 g/g) with manure rate. Beyond the season of application to droughty Ultisols, poultry-droppings manure at heavy rates (50-75 t/ha) can still be promoting their SOM and P-fertility but not macro-aggregation. Up to 75 t/ha of the manure may be required for similar residual effects on soil hydraulic properties (including FCtime and FCwater), with FCtime seemingly varying not just with SOM but also the associated rainfall’s characteristics.

The research investigated the distribution of soil fertility indices among aggregate-size fractions across three land-use types (forested, cultivated, and fallow lands) for loamy sand at Ede-Oballa, a derived savannah in southeastern Nigeria. Surface soil samples from these land-use types were air-dried and separated into <0.25 mm, 0.25-0.5 mm, 0.5-1.0 mm, 1.0-2.0 mm, 2.0-4.0 mm, and 4.0-8.0 mm aggregates before analyses. There were significant interaction effects of land-use type and aggregate-size fraction on the distribution of soil particle sizes (sand, silt, and clay) and contents of total nitrogen, available phosphorus, exchangeable bases (Mg2+ and Na+), and exchangeable acidity as well as apparent cation exchange capacity (CEC). Land-use type affected soil pH, K+, Ca+, and percent base saturation but not soil organic carbon (SOC), which was rather unevenly distributed among the aggregate-size fractions. The distribution of most fertility indices, those defining CEC, and SOC content favoured >2.0 mm (large), 0.25-0.5 mm and <0.25 mm (micro-) aggregates, respectively, under cultivated land. However, soil total nitrogen and exchangeable acidity contents were best improved in the largest (4.0-8.0 mm) aggregates under forested and fallow lands, respectively. The study highlights the potential of land-use types generally and arable-crop cultivation specifically to engender aggregates of different sizes with specific roles in improving soil quality and fertility.

Phosphorus (P) is one of the most abundant and essential nutrient elements for crop growth; however, it is often not available to plants in sufficient quantity. Organic amendment can enhance its availability in acid tropical soils. In this study, we investigated the effects of manure type on the pattern of P release in highly weathered tropical soils of contrasting lithology and mineralogy. Two fallow-land soils from Isinyi-Nando and UNN-Nsukka in southeastern Nigeria with antecedent Bray-2 available P of 3.73 and 20.83 mg kg-1, respectively, were amended with poultry droppings, pig slurry and cattle dung at soil-manure mass ratio of 100:6 and incubated. They were sub-sampled at 0, 1, 2, 4, 8, 16 and 28 weeks of incubation. In the two soil types, the highest increases in soil pH and available P always occurred with poultry droppings, with P release peaking at 16 weeks. Manure-induced increases in available P were greater in the less oxide-rich Isinyi-Nando soil compared with the more oxide-rich UNN-Nsukka soil over the incubation duration, with lower values in the former than the latter without amendment (5.79 and 23.57 mg kg-1, respectively) and across treatments (40.59 and 45.16 mg kg-1, respectively). The P release patterns were defined more by increases and decreases in soil pH and organic carbon with incubation time, respectively, and less by soil aggregation. Poultry droppings could enhance P release around 16 weeks post-amendment of these soils. Further studies are needed on its synchronized bioavailability and the prospects of slow-mineralizing cattle dung beyond 28 weeks.