Abstract 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.

Keywords: Topsoil-Amendment ratio, Organic agricultural waste, Composted organic amendment, Varietal effect, Sustainable horticultural production

Citation:  Chizaram-Ndubuaku, C.A., Obalum, S.E., and Baiyeri, K.P. 2026. Partial substitution of coarse-textured topsoil in composted poultry manure-based media with composted rice husk and responses of two Daucus carota varieties. Natural and Life Sciences Communications. 25(4): e2026074.

Graphical Abstract:

INTRODUCTION

Sustainable agricultural practice is essential for ensuring food security while minimising the environmental impact of food production (Ajayi et al., 2022; Dickson et al., 2023; Igwe and Obalum, 2023). With the increasing growth in population of Africa, which must be catered for, there is a need for intensification of food production on existing croplands with agro-technology involving appropriate proportions of agricultural wastes as sources of plant nutrients in the soil (Adeniyan et al., 2011; FAO, 2012; Nwite et al., 2012a, 2012b; Nwite et al., 2016; Baiyeri et al., 2020; Uzoh et al., 2020; Ebido et al., 2024; Ogumba et al., 2024a, 2024b). This is more so as such intensification of agricultural production systems to feed the steadily growing population has resulted in palpable physical, chemical and biological changes of agroecological zones like the derived savannah (Achterbosch, 2005).

The dominant Ultisols of the derived savannah agroecology in the South-East of Nigeria that typifies a humid tropical environment are characterised by coarse texture, high soil acidity levels, low cation exchange capacity (CEC) and/or base saturation, translating into low fertility status (Nwite et al., 2011; Obalum et al., 2013). As such, these soils represent poor growing media when used alone, and this situation potentially can limit crop production (Ugwu et al., 2020; Ogumba et al., 2024a). Based on the initial estimate and projected expansion rate of commercial poultry litter proposed by the Food and Agriculture Organization of the United Nations in the mid-1990s, ca. 10,000 metric tonnes of poultry litter is, to date, generated in Nigeria per annum. Rice husk is another agro-waste that is currently generated in the country (Ebido et al., 2025), and this is to the tune of ca. 1.80 million metric tonnes annually. These enormous wastes can be profitably harnessed in the blending of potting mixes as growth media for the production of root vegetable crops.

Carrot (Daucus carota var sativus) is a cool-weather, day-neutral root crop that is widely accepted as a vegetable that offers an affordable supply of vitamins, minerals and fibre; its field cultivation is usually in the dry or high altitudinal regions where stable to more effective management practices are implemented all-year-round (Sarkindiya et al., 2006; Nuñez et al., 2008). In Nigeria, consumption of carrot roots accounts for about 30% of the average per capita daily consumption of vegetable (Nuez and Probens, 2008; Muhie, et al., 2023), necessitating its increased production in the country. The utilisation of Thema and Touchon, which are the most two accessible varieties of carrot for its production in Nigeria, supports early harvesting, high yields, pest resistance, and strong market potential.

However, the soil types and local agricultural practices in Nigeria offer unique challenges and opportunities for sustainable carrot crop production systems. To enhance carrot production in Nigeria, expansion beyond the fairly dry core savannah zones of the northern region is needed. The bulk of the South-East geopolitical zone is derived savannah, but the city/suburb dwellers here have increasingly depleted green spaces and have limited access to fresh food due to land fragmentation and insecurity. There have been studies on the use of poultry manure (PM) and rice husk (RH) organic soil amendments due their abundance in Nigeria, with some of these studies demonstrating their effectiveness in horticulture when co-applied at different rates and combining ratios and hence exhibiting varying extents of decomposition and mineralisation (Ugese et al., 2011; Baiyeri et al., 2015; Ede et al., 2015; Adubasim et al., 2018; Baiyeri et al., 2019; Baiyeri et al., 2020; Ugwu et al., 2020).

While previous studies have demonstrated the benefits of using raw PM and RH in growth media, a clear research gap exists (Baiyeri et al., 2019). Specifically, there is a lack of empirical research data on the effects of using composted forms of PM and RH, particularly in ratios that represent using the latter for a progressive partial substitution of the low-fertility topsoils that are common in the South-East zone of Nigeria. Moreover, despite the dependence of growth media amendment strategies on specific crop requirements (Yüksel et al., 2020), the optimal substitution rate for enhancing soil properties and carrot productivity has not been established, and it remains unknown if different carrot varieties exhibit varied responses to these modified PM-based media. By their appropriate formulation and composition, such growth media have very low densities thereby assuring easy root penetration and influencing the quality of vegetable seedlings, while reducing the incidence of soil-borne diseases (Baiyeri, 2003; Baiyeri and Mbah, 2006; Robbins and Evans, 2011).

Probably because of the ability of RH to double as nutrient source and aerator in seedlings growth media (Adubasim et al., 2018), it has been demonstrated to effectively replace PM in such media (Ugwu et al., 2020). Considering the positive synergy of PM and composts in agronomic production (Ogumba et al., 2024a), this effect may be more evident when RH is composted. In the present study, we hypothesized that the quality of composted PM-based growth media and carrot productivity would increase with increasing level of substitution of their topsoil component (characterised by inherently low-fertility status) using composted RH. Therefore, the aim of the study was to evaluate the effects of progressive partial substitution of topsoil (at a fixed relative proportion of composted PM) with composted RH on soil fertility of the ensuing growth media using performance of carrot.

MATERIALS AND METHODS

Description of the experimental site

The experiment was conducted at Edem in Nsukka area of the South East geopolitical zone of Nigeria in 2023 and 2024 cropping seasons under rainfed conditions. Geographically, the area is located by 06° 51' 43'' N and 07° 20' 21'' E, with an altitude of 400 m above sea level. The area falls within the Derived Savannah zone. The data for rainfall, temperature and relative humidity that prevailed in the two years of the study are presented (Table 1), showing a bimodal rainfall distribution pattern with peaks in June-July and September-October and a short dry spell around July-August, the exact month for these incidents varying between the two years. The soils, derived mostly from sandy deposits of false-bedded sandstones, are typically of deep (> 2 m), red to brownish red, porous and well-drained profiles, with the solum (topsoil and subsoils) being moderately to severely acidic and generally showing low values for soil organic matter content and CEC (Obalum et al., 2012, 2013).

Table 1. Rainfall, temperature and relative humidity at the study site in the two years of the study.

Procurement of experimental materials

Coarse-textured topsoil was collected at the depth of 0-20 cm from the farm site at Edem in Nsukka area of the derived savannah of the South-East, Nigeria. The organic amendments of this study being poultry manure (poultry farm wastes) and rice husk (rice-mill wastes) were collected from the poultry farm of the Department of Animal Science of the University of Nigeria Nsukka and rice mills at Adani, respectively, both in Enugu State in the South-East, Nigeria. The study utilised two most accessible and contemporary varieties of carrot in Nigeria; Thema and Touchon, viable seeds of which were sourced from East-West Seed Company, Enugu State. Samples of the coarse-textured topsoil as well as of the fresh poultry manure (PM) and rice husk (RH) were collected, air-dried to constant weight and crushed. The air-dry topsoil samples were passed through a 2-mm sieve while the air-dry PM and RH samples were ground with pestle and mortar. All three samples were stored for laboratory analyses.

Potting mix preparation and layout of the experiment

The potting mixes meant to serve as growth media were formulated based on volume ratios of coarse-textured topsoil, composted PM and composted RH. The topsoil was pulverised and debris/gravels/stones removed by passing it through the 2-mm sieve. The two organic agro-wastes (PM and RH) were composted separately for six weeks. They were turned thoroughly with a shovel, bagged, moistened and covered properly with a black polyethylene bag to prevent further wetting. During the composting period, the substrates were re-mixed biweekly, while their temperatures reduced with time until the mixtures became homogenous. At the end of the six-week period, appropriate quantities of the coarse-textured topsoil, composted PM and composted RH were measured for blending, using a volume-calibrated vessel. So, the blending of the growth media was done on a volume basis.  

The blending of the growth media comprising topsoil and composted PM in the percentage volume ratio of 70:30 involved progressive partial substitution of the topsoil (TS) with composted RH, with the relative proportion of PM remaining fixed (30%). Six partial substitutions were done, achieving TS-PM-RH volume ratios of 70:30:0, 60:30:10, 50:30:20, 40:30:30, 30:30:40, and 20:30:50. The ensuing six blends were coded TS₇₀:PM₃₀:RH₀, TS₆₀:PM₃₀:RH₁₀, TS₅₀:PM₃₀:RH₂₀, TS₄₀:PM₃₀:RH₃₀, TS₃₀:PM₃₀:RH₄₀, and TS₂₀:PM₃₀:RH₅₀, respectively (Table 2), among which the control growth medium was TS₇₀:PM₃₀:RH₀.

Table 2. Ratios of the soil and organic agro-waste materials [coarse-textured topsoil (TS), composted poultry manure (PM), and composted rice husk (RH)] in the composition of the potting mixes and the codes assigned to the treatments.

Treatments of growth media were thoroughly blended, after which they were placed in 20-L brown polythene bags for planting. The average mass of the bagged growth media was 30 kg. Because the study evaluated media effects on growth of two varieties of carrot, it was executed as a 6 × 2 factorial experiment. The 12 media/variety combinations serving as treatments were replicated 15 times in a Completely Randomized Design (CRD), giving 180 observations.

Before bagging in the first year, the growth media were sampled and processed in the same manner as for the topsoil sample. These processed samples, together with the stored air-dry samples (of topsoil, composted PM and composted RH), were analysed in triplicate at the Laboratory of the Department of Soil Science of the University of Nigeria Nsukka, following the procedures described by Estefan et al. (2013). Base saturation of the soil and growth media was computed as the percentage ratio of the sum of the exchangeable bases (K, Ca, Mg and Na) to the effective CEC.

Planting and cultural practices

The surfaces of the bagged growth media were leveled to avoid variations in growth pattern. Fresh media were prepared in planting seasons of year 2023 and 2024 when the open-field horticultural trials were carried out. Seeds were planted shallowly (about 1 cm depth) and thinly covered with soil. Planting of 10 seeds per bag was done in June immediately after blending and bagging the composted growing media, coinciding with the stabilisation of rains in early June in both years of the study. Carrot seedlings were thinned down to five plants per bagged medium one month after seedling establishment. Weeding and pest control were done manually. Harvesting of carrot roots was done three months after planting, specifically in September.  

Data collection

Data collection on seedling emergence and on yield parameters 90 days after planting was done on five randomly selected plants of similar vigour. The number of days to the emergence of carrot seedlings after planting was counted and recorded as days to seedling emergence. Number of harvested roots was recorded at maturity and harvest. Number of marketable roots was also recorded after examining the carrot roots and removing those affected by malformation and rot.

Whole-plant biomass distribution was also determined as the total fresh weight of root and foliage biomass immediately after harvest. Root weight was determined as the fresh below-ground biomass and foliage weight as the fresh above-ground biomass of the harvested plants immediately after harvest. All measurements of weight were done using a sensitive weighing scale and expressed in g/bag.

Also, percent marketable roots, defined as the proportion of the harvested carrot roots that can be marketed, was calculated using the formula (Kanju et al., 2019):

Finally, harvest index on fresh weight basis was calculated thus (Eze et al., 2024):

Statistical analysis

The agronomic data underwent analysis of variance using the software GenStat 12^th edition (VSN International Ltd., Hemel Hempstead, UK), adopting the option deemed appropriate for a factorial experiment in CRD. Where this analysis showed the effects of growth medium and/or carrot variety to be significant, the means were separated using the Least Significant Difference test at a 5% level of probability (LSD_0.05).

RESULTS

Initial soil properties and organic amendments composition

The physicochemical properties of the experimental soil are presented in Table 3. The soil is in the texture class of sandy clay loam and hence coarse-textured. Being well-drained, the soil was acidic (pH-H₂O of 5.4). Soil organic matter content, total nitrogen, available phosphorus, the exchangeable bases, base saturation, and apparent CEC all indicated that the concentrations of major nutrient elements and hence the fertility status of the coarse-textured topsoil at the beginning of the study were low. Table 3 also shows some chemical properties of the fresh organic agro-wastes (PM and RH) whose composted forms were used in blending the growth media.

Table 3. Physical and physicochemical properties of the coarse-textured topsoil and fresh forms of the organic agro-wastes (n = 3).

Table 4. Physicochemical properties of the composted poultry manure-based potting mixes as affected by partial substitution of the coarse-textured topsoil with composted rice husk (n = 3).

Effects of partial substitution of coarse-textured topsoil in composted poultry manure-based potting mix using composted rice husk on their physicochemical properties

The effects of partial substitution of the coarse-textured topsoil in the composted PM-based potting mix using composted RH are shown in Table 4. The partial substitution had effects on the post-cropping physicochemical properties of the growing media. The pH of the media was improved in all treated growth media including the control which did not receive composted RH substitution compared to the pH of the topsoil (5.4) before amendment. The highest value of soil pH-H₂O (7.4) was obtained in treatments TS₇₀:PM₃₀:RH₀ and TS₄₀:PM₃₀:RH₃₀. Notably, TS₂₀:PM₃₀:RH₅₀ was the only growth medium with a pH (6.9) within the range (6.5-7.0) often considered optimal for high crop yields in carrot production.

There were also considerable improvements in organic matter content of the growth media with TS₂₀:PM₃₀:RH₅₀ showing the highest values, while TS₆₀:PM₃₀:RH₁₀ had similar values as the pre-cropping topsoil. The results also indicated enhancement of total nitrogen content and base saturation of the growth media compared to the pre-cropping topsoil. The available phosphorus value increased from 38.05 to 54.96 mg/kg as the relative proportion of composted RH in the growth media progressively increased from 0% to 40%, and thereafter decreased at 50%.

Main effects of the partial substitution and crop variety on carrot performance

The growth and yield of the two carrot varieties as influenced by admixing coarse-textured topsoil with composted PM at 30% proportion (v/v) and partially substituting the former with composted RH are presented in Table 5. This treatment of partial substitution influenced (P < 0.05) days to emergence of seedlings in 2023. Emergence of carrot seedlings was earliest in TS₂₀:PM₃₀:RH₅₀ (8.4 days) followed by TS₇₀:RH₀:PM₃₀ (8.6 days), values of which were similar. Seedling emergence was latest in TS₅₀:PM₃₀:RH₂₀ (9.8 days). Though treatment effect on days to seedling emergence was not evident in 2024, the quickest emergence tended to be in TS₇₀:RH₀:PM₃₀ (7.1 days) and the slowest in TS₂₀:PM₃₀:RH₅₀ (10.1 days).

In 2023, growth medium significantly affected the yield and yield components of carrot; number of marketable roots, foliage weight, root weight, percent marketable roots, whole-plant biomass and harvest index, but had no effect on the number of harvested roots per bagged medium (Table 5). Treatment TS₂₀:PM₃₀:RH₅₀ had the highest values of foliage weight (130.5 g) and whole-plant biomass per bagged medium (317.0 g), followed closely by TS₃₀:PM₃₀:RH₄₀ (119.9 and 304.3 g, respectively) and TS₆₀:PM₃₀:RH₁₀ (106.4 and 305.8 g, respectively). Root weight per bagged medium was heaviest for carrot plants grown in TS₆₀:PM₃₀:RH₁₀ (197.3 g) which, however, was similar to TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ (183.4 and 182.0 g, respectively); the lowest values were recorded in the control TS₇₀:PM₃₀:RH₀. For percent marketable roots, remarkably lower values were recorded in the control compared to growth media with varying proportions of composted RH. Harvest index was also lower in the control compared to the rest for which values were similar.

In 2024, TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ recorded the highest values for number of marketable roots, foliage weight, root weight, whole-plant biomass and percent marketable roots. The control (TS₇₀:PM₃₀:RH₀) showed the lowest values of most yield and yield components. We observed visually throughout the study that this control growth medium without composted RH produced shorter plants with fewer leaves and thinner stems and roots, thus giving abysmally poor rooting than the rest with a fixed proportion of composted PM and varying proportions of composted RH.

For varietal effect, days to seedling emergence and number of harvestable roots significantly differed in 2023 when Thema seedlings emerged earlier and had more harvestable roots than Touchon seedlings (Table 5). In both years, crop variety influenced not only foliage weight (Thema > Touchon), but also root weight, harvest index (Touchon > Thema), and whole-plant biomass (inconsistent).

Interaction effects of the partial substitution and crop variety on carrot performance

The interaction effects of growth media composition (admixing topsoil with composted PM at a fixed relative proportion of 30% (v/v) and partially substituting the former with composted RH) and crop variety on carrot performance are shown (Table 6). In 2023, the interaction effect was significant (P ≤ 0.05) for days to emergence. The fastest emergence was observed in Thema variety grown with TS₂₀:PM₃₀:RH₅₀ and TS₃₀:PM₃₀:RH₄₀ (7.0 days); the slowest occurred with Touchon-×-TS₅₀:PM₃₀:RH₂₀ (11.2 days) and Touchon-×-TS₆₀:PM₃₀:RH₁₀ (11.1 days). Notably, these media-variety combinations were similar to Thema-×-TS₇₀:PM₃₀:RH₀ even with this growth medium seemingly representing less-than-ideal soil conditions.

Also in 2023 but not 2024, the interaction between growth medium and crop variety indicated significant (P ≤ 0.05) effects on number of harvested roots per bagged medium (Table 6). This quantitative agronomic variable with implication for root yield depended almost entirely on crop variety, as its highest values were achieved with Touchon-×-TS₇₀:PM₃₀:RH₀ and lowest with Thema-×-TS₇₀:PM₃₀:RH₀.

In both years of the open-field study, we observed that Thema-×-TS₃₀:PM₃₀:RH₄₀ and Thema-×-TS₂₀:PM₃₀:RH₅₀ showed the highest foliage weight, but with the former being superior to the latter in the second year (Table 6). Notably, these two treatments were followed by the trio of Thema-×-TS₆₀:PM₃₀:RH₁₀, Thema-×-TS₅₀:PM₃₀:RH₂₀ and Thema-×-TS₄₀:PM₃₀:RH₃₀ which were consistently similar to Touchon-×-TS₂₀:PM₃₀:RH₅₀. The lowest foliage weights were from Thema-×-TS₇₀:PM₃₀:RH₀ which was similar to Touchon-×-TS₇₀:PM₃₀:RH₀ in the second year.

Treatment effects on root weight and whole-plant biomass were not significant; however, Touchon grown with all RH-containing media tended to show the highest root weight (213.0-249.8 g per bagged medium) and the highest whole-plant biomass (294.8-346.8 g per bagged medium), while Thema-×-TS₇₀:PM₃₀:RH₀ tended to show the lowest values of these two yield-related agronomic variables (1.6 and 5.8 g per bagged medium, respectively). Treatments were also similar in terms of number of marketable roots, percent marketable roots and harvest index.

In 2024, treatment effects on most of the agronomic variables were not significant (P > 0.05). For root weight, Touchon grown with all RH-containing media still tended to be the best, with Touchon-×-TS₅₀:PM₃₀:RH₂₀ leading (307.0 g per bagged medium). For whole-plant biomass, however, Thema grown with all RH-containing media tended to show the highest values, with Thema-×-TS₃₀:PM₃₀:RH₄₀ and Thema-×-TS₂₀:PM₃₀:RH₅₀ leading (611.0 and 540 g per bagged medium, respectively). Treatments Thema-×-TS₇₀:PM₃₀:RH₀ and Touchon-×-TS₇₀:PM₃₀:RH₀ were among those tending to show the lowest values of these two yield-related variables (34.5-61-2 and 110-114.0 g per bagged medium, respectively).

Generally, the partial substitution of the coarse-textured topsoil in the potting mixes having 30% composted PM with composted RH enhanced carrot rooting, with TS₆₀:PM₃₀:RH₁₀ outperforming TS₄₀:PM₃₀:RH₃₀ and Touchon outperforming Thema.

DISCUSSION

The topsoil of this study was acidic. This was due to its coarse texture and hence ‘porous’ nature, its long weathering history leading to deep profiles, and the prevailing high rainfall regimes, all of which render the soil excessively leached of base-forming cations (Obalum et al., 2011, 2024a). This phenomenon reflected as low fertility status of the topsoil. In this situation, slow-mineralising manures could help reduce nutrient losses and improve soil structure towards enhancing crop productivity. Such manures often require that their high carbon-to-nitrogen (CN) ratio be lowered. This ratio, an index of decomposability of organic materials, is usually much lower in animal manures particularly PM than RH (Nwite et al., 2012a; Igwe et al., 2013; Adubasim et al., 2018; Uzoh et al., 2020; Nnadi et al., 2021), but PM was surprisingly higher than RH here (12.78 and 10.68, respectively). However, there is ample evidence that RHs from the same source as the present study are typically of higher, wide-range (18-47) CN ratio (Adubasim et al., 2018; Ebido et al., 2021), such that they need composting before soil application (Baiyeri et al., 2020; Ndzeshala et al., 2025a).

In this study, topsoil and composted PM in the volume ratio of 70:30 without partial substitution of the former using composted RH was designated the control growth medium. All six growth media including the control showed improvements in pH-H₂O relative to the pre-study topsoil with a value of 5.4. The highest value of 7.4 was obtained in media TS₇₀:PM₃₀:RH₀ and TS₄₀:PM₃₀:RH₃₀. So, these increases in pH-H₂O could be attributed not only to the liming effect of PM (Obalum et al., 2020; Obalum et al., 2024a, 2024b; Ogumba et al., 2024b; Ugwu et al., 2024), but also to the co-application of composted PM and composted RH both with higher concentrations of K, Ca, Mg and Na than the topsoil (Nwite et al., 2012a). According to Nuñez et al. (2008) and Paye et al. (2024), maximum yield of carrot and most vegetables is expected at soil pH in the range of 6.0-7.0, which is slightly acidic to neutral pH. Notably, potting mix TS₂₀:PM₃₀:RH₅₀ was the only growth medium with a pH (6.9) within this range. The inclusion of organic materials among the amendments for acid tropical soils raises soil pH by reducing exchangeable aluminum thus allowing the uptake of more exchangeable bases (Onwuka et al., 2007; Chukwuma et al., 2024), which might promote whole-plant fresh matter production of carrot.

The improvements in organic matter and total nitrogen contents including base saturation of the RH-containing growth media compared to the control treatment are noteworthy. With respect to phosphorus availability which remains a critical factor in acid tropical soils (Chukwuma et al., 2024; Ugwu et al., 2024; Ndzeshala et al., 2025b), the data attained show that the optimum level of such substitution is achieved between TS₅₀:PM₃₀:RH₂₀ and TS₃₀:PM₃₀:RH₄₀ (which translates into partial substitution of topsoil with composted RH giving 20-40% of the latter and a fixed 30% relative proportion of composted PM in seedlings growth media). It appears that combining a fixed rate of PM with especially rates of RH close to the said fixed rate is a way to increase available phosphorus in coarse-textured soils of the derived savannah (Nwite et al., 2012a). Also, as increases in soil organic matter imply increases in CEC (Obalum et al., 2013; Onah et al., 2023), the relative amount of exchangeable acidity is expected to decrease leading to increases in base saturation.

Growth medium influenced days to emergence of carrot seedlings in 2023. Stored energy in seeds enables them to sprout and grow without rapid access to nutrients from the soil; however, some plants may require specific nutrients to germinate even if all other germination conditions are met (Nascimento et al., 2008; Licata et al., 2023). These germination conditions viz. warmth, moistness and aeration depend on the growth medium, and our data suggest that factors favouring such conditions are not totally absent in the growth media without RH. The inclusion of RH in the blending of growth media has been suggested to improve aeration (Adubasim et al., 2018), but it appears that composted RH may not have similar effect.

However, composted RH-containing media enhanced growth of carrot plants. Composted PM-RH can serve as an effective soil amendment, enhancing soil porosity for moisture retention and contents and availability of plant nutrients (Nwite et al., 2012b; Adubasim et al., 2018; Baiyeri et al., 2019, 2020). Such enhancements in growth media quality due to composted RH favours root development in container-grown crops (Nuñez et al., 2008; Jeon et al., 2010; Varela et al. 2013; Baiyeri et al., 2020; Ugwu et al., 2020). In terms of nutrient bioavailability in the growth media, the composted PM and composted RH probably enhanced mainly phosphorus availability (Ugwu et al., 2020), which often favours root crop production in coarse-textured tropical soils (Mwangi et al., 2020; Obalum et al., 2020). In the present location, concurrent soil fertility management-induced increases in sweet potato tuber yields and soil available phosphorus have been separately reported (Nnadi et al., 2020; Ogumba et al., 2024b). Therefore, the poor rooting of the control TS₇₀:PM₃₀:RH₀ and, to a much lesser extent, TS₄₀:PM₃₀:RH₃₀ of this study could be a reflection of the alkalinity-induced low phosphorus bioavailability (Table 4).

In their study involving varying RH proportions in PM-based media, Baiyeri et al. (2019) reported consistently low root weight, wet and dry biomasses, and harvest index of carrot with the highest quantity of RH. By contrast, the pre-blending composting of RH here facilitated the mineralisation of the composted PM in the growth media, while the composted RH itself added and helped stabilize organic matter, both leading to increases in porosity and soil moisture and nutrient availability that reflected in carrot performance (Agbede and Oyewumi, 2023). The effectiveness of the topsoil substitution here could thus be linked to the composting of RH, leading to lowered CN ratios of the RH-containing media. Generally, CN ratios around 10 have been targeted at for coarse-textured tropical soils (Ndzeshala et al., 2023; Ameh et al., 2025), and values farther away below are deemed unfavourable. This role of CN ratio was evident in the second year when the said poor rooting in growth media TS₇₀:PM₃₀:RH₀ (the control) and TS₄₀:PM₃₀:RH₃₀ reflected their respective CN ratios (Table 4).

On the varietal effect, Thema outperformed Touchon only for foliage weight and vice versa for root weight and harvest index. Under optimal conditions, responses to environmental variables (viz. soil moisture, temperature, and light and nutrient availability), ability to respond to stress, ability to utilise nutrients and yield potential of cultivated varieties depend on their genetic makeup/adaptation and influence their productivity (Nascimento et al., 2008; Licata et al., 2023).

The interaction of growth medium and carrot variety influenced days to seedling emergence only in the first year. Plant germination depends on both the growth medium’s critical germination factors and plant’s adaptability (Nascimento et al., 2008; Licata et al., 2023). Notably, growth medium had lesser influence than carrot variety on number of harvested roots per bagged medium. The interaction being significant for foliage weight in the two years of the study implies that this yield parameter depended not just on growth medium but also on carrot variety; hence, the two best-performing growth media (TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀) gave the highest values for foliage weight with the Thema variety.

From economic and food security perspectives, the major indicators of carrot performance should be foliage weight and root weight. For both varieties, TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ generally outperformed the other growth media in terms of foliage weight, while all RH-containing media outperformed the control TS₇₀:PM₃₀:RH₀ in terms of root weight, with TS₆₀:PM₃₀:RH₁₀, TS₅₀:PM₃₀:RH₂₀, TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ (PM-to-RH ratios of 3, 1.5, 0.75 and 0.60, respectively) being generally superior to TS₄₀:PM₃₀:RH₃₀ with a PM-to-RH ratio of 1. Therefore, the required partial substitution of acid coarse-textured topsoils (already imbued with composted PM at a fixed volume proportion of around 30%) with composted RH should be done either frugally or generously, to achieve a PM-to-RH ratio within the ranges of 3.0-1.5 and 0.75-0.60, respectively. Such suitable potting mixes for container production of carrots should be used to grow either of the two varieties, preferably Touchon, for increased root yield; however, those of ratios within 0.75-0.60 should be used to grow specifically Thema for increased foliage yields. Adequate knowledge of the yield potential and nutritional requirements of these varieties vis-à-vis the nutrient-supplying ability of these suitable potting mixes may assist growers of a given variety apply supplementary nutrients, where necessary, towards enhancing productivity (Santos et al., 2012; Sharma et al., 2012).

CONCLUSION

The evaluation of the effects of partial substitution of acid coarse-textured topsoil imbued with a fixed relative proportion of composted poultry manure (PM) using different levels of composted rice husk (RH) on the properties of potting mix for horticultural purposes and the performance of two carrot varieties showed its prospects to give potting mixes (growth media) that could support container production of carrot. Most agronomic traits showed improvements in growth media with composted RH relative to the control treatment without. Amending such topsoils (TS) with composted PM and composted RH such that the PM-to-RH volume ratios in the growth media become 0.75 (TS₃₀:PM₃₀:RH₄₀) would improve their fertility status. However, this particular growth medium was somewhat similar to option TS₂₀:PM₃₀:RH₅₀ (PM-to-RH ratio of 0.6) as well as to options TS₆₀:PM₃₀:RH₁₀ (PM-to-RH ratio of 3) and TS₅₀:PM₃₀:RH₂₀ (PM-to-RH ratio of 1.5) in terms of carrot growth and yield responses.

Of the two carrot varieties, Touchon generally outperformed Thema; however, the former is preferred for increasing root yield and the latter to be adopted only for increasing foliage yield. The data attained support the use of any of the RH-containing media (except TS₄₀:PM₃₀:RH₃₀ with a PM-to-RH ratio of 1) to grow either of the two carrot varieties, preferably Touchon, for increased root yield, or either of TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ to grow Thema where greater premium is placed on foliage yields. Growth-medium options TS₆₀:PM₃₀:RH₁₀ and TS₅₀:PM₃₀:RH₂₀ represent potting mixes where the necessary partial substitution of topsoil on volumetric basis (while fixing the relative proportion of composted PM at 30%) with composted RH is done frugally, to achieve a higher PM-to-RH ratio of, say x, such that 3.0 ≥ x ≥ 1.5. Options TS₃₀:PM₃₀:RH₄₀ and TS₂₀:PM₃₀:RH₅₀ with the highest relative proportions of composted RH in this study imply achieving a lower PM-to-RH ratio in the potting mix of, say y, such that 0.75 ≥ y ≥ 0.60. These targets of PM-to-RH ratios within the ranges represented with x and y in the dominant coarse-textured topsoils of the savannah region are suggested, as applicable, for enhanced and sustainable bagged-media production of carrot in the concerned agro-ecologies.

This study analysed the soil physicochemical properties of the blended growth media defining fertility status; their key hydraulic properties were not determined. Future studies should examine the contributions of reduced bulk density and hence enhanced porosity and associated moisture retention and aeration to the effectiveness of RH-containing PM-based growth media. This might also help to provide further insight into the generally poorer growth of carrot in bagged media with PM-to-RH volume ratio of 1 compared to those with higher and lower ratios.

AUTHOR CONTRIBUTIONS

Chiamaka A. Chizaram-Ndubuaku: Resources (Lead), Investigation (Lead), Data Curation (Supporting), Writing–Original Draft (Lead); Sunday E. Obalum: Formal Analysis (Lead), Software (Lead), Data Curation (Lead), Visualization (Lead), Writing–Review & Editing (Equal); Kayode P. Baiyeri:  Conceptualisation (Lead), Resources (Supporting), Supervision (Lead), Project Administration (Lead), Validation (Lead), Writing–Review & Editing (Equal). All three authors have read and approved of the final manuscript.

CONFLICT OF INTEREST

The authors declare that they have no conflicts of interest.

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