Abstract Enhancing soil quality and crop productivity through sustainable nutrient management is crucial for agricultural sustainability. This study assessed the impacts of organic manures (poultry manure (PM), cattle manure (CM), swine manure (SM), goat manure (GM), Chromolaena odorata (Chromolaena) and NPK fertilizer, applied individually or in combination, on soil properties, carrot (Daucus carota L.) growth, yield, and nutrient content. The treatments were arranged in a randomized complete block design with three replications. Results showed that organic manures significantly improved soil physical properties by reducing bulk density, increasing porosity, and enhancing moisture content, with GM being the most effective due to its higher organic carbon content (31.8%). Carrot yield was more influenced by soil physical properties than by chemical attributes. Among the treatments, PM produced the tallest plants (24.5 cm) and the most leaves (12.6), while GM resulted in the largest root diameters (1.4 cm), longest roots (22.1 cm), and heaviest root weights (40.6 t ha^-1). The combined application of organic manures and NPK fertilizer exhibited synergistic effects, improving both soil physical properties and nutrient availability. The GM+NPK combination recorded the highest root yield (45.6 t ha^-1) in 2021 and (45.9 t ha^-1) in 2022 and enhanced Vitamin A (β-carotene) and Vitamin C (ascorbic acid) contents. The co-application of organic manures and NPK fertilizer, as an integrated nutrient management approach, consistently proved superior to the sole application of either input by improving soil fertility and reducing dependence on chemical fertilizers, making it a viable strategy for sustainable carrot production in tropical regions.

Keywords: Carrot, NPK fertilizer, Organic manures, Soil properties, Vitamin A and Vitamin C, Sustainable nutrient management

Citation:  Adekiya, A.O., Agbede, T.M., and Lawal, T.E. 2025. Effects of co-application of animal manures, Chromolaena odorata and NPK fertilizer on soil properties, carrot yield and nutrient content in a tropical agroecosystem. Natural and Life Sciences Communications. 24(4): e2025073.

INTRODUCTION

Carrot (Daucus carota L.), a widely cultivated root vegetable, plays a crucial role in global food security due to its high nutritional value and associated health benefits. It is particularly rich in beta-carotene, a precursor to vitamin A (Kopsell and Kopsell, 2006; Ikram et al., 2024), as well as fiber, potassium, and antioxidants (Kamel et al., 2023). Carrots are commonly consumed raw as snacks, in salads, or as crudités served with dips, with their crisp texture making them a popular choice for fresh consumption (Pant and Manandhar, 2007; McGreal, 2018). Carrot purées are commonly served as a side dish or incorporated into baby food, while their natural sweetness and moisture make them a popular ingredient in baked goods such as carrot cake, muffins, and cookies. Fresh carrot juice is valued for its health benefits and is often blended with other fruits and vegetables in smoothies (Ikrama et al., 2024). In the cosmetics industry, carrot seed oil and extracts are used in skincare products for their moisturizing and anti-aging properties (Staniszewska and Kula, 2001; Musnaini et al., 2022). Additionally, carrot peels, tops, and surplus or damaged carrots provide a valuable feed source for livestock (Ikram et al., 2024).

In Nigeria, carrot cultivation is primarily concentrated in the middle belt and northern states, covering approximately 27,000 hectares of farmland but with relatively low yields (Ajayi et al., 2022). The average carrot yield in Nigeria is about 15 t ha⁻¹, significantly lower than the 170 t ha⁻¹ reported in the United States (FAOSTAT, 2012).

The Nigerian savannah soils, where carrots are predominantly grown, face significant challenges that hinder optimal yields. These soils are typically low in essential nutrients and organic matter, have poor structure, and exhibit reduced fertility (Adegbite et al., 2019; Adekiya et al., 2022). In high-rainfall areas, acidification caused by nutrient leaching and prolonged use of acid-forming fertilizers further limits nutrient availability (Agegnehu et al., 2021). As a result, low soil fertility often leads to poor crop yields on farmers’ fields.

Carrots, classified as medium nutrient feeders (Thapa et al., 2023), require balanced fertilization for optimal growth. According to the Federal Ministry of Agriculture (2002), the recommended fertilizer application for carrots in northern Nigeria is 50 kg N ha⁻¹, 38 kg P₂O₅ ha⁻¹, and 60 kg K₂O ha⁻¹ to achieve optimal yields. However, research has shown that carrots respond well to both organic and inorganic fertilizers (Sani et al., 2016; Kushwah et al., 2019; Mahari et al., 2024).

In many agricultural systems, chemical fertilizers, such as NPK, are widely used to meet crop nutrient demands. However, excessive and indiscriminate use of these fertilizers has raised concerns about soil degradation, environmental pollution, and reduced soil biological activity (Getu and Teshager, 2015). Additionally, in Nigeria, the rising cost of chemical fertilizers and their limited availability during critical planting periods pose significant challenges for farmers (Adekiya et al., 2022).

Organic manures, such as cattle manure, poultry manure, goat manure, swine manure, and green manure, are widely recognized for their ability to improve soil physical, chemical, and biological properties (Adekiya et al., 2022). These amendments not only supply essential nutrients but also enhance soil organic matter content, water retention, and microbial activity. A study conducted in Akure, southwest Nigeria, found that cattle manure application reduced soil bulk density, dispersion ratio, and soil temperature while increasing porosity, moisture content, and maize growth and yield compared to untreated soil (Adekiya et al., 2016). Similarly, Daba et al. (2018) reported that cattle manure application improved carrot growth and yield. Habimana et al. (2015) also observed a significant increase in carrot yield due to poultry manure application. Poultry manure has been shown to increase soil organic matter, nitrogen (N), and phosphorus (P) levels, as well as improve tomato growth and yield while reducing soil bulk density and moisture content as application rates increased (Ewulo et al., 2008; Adekiya and Agbede, 2009; Agbede and Oyewumi, 2023). Swine manure application was found to enhance soil chemical properties and maize yield (Diri and Kedoneojo, 2024). Similarly, Awosika et al. (2014) reported increases in soil nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg), along with improved tomato yield following pig manure application. Goat manure application significantly increased soil pH, organic matter content, macronutrient levels (N, P, K, Ca, Mg), cation exchange capacity (CEC), and maize yield (Uwah and Eyo, 2014). Additionally, Nawaz and George (2004) found that applying Chromolaena odorata biomass increased okra yield.

Despite their benefits, organic manures typically release nutrients gradually over time and large quantities are needed to meet the same nutrient level as chemical fertilizer (Adekiya et al., 2022). This slow release may not always meet the immediate nutrient needs of high-demand and short-duration crops like carrots, which require rapid nutrient availability for optimal growth. Integrating organic and inorganic fertilizers can help balance the need for both immediate nutrient uptake and long-term soil health. Organic manures supply essential micronutrients, improve soil structure, increase organic matter content, and enhance nutrient retention and microbial activity (Adekiya, 2019). In contrast, inorganic fertilizers (NPK) provide readily available macronutrients (nitrogen, phosphorus, and potassium) needed for immediate crop uptake. By combining both organic and inorganic fertilizers, farmers can achieve a more balanced and sustained nutrient supply throughout the growing season, reducing the risk of nutrient deficiencies or excesses while also minimizing the amount of organic manure required.

Previous studies have demonstrated the benefits of integrated nutrient management in enhancing soil fertility, promoting plant growth, and improving crop yields across various crops. In an experiment assessing the effects of poultry manure, NPK fertilizer, and irrigation intervals on carrot (Daucus carota) productivity, Sani et al. (2016) found that integrating poultry manure with NPK fertilizer resulted in the highest carrot yield compared to the sole application of either organic or inorganic fertilizers. Similarly, Sarkin et al. (2019) reported that combining cow dung with NPK fertilizer produced the highest maize yield relative to the sole application of either cow dung or NPK. Additionally, Frank et al. (2020) found that the combined application of cow dung and NPK fertilizer significantly improved soil properties, including organic carbon content, available phosphorus (P), soil pH, total nitrogen (N), exchangeable bases, and effective cation exchange capacity. This combination also led to increased yields of fluted pumpkin compared to the control.

However, limited research has been conducted on the specific effects of different organic manure sources combined with NPK fertilizers on carrot growth and yield. Additionally, the impact of these nutrient sources on soil properties, such as pH, organic matter content, nutrient availability, and bulk density, remains underexplored, particularly in tropical regions. Furthermore, various organic manures and NPK fertilizer, as well as their combinations, may influence the vitamin content of carrots differently, a factor that has not been adequately investigated. This study aims to evaluate the individual and combined effects of organic manures and NPK fertilizer on soil properties, carrot growth, yield, and vitamin content. It was hypothesized that soil physical and chemical properties, as well as carrot growth, yield, and vitamin content, would respond differently to the individual and combined applications of organic manures and NPK fertilizer. To test this hypothesis, experiments were conducted to assess these effects.

MATERIALS AND METHODS

Site description and experimental layout

Field experiments were conducted between June and November in both 2021 and 2022 at the Teaching and Research Farm of Landmark University, Omu-Aran, Kwara State, Nigeria (latitude 8° 7' 26.21388" N and longitude 5° 5' 0.1788" E). The study aimed to investigate the combined influence of organic manures and NPK fertilizer on soil properties, carrot growth, and yield. The experimental location experiences a bimodal rainfall distribution with an average annual rainfall of approximately 1,300 mm and a mean annual temperature of 32°C. The site lies within Nigeria's derived savanna ecological zone. The soil type is classified as an Alfisol, specifically Oxic Haplustalf or Luvisol (Adegbite et al., 2019). Before cultivation, the dominant vegetation at the sites consisted primarily of guinea grass (Panicum maximum).

The experiment was a single-factor experiment comprised of: control, poultry manure (PM), cattle manure (CM), swine manure (SM), goat manure (GM), green manure - Chromolaena odorata (Chromolaena), NPK 15-15-15 fertilizer (NPK), PM + NPK, CM + NPK, SM + NPK, GM + NPK and Chromolaena + NPK. The twelve (12) treatments were arranged in a randomized complete block design, replicated three times. The size of each plot was 3 × 3 m. All amendments were applied at the rate of 50 kg N ha⁻¹ (FMA, 2002). These rates were equivalent to 1,661.13 kg ha⁻¹, 2,840.91 kg ha⁻¹, 1,908.40 kg ha⁻¹, 3,787.88 kg ha⁻¹, 1,992.03 kg ha⁻¹, and 333.33 kg ha⁻¹ for PM, CM, SM, GM, Chromolaena, and NPK fertilizer, respectively.

Land preparation, application of manures and carrot establishment

Land preparation for carrots involves clearing the area, removing debris, and deep ploughing to a depth of 30 cm. The soil was then harrowed to ensure proper drainage. Afterward, the land was divided into blocks and plots, with each plot measuring 3 × 3 m. The manures were weighed and incorporated into the soil according to the treatment plan, to a depth of approximately 10 cm. The manures were then allowed to mineralize for two weeks before sowing carrot seeds. Emerged weeds at this period were manually removed using a hoe. The carrot variety Touchon, purchased from an agricultural store in Ilorin, Kwara State, was used for this experiment. Sowing took place on April 16 each year, with seeds drilled in rows spaced 30 cm apart. The seeds were immediately covered with palm fronds to conserve moisture and protect them from excessive heat. The palm fronds were removed one week after sowing. Three weeks after sowing, thinning was carried out to achieve a spacing of 15 cm within rows, resulting in a plant population of approximately 200 plants per plot. NPK fertilizer was applied using the band method, 10 cm away from the carrot plants along the rows after thinning. No irrigation was provided. Weeding was done manually every four weeks by hand-pulling emerging weeds. Additionally, earthing up was performed regularly to support the growth of the carrot plants.

Soil and manures analysis

In 2021 and 2022, prior to the commencement of the experiment, five intact soil samples were randomly collected from five different locations at each site using a core steel sampler at a depth of 0-15 cm. These samples were used to determine bulk density after being oven-dried at 100°C for 24 hours. Additionally, soil samples from the topsoil layer (0–15 cm) were randomly collected from various locations within the study areas and combined to form a composite sample. Both soil and manure samples were air-dried, ground, and passed through a 2-mm sieve before their nutrient compositions were analyzed using standardized methods (Carter and Gregorich, 2007). At the end of the experiment, additional soil samples were collected from each plot at each site for chemical analysis, excluding soil texture analysis.

The Bouyoucos (1962) hydrometer method was employed to determine soil texture (Mozaffari et al., 2024). Organic carbon content was estimated using the Walkley-Black oxidation technique (El Mouridi et al., 2023). Total nitrogen content was assessed via the micro-Kjeldahl method (Gautam et al., 2023). Available phosphorus was extracted using Bray 1 extractant (0.03 N NH₄F + 0.025 N HCl), following the procedure described by Dermawan et al. (2024). Exchangeable potassium (K), calcium (Ca), and magnesium (Mg) were extracted using neutral 1N ammonium acetate (NH₄OAc). Potassium levels were quantified through flame photometry, while calcium and magnesium concentrations were analyzed via atomic absorption spectrophotometry, following standardized procedures (Mustapha et al., 2020).

Soil physical properties

Two weeks following the application of fertilizer (five weeks after sowing), measurements of bulk density, total porosity, gravimetric water content and soil temperature were initiated across all plots and repeated at three-week intervals. Bulk density was assessed using the previously outlined method. Total porosity was calculated from the values of bulk density by using particle density of 2.65 Mg m^-3. Soil moisture content was evaluated gravimetrically by drying samples in an oven at 105°C for 24 hours. Soil temperature was measured at 3:00 pm using a soil thermometer inserted to a depth of 15 cm. Three readings were taken from each plot during every sampling session and their average was then computed.

Analysis of manures  

The nutrient profile of poultry manure (PM), cattle manure (CM), swine manure (SM), goat manure (GM) and Chromolaena odorata (Chromolaena) was evaluated following sieving through a 2 mm mesh. The evaluations covered total organic carbon, N, P, K, Ca and Mg according to the techniques outlined by the Association of Analytical Chemists (AOAC, 2023).

Determination of growth and yield parameters of carrot

Fifteen carrot plants were selected for data collection on growth and yield. Carrot height was measured at 15-day intervals by measuring the height of the plant from the soil surface to the tip of the tallest leaf using a ruler. Additionally, every two weeks, the number of leaves per plant was determined by counting all the leaves on a single plant, ensuring that both mature and younger leaves were included. Carrots were ready for harvest 90 days after sowing, indicated by the leaves beginning to turn yellow. The soil around the carrots was gently loosened using a trowel, after which the foliage close to the top of the carrot was grasped and gently wiggled back and forth while pulling upwards. After uprooting, excess soil attached to the carrot was shaken off. The diameter of the root at its thickest point was measured using calipers. Root length was determined by measuring the carrot from just below the foliage to the tip of the root. The fresh weight of the carrots was determined using a balance.

Determination of vitamin A and C contents in carrots

The determination of vitamin A (β-carotene) in carrots followed the method outlined by Amorim-Carrilho et al. (2014). The vitamin C content of the carrot was determined by using the indophenol dye method (Singh et al., 2007).

Statistical analysis

Data collected were subjected to analysis of variance (ANOVA). Significant differences between treatment means were identified using Duncan's Multiple Range Test (DMRT) at a level of P < 0.05.

RESULTS

Pre-cropping soil and chemical analysis of manures used

The results of the soil physical and chemical properties before carrot cropping in 2021 and 2022 are presented in Table 1. The soils had a sandy loam texture (78% sand, 13% silt, 9% clay in 2021; and 77.2% sand, 13.6% silt, 9.2% clay in 2022), with low organic carbon (1.22% and 1.25%), nitrogen (0.11% and 0.10%), phosphorus (8.6 and 8.8 mg kg⁻¹), potassium (0.13 and 0.14 cmol kg⁻¹), and calcium (1.85 and 1.78 cmol kg⁻¹), in 2021 and 2022, respectively. Magnesium was moderate in both years (0.44 and 0.48 cmol kg⁻¹), while the soils were slightly acidic, with pH values of 5.98 and 5.91. Bulk density was moderately high in both years.

Table 2 presents the nutrient composition of the different organic amendments. Goat manure had the highest organic carbon content (31.8%), followed by cattle manure (27.2%), Chromolaena odorata (24.3%), swine manure (21.6%), and the least was poultry manure (18.1%). Poultry manure had the highest concentrations of nitrogen (3.01%), phosphorus (1.86%), potassium (3.74%), calcium (3.41%), magnesium (0.8%), and the lowest C:N ratio (6.01).

Table 1. Initial soil characteristics of the experimental site before sowing carrot.

Table 2. Chemical compositions of the organic manures used.

Note: Values followed by similar letters under the same column are not significantly different at P < 0.05 according to Duncan’s multiple range test (DMRT).

Effects of organic manures and NPK fertilizer on soil physical properties

The results of the effects of organic manures and NPK fertilizer on physical properties are presented in Table 3. In both years, the control and NPK fertilizer treatments had the highest bulk densities (1.54 and 1.52 Mg m^-3), soil temperatures (32.7 and 31.5°C), lowest porosities (41.9 and 42.9%) and moisture contents (9.7 and 10.2%) compared with other treatments. The values of bulk density, porosity, moisture content and soil temperature for the control and NPK fertilizer were not significantly different from each other. Organic manure treatments, particularly GM and CM, had significantly lower bulk densities (1.22 and 1.23 Mg m^-3) and soil temperatures (25.8 and 26.0°C), and highest porosities (54.1 and 53.6%) and moisture contents (15.8 and 15.3%). Also, the values of bulk density, porosity, moisture content and soil temperature between GM and CM were not significant, suggesting improved soil structure and reduced compaction. The combined organic manure + NPK treatments had moderate bulk densities, ranging from 1.27 to 1.37 Mg m^-3. These combinations exhibited moderate improvements in soil physical properties, with the GM + NPK treatment showing the lowest bulk density (1.28 Mg m^-3) and soil temperature (27.3°C), as well as relatively high porosity (52.0%) and moisture content (12.9%). Combining organic manures with NPK fertilizer provided some improvement in soil physical properties relative to the control but was not as effective as sole organic manure application.

Table 3. Effects of organic manures and NPK fertilizer on soil physical properties.

Note: Values followed by similar letters under the same column are not significantly different at P < 0.05 according to Duncan’s multiple range test (DMRT); PM = poultry manure; CM = cattle manure; SM = swine manure; GM = goat manure; NPK = NPK fertilizer; Chromolaena = Chromolaena odorata.

Effects of organic manures and NPK fertilizer on soil chemical properties

The effects of organic manures and NPK fertilizer on soil chemical properties are shown in Table 4. In both years, sole organic manure treatments increased soil pH to 6.19, compared to 5.84 recorded under NPK fertilizer and control treatments, which slightly reduced the pH. The combined treatments generally maintained a pH (6.09) close to the individual manure treatments. The highest values for OC (1.98% and 2.02%, respectively) were observed in soils treated with goat manure (GM) alone in both years. The addition of NPK fertilizer to manure treatments generally decreased the OC content compared to manure alone (Table 4). Among addition of NPK fertilizer, GM + NPK had the highest values of OC (1.64% and 1.78%). Addition of soil amendments increased soil N, P, K, Ca and Mg contents compared with the control (Table 4). NPK fertilizer did not increase Ca and Mg contents of the soil relative to the control. Among individual applications, PM had the highest values of N (0.28%), P (25.9 mg kg^-1), K (0.41 cmol kg^-1), Ca (5.60 cmol kg^-1), and Mg (0.88 cmol kg^-1) compared to CM, SM, GM, and Chromolaena in both years. Among all treatments, PM + NPK had the highest values of P (31.2 mg kg^-1), and K (0.64 cmol kg^-1). Among the combined amendments, PM + NPK had the best values of Ca (3.51 cmol kg^-1) and Mg (0.65 cmol kg^-1). Again, combining manure with NPK fertilizer often resulted in a synergistic effect, with higher values for available P and exchangeable K compared to the individual treatments (Table 4). 

Table 4. Effects of organic manures and NPK fertilizer on soil chemical properties.

Note: Values followed by similar letters under the same column are not significantly different at P < 0.05 according to Duncan’s multiple range test (DMRT); PM = poultry manure; CM = cattle manure; SM = swine manure; GM = goat manure; NPK = NPK fertilizer; Chromolaena = Chromolaena odorata.

Effects of organic manures and NPK fertilizer on carrot growth and yield characteristics

The results in the Table 5 show the variations in carrot’s growth and yield parameters due to organic amendments and NPK fertilizer. Addition of soil amendments increased the growth and yield characteristics of carrot in years 2021 and 2022 relative to the control. Among individual applications, PM resulted in the highest plant height (24.2 cm) in 2021 and (24.8 cm) in 2022, as well as the highest number of leaves (12.4) in 2021 and (12.8) in 2022 relative to CM, SM, GM, Chromolaena, and NPK fertilizer. Whereas, GM significantly recorded the highest root diameter (1.4 cm) in 2021 and 2022, root length (22.2 cm) in 2021 and (21.9 cm) in 2022, and carrot root weight (40.6 t ha^-1) in 2021 and (40.5 t ha^-1) in 2022 relative to PM, CM, SM, Chromolaena and NPK fertilizer. All organic manures increased the growth and yield of carrot relative to NPK fertilizer. When organic amendments were combined with NPK fertilizer, the highest plant height was recorded in the PM + NPK treatment (31.6 cm) in 2021 and (32.8 cm) in 2022, followed by SM + NPK (28.4 cm) in 2021 and (28.9 cm) in 2022. PM + NPK produced the highest number of leaves (16.3), suggesting improved vegetative growth. The GM + NPK treatment had the largest root diameter (1.7 cm) in 2021 and (1.8 cm) in 2022, followed closely by CM + NPK (1.5 cm) in 2021 and (1.6 cm) in 2022. The longest roots were recorded in the GM treatment (22.2 cm) in 2021 and (21.9 cm) in 2022. GM + NPK resulted in the highest root yield (45.6 t ha^-1) in 2021 and (45.9 t ha^-1) in 2022, followed by CM + NPK (41.4 t ha^-1) in 2021 and (40.9 t ha-1) in 2022. In 2021, relative to Chromolaena + NPK, SM + NPK, CM + NPK, PM + NPK, NPK fertilizer alone, Chromolaena alone, GM alone, SM alone, CM alone, PM alone and control, GM + NPK increased fresh weight of carrot by 19.7%, 36.9%, 10.1%, 34.9%, 93.6%, 34.9%, 12.3%, 51.5%, 24.6%, 67.0% and 104.5%, respectively. Similarly, in 2022, the respective values were: 19.5%, 37%, 12.2%, 37.0%, 86.6%, 35.4%, 13.3%, 49.5%, 24.7%, 67.5% and 111.5% for Chromolaena + NPK, SM + NPK, CM + NPK, PM + NPK, NPK fertilizer alone, Chromolaena alone, GM alone, SM alone, CM alone, PM alone and the control.

Table 5. Effects of organic manures and NPK fertilizer on growth and yield of carrot.

Note: Values followed by similar letters under the same column are not significantly different at P < 0.05 according to Duncan’s multiple range test (DMRT); PM = poultry manure; CM = cattle manure; SM = swine manure; GM = goat manure; NPK = NPK fertilizer; Chromolaena = Chromolaena odoratav.

Effects of organic manures and NPK fertilizer on the vitamin A and C content of carrot

The results presented in Figures 1 and 2, respectively illustrates the effects of organic manures and NPK fertilizer and their combinations on the Vitamin A (β-carotene) and Vitamin C content of carrots in 2021 and 2022. The carrots grown without any treatment (control) had the lowest levels of vitamin A and vitamin C, that is, application of amendments enhanced the level of vitamins A and C relative to the control. In both years, cattle manure (CM) and goat manure (GM) significantly resulted in the highest concentrations of vitamin A (12.57 mg 100 g^-1) and vitamin C (31.95 mg 100 g^-1), respectively. These values were notably higher compared to those recorded under PM (8.74 mg 100 g^-1) for vitamin A and (25.08 mg 100 g^-1) for vitamin C, SM (8.40 mg 100 g^-1 and 26.56 mg 100 g^-1), Chromolaena (11.69 mg 100 g^-1 and 28.43 mg 100 g^-1), and NPK fertilizer (7.56 mg 100 g^-1 and 12.57 mg 100 g^-1), respectively. NPK fertilizer alone significantly increased both vitamins compared to the control but was less effective than organic manures. Organic manure + NPK fertilizer treatments increased the vitamin A and C contents of carrot compared to their individual applications. Among all treatments, GM + NPK significantly resulted in the highest vitamin A (16.55 mg 100 g^-1 and 16.88 mg 100 g^-1) in 2021 and 2022, respectively and vitamin C (41. 66 mg 100 g^-1 and 42.07 mg 100 g^-1) in 2021 and 2022, respectively.

Figure 1. Effects of organic manures and NPK fertilizer on vitamin A content of carrot. Vertical bars show standard error of paired comparisons. Bars marked with different letters show means significantly different at 5% level using Duncan's multiple range test (DMRT); PM = poultry manure; CM = cattle manure; SM = swine manure; GM = goat manure; NPK = NPK fertilizer; Chromolaena = Chromolaena odorata.

Figure 2. Effects of organic manures and NPK fertilizer on vitamin C content of carrot. Vertical bars show standard error of paired comparisons. Bars marked with different letters show means significantly different at 5% level using Duncan's multiple range test (DMRT); PM = poultry manure; CM = cattle manure; SM = swine manure; GM = goat manure; NPK = NPK fertilizer; Chromolaena = Chromolaena odorata.

DISCUSSION

The low nutrient status of the soil at each site is attributed to its sandy loam texture, which limits nutrient retention, along with high acidity, moderate bulk density, and low porosity, all of which restrict root development and nutrient uptake. Key fertility indicators—organic carbon, nitrogen, phosphorus, and essential cations (K and Ca)—fall below the recommended critical levels (Akinrinde and Obigbesan, 2000). Consequently, external inputs are required to enhance soil fertility and support sustainable crop production.

The differences in bulk density, soil temperature, porosity, and moisture content between the control/NPK treatments and organic manure treatments can be attributed to the decomposition of the organic manures to organic matter, which improves the soil physical properties. The lower bulk densities observed in organic manure-treated soils are due to the incorporation of light and porous materials from the organic matter, which reduces soil particle packing (Rayne and Aulam, 2020). Additionally, microbial activity stimulated by organic manures leads to the production of polysaccharides and humic substances, which bind soil particles into aggregates, thereby reducing soil compaction (Brady and Weil, 2008; Dhaliwal et al., 2019). This process improves soil structure, resulting in lower bulk density, enhanced aeration, and easier root penetration. Similar research indicates that incorporating manure can positively alter soil properties. Celik et al. (2010) found that adding manure at a rate of 25 Mg ha⁻¹ year⁻¹ resulted in the lowest bulk densities when compared to synthetic fertilizer. Meng et al. (2019) demonstrated that applying manure over 20 years increased the total soil porosity by 11.9% and decreased bulk density by 13.1% compared to a control treatment.

The improvement in soil total porosity due to manure application can be attributed to the role of organic matter in promoting the formation of stable soil aggregates, which create more pore spaces and enhance soil porosity (Lehmann et al., 2020). Decomposed organic matter increases the proportion of macropores and micropores, facilitating better air circulation and water infiltration. Additionally, earthworm and microbial activity contribute to porosity by burrowing and breaking down organic materials (Ahmed and Al-Mutairi, 2022; Bilong et al., 2022).

The increase in soil moisture content observed with manure application may be due to the high water retention capacity of humus formed from decomposed organic matter, which helps the soil retain moisture for plant growth. Organic matter can create a sponge-like structure that absorbs and holds water in the root zone, reducing drought stress (Clark, 2025). Improved soil aggregation also minimizes surface runoff, thereby enhancing water infiltration and storage (Du et al., 2022). Furthermore, organic amendments may act as an insulating layer, reducing the direct impact of solar radiation on the soil surface (Komariah et al., 2011). This shading effect helps moderate soil temperature by preventing excessive heat absorption. Additionally, improved soil structure can prevent crust formation, further reducing heat buildup and enhancing water retention, which collectively contributes to a more stable soil environment.

The application of GM and CM, either individually or in combination with NPK fertilizer, resulted in significantly lower bulk densities and soil temperatures, along with the highest porosity and moisture content. The improvement in soil physical properties observed with CM and GM, compared to other treatments, is likely due to their higher soil organic matter (SOM) content. This finding is consistent with previous studies showing a negative correlation between soil bulk density and SOM (Agbede et al., 2019), as SOM enhances soil aggregation and increases macroporosity. These improvements can occur both directly, due to the lower density of organic matter itself, and indirectly, through the stimulation of soil biological activity (Franzluebbers, 2002). Although CM and GM exhibited statistically similar effects, the slightly higher organic carbon content of GM (Table 2) may explain its marginally better performance in enhancing soil physical properties. A significant correlation was observed between soil bulk density, porosity, moisture content, soil temperature, and SOM. The correlation coefficients (R-values) were -0.90*, 0.89*, 0.97**, and -0.97** for soil bulk density, porosity, moisture content, and soil temperature, respectively, indicating strong relationships between these soil properties and SOM levels.

The application of PM, CM, SM, GM and Chromolaena biomass, either individually or in combination with NPK fertilizer, significantly increased soil pH, soil organic matter (SOM), and the concentrations of N, P, K, Ca, and Mg compared to the control. This indicates that nutrients stored in these organic materials were gradually released into the soil upon decomposition. Adekiya et al. (2019) also reported that poultry manure is an effective source of plant nutrients, as it increases soil organic matter, enhances soil fertility, and improves the yield of green amaranth (Amaranthus hybridus). Similarly, Yakub and Alex (2024) found that cow dung manure improved soil organic matter, total nitrogen, effective cation exchange capacity (ECEC), and available phosphorus in a study conducted in Wukari, Taraba State, Nigeria. Abdulkareem et al. (2024) reported that the application of goat dung increased soil pH, organic matter, total nitrogen, available phosphorus, cation exchange capacity (CEC), and exchangeable K, Na, Ca, and Mg. In another study, Okon et al. (2016) found that applying goat manure alone increased soil total nitrogen by 0.24%, representing a 166.67% increase over the control. Soil organic carbon, available P and K, and total P were found to increase in the soil with the application of pig manure (Shakoor et al., 2022). Additionally, Chromolaena biomass manure has been reported to enhance soil nutrient levels (Aboyeji, 2019).

The observed increase in soil pH with organic manure application can be attributed to the presence of base cations such as Ca²⁺, Mg^2⁺, K⁺, and Na⁺, which displace hydrogen (H⁺) and aluminum (Al³⁺) ions from soil exchange sites, thereby reducing soil acidity and raising pH. This finding aligns with Okon et al. (2016), who reported that poultry and goat manure, as well as their combination (GM + PM), increased soil pH from an initial value of 4.24 in the control plot to a range of 5.48-5.72. In contrast, soils treated with NPK fertilizer exhibited the lowest pH, likely due to the leaching of base cations from the soil surface.

The increase in SOM following organic manure application is due to the direct contribution of organic carbon from these amendments (Table 2). Okon et al. (2016) also reported that poultry manure increased soil organic matter content by 126.53% over the control. Among the organic manures, PM had the highest impact on improving soil chemical properties, likely due to its lower carbon-to-nitrogen (C:N) ratio (Table 2). The C:N ratio plays a crucial role in nutrient release, as it influences the rate of decomposition and nutrient availability. Manures with a low C:N ratio, such as poultry manure, decompose rapidly, leading to faster mineralization and nutrient release for plant uptake. In contrast, GM had the highest C:N ratio among the organic manures, which explains its greater contribution to SOM accumulation. Organic materials with high C:N ratios decompose more slowly, allowing for long-term soil organic matter buildup (Adekiya, 2018). Since high C:N materials contain more carbon relative to nitrogen, they resist microbial breakdown, resulting in the retention of stable organic residues in the soil for extended periods. While low C:N materials provide immediate nutrient availability, but leaving less long-term organic matter behind, high C:N materials are more effective in maintaining and improving soil organic matter over time.

The significant increase in soil OM, and other nutrients, particularly N, P, and K in plots treated with organic manures compared to NPK-treated plots was attributed to reduced nutrient losses through leaching. NPK-treated plots exhibited lower levels of Ca and Mg due to the absence of these nutrients in the NPK fertilizer formulation. This highlights the advantage of organic amendments in providing a more balanced and sustained nutrient supply while improving soil structure and fertility.

The application of manure significantly enhances the growth and yield of carrots compared to the control. Organic manures such PM, CM, SM, Chromolaena and GM are rich in essential nutrients, including nitrogen, phosphorus, and potassium. These nutrients are crucial for plant growth and development. Manure application improves soil fertility by increasing the levels of both macro and micronutrients available in the soil to plants (Wang et al., 2023). This enhancement leads to better soil structure, which facilitates root expansion and nutrient uptake (Verma et al., 2024).  Improved soil conditions due manures also promote microbial activity that further aids in nutrient cycling, making more nutrients accessible to the carrot plant (Chen et al., 2024). The availability of these nutrients promotes increased root length, diameter, and overall plant vigour.

The addition of manure enhances the physical properties of the soil, such as lower bulk density and soil temperature and increased porosity and moisture content of the soil. This improved soil physical properties allows carrot roots to penetrate deeper into the soil profile without obstruction, resulting in longer and thicker roots compared to control plots. Research indicates that carrots grown with manure exhibit superior growth parameters, including greater plant height and leaf number. For instance, carrots treated with 20 tons per hectare of chicken manure showed significant increases in root length (7.3%), root diameter (22.7%) and root weight (57%), compared to control treatments (Dawuda et al., 2011). In a separate experiment involving a control and four different soil amendments—10 t ha^-1 Mucuna pruriens, Chromolaena odorata, Gliricidia sepium, and 300 kg ha^-1 NPK (15-15-15) (Appiah et al., 2017)—the results showed that these amendments enhanced the vegetative growth of carrots and facilitated the translocation of assimilates, leading to higher gross and marketable root yields compared to the control (Akter et al., 2024).

Table 6. Correlation coefficient between growth and yield parameters of carrot and soil properties.

Note: **. Correlation is significant at the 0.01 level; *. Correlation is significant at the 0.05 level

Poultry manure (PM) significantly increased plant height and the number of leaves compared to CM, SM, GM, Chromolaena, and NPK fertilizer. This effect is attributed to PM's higher soil nutrient concentrations (N, P, K, Ca, and Mg) and lower C:N ratio (Adekiya et al., 2022). However, while PM enhanced vegetative growth, it did not translate into improved yield parameters. In contrast, GM, despite having lower nutrient content, contributed more to yield performance due to its superior impact on soil physical properties. This suggests that carrot yield in this experiment was more sensitive to soil physical characteristics than to chemical properties. As shown in Table 6, plant growth parameters (plant height and number of leaves) were primarily influenced by soil chemical properties (OM, N, Mg, and pH), as well as soil moisture and temperature. On the other hand, yield parameters (root length, root diameter, and root weight) were predominantly affected by soil physical properties, including bulk density, porosity, moisture content, and temperature, with only a few chemical properties (OM and N) playing a role. Carrot root growth was more sensitive to soil physical properties than chemical ones due to the following reasons: Carrots, being root crops, require loose, well-aerated soil for optimal elongation (Johansen et al., 2014). High bulk density or compacted soil restricts root penetration, leading to shorter, misshapen, or forked roots (Johansen et al., 2014). The addition of GM, which is rich in organic carbon (OC), significantly improves soil physical properties by reducing bulk density, enhancing porosity, and maintaining moisture balance. This creates an optimal environment for carrot root elongation, ensuring proper drainage and promoting long, straight, and marketable roots. Even moisture distribution is critical, as inadequate moisture slows down root extension (Bengough et al., 2011). Therefore, for optimal carrot root yield, soil physical properties—such as bulk density, porosity, and moisture retention—are more crucial than chemical properties. This underscores the importance of deep tillage, organic matter addition (to improve soil structure), and proper irrigation management in producing high-quality carrot roots.

The improved growth and yield of carrots observed when organic manures were combined with NPK fertilizer, compared to their individual applications, can be attributed to the synergistic effects of these nutrient sources. Organic manure not only enhances soil physical properties but also provides a slow-release source of N, P, and K (Ullah et al., 2023). In contrast, NPK fertilizer supplies nutrients in readily available forms. The combination of both ensures that carrot plants have immediate access to nutrients from NPK while benefiting from the continuous nutrient supply and soil structure improvement provided by organic manure. This enhances nutrient retention and reduces the leaching often associated with NPK fertilizers (Adekiya, 2019). Additionally, organic manure helps buffer soil pH (Assefa and Tadesse, 2019), mitigating the acidifying effects of NPK fertilizer. As a result, soil pH in combined treatments tends to be closer to neutral compared to the use of NPK alone. The GM + NPK treatment resulted in the highest carrot root weight, which can be attributed to improved soil physical conditions and optimal nutrient availability. Beyond improving soil structure, the humus formed from the slow decomposition of goat manure (GM), which contains high organic carbon, may also aid in retaining nutrients released from NPK within the root zone (Adekiya, 2019). This retention enhances nutrient uptake efficiency, ultimately leading to higher carrot yields compared to other treatments. The application of organic manures enhances the vitamin A (β-carotene) and vitamin C (ascorbic acid) content in carrot roots by enriching the soil with essential nutrients, including N, P, and K, as well as micronutrients such as Zn and Fe (Adekiya et al., 2022). These nutrients play a crucial role in the biosynthesis of secondary metabolites, including carotenoids (precursors of vitamin A) and ascorbic acid (vitamin C) (Adekiya et al., 2019). NPK fertilizer also increased vitamin A and C levels relative to the control, primarily due to its role in improving leaf chlorophyll content and enhancing photosynthesis, which leads to increased sugar production (Shah et al., 2024). Since vitamin C (ascorbic acid) is synthesized through glucose metabolism (Valdés, 2006), higher carbohydrate production directly contributes to greater vitamin C accumulation. Similarly, β-carotene (a precursor of vitamin A) is produced through photosynthetic pathways (Grune et al., 2010), meaning that increased photosynthesis results in higher carotenoid levels. Among all treatments, the GM + NPK combination resulted in the highest vitamin A and C content in carrot roots. This was attributed to improved soil physical conditions and optimal nutrient availability under this treatment. The beneficial effects of manure and NPK fertilizer in this study on carrot growth, yield, and vitamin content were largely associated with improvements in soil structure rather than solely chemical properties. This study highlights that the combined use of organic amendments and NPK fertilizer can effectively enhance soil quality and boost carrot productivity. By adopting this integrated management approach, farmers in tropical regions can reduce their reliance on expensive chemical fertilizers, making carrot cultivation more sustainable and cost-effective.

CONCLUSION

This study demonstrated that the application of organic manures and NPK fertilizer, whether individually or in combination, significantly improved soil properties, carrot growth, yield, as well as the vitamin A (β-carotene) and vitamin C (ascorbic acid) content of carrot roots compared to the control. Organic manures, particularly goat manure (GM) and cattle manure (CM), contributed to lower soil bulk density, increased porosity, and improved soil moisture retention, creating favorable conditions for root elongation and expansion. Among these, GM, due to its higher organic carbon content, was the most effective in enhancing soil physical properties. The findings suggest that carrot yield was more influenced by soil physical attributes, such as bulk density, porosity, and moisture content, than by soil chemical properties. Among the individual treatments, poultry manure (PM) promoted the tallest plants and the highest number of leaves, likely due to its high nutrient content and rapid mineralization. However, GM resulted in the largest root diameters, longest roots, and heaviest root weights, underscoring the importance of soil structure in carrot productivity. The combined application of organic manures with NPK fertilizer produced synergistic effects, improving both soil structure and nutrient availability. The GM + NPK treatment yielded the highest carrot root production (45.6 t ha^-1 in 2021 and 45.9 t ha^-1 in 2022) and the highest vitamin A (16.55 mg 100 g^-1 and 16.88 mg 100 g^-1) and vitamin C (41.66 mg 100 g^-1 and 42.07 mg 100 g^-1) content. This integration enhanced soil porosity and ensured a sustained nutrient supply, optimizing carrot growth and nutritional quality. The co-application of organic manures and NPK fertilizer was consistently more effective and superior to individual applications, providing a sustainable strategy for improving soil quality, increasing carrot productivity, and enhancing nutrient content. This approach reduces dependence on expensive chemical fertilizers while maintaining soil fertility. For sustainable carrot production in tropical regions, farmers should adopt integrated soil fertility management practices. Future research should focus on the long-term effects of these nutrient combinations on soil health and overall crop performance.

AUTHOR CONTRIBUTIONS

Aruna Olasekan Adekiya and Taiwo Michael Agbede designed the experiments, and conducted the experiments, performed the statistical analysis and data visualization, and wrote the manuscript. Tunde Ezekiel Lawal conducted the experiments, and wrote the manuscript. All authors have read and approved the final manuscript.

CONFLICT OF INTEREST

The authors declare that they hold no competing interests.

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