INTRODUCTION

Cacao is cultivated in 70 nations mostly situated in tropical regions and sustains the livelihoods of about 5 million small-scale farmers (^{Rafflegeau et al. 2015}). Colombian cacao has been qualified as fine and aroma, which is very important, given that only 5% of the total cocoa produced and marketed on the planet is classified in this ranking. There are around 65,000 cocoa families supported by this crop (^{Fedecacao, 2020}).

In Nariño departamento, Tumaco has 93% of the cacao crops in approximately 17,809 hectares (^{Agronet, 2024}). In Tumaco cacao beans are recognized specially by its aroma with fruity and floral tones and chocolate flavor (^{Casa Luker, 2011}). Unfortunately, most of the cocoa crops is cultivated with very low technology. For instance, seedlings come from empirical selection without any care in their initial stages.

The average yearly output in Tumaco has been almost constant, ranging from 210 to 500 kg ha⁻¹ (^{Agronet 2024}). Potential yield estimations exceed 2000 kg ha⁻¹ according to ^{Fedecacaco (2020)}. Considerable yield gaps are believed to arise from cultivation in climatically unfavorable regions (^{Asante et al., 2021}), old plantations (^{Wessel & Quist-Wessel, 2015}), inadequate farm management techniques (^{Aneani & Ofori-Frimpong, 2013}) coupled with high pests and disease pressure and insufficient fertilizers application (^{Abdulai et al., 2020}; ^{Wessel & Quist-Wessel, 2015}). Likewise, mechanisms underlying yield responses to increased nutrient availability are still largely unknown (^{Goudsmit et al. 2023}).

An essential determinant of plant quality is the early nutrition (^{Hartmann et al., 1997}). However, In Colombia scientific literature is scarce on cocoa nutrition during the nursery phase. As similar to other countries, the extension services continue to suggest exclusive formulas that fail to consider differences in soil composition, environmental conditions, safety factors, or the nutrient balance required according to the context (^{Snoeck et al., 2016}). For this reason, an adequate cocoa nutrition is crucial for crop growth to achieve the wanted yields.

The main disadvantage in cacao crop performance is sometimes the lack of understanding about edaphic and foliar fertilization of early plants (seedlings), which determines yield and profitability. To ensure sufficient and well-proportioned nutrient levels for support of cacao growth and production, researchers and cocoa growers must take note of the specific requirements of cacao, the existing soil composition, and the methods for making these soil nutrients accessible to the cacao trees (^{Snoeck et al. 2016}).

Every plant species has unique nutritional requirements and individual mechanisms for absorbing nutrients from the soil. The physiological development of plants relies on the presence of 16 key elements that constitute their structural composition (^{Snoeck et al. 2016}).

Furthermore, a significant number of small-scale farmers remain doubtful about the necessity and impact of fertilizer on the productivity of cocoa beans (^{Kenfack Essougong et al., 2020}). The observed outcome might be attributed to the considerable variability in the impact of fertilizer application, which can range from a twofold increase in crop production to no impact at all (^{Dossa et al., 2018}). This variability is further compounded by the absence of definitive fertilizer guidelines supported by reliable scientific evidence (^{Kenfack Essougong et al., 2020}).

The aim of this study was to assess the impact of various doses fertilizers on the growth and development of cocoa seedlings (FEAR-5) in a nursery in the municipality of Tumaco. The study also included a cost analysis of to enhance understanding of the needs of these dosage on seedlings from their early stages.

MATERIAL AND METHODS

Location

The study was carried out at the Asprocat center, in the municipality of Tumaco, Vereda Inguapi del Carmen Kilometer 22, via Tumaco-Pasto, Colombia, at 1°40'24” north latitude and 78°45'25” West longitude, at 18.82 m.a.s.l. There, the climate is as follows: precipitation 3,050 mm, temperature 25.5 °C, and relative humidity of 88% (^{IGAC, 2014}). The site study belongs to the life zone of humid tropical rain forest (^{Holdridge, 1982}).

Experimental material

Cocoa seedlings were propagated by seeds from a self-compatible clone FEAR-5. Mature pods were harvested, the central beans were taken, the mucilage was cleaned with wood sawdust and left to dry in the shade. Then they were sown in a germinator. After 10 days the better seedlings were planted in a special bag. Before placing the seeds in the germinator, we applied fungicide and insecticide to the soil to prevent de seedlings from pests and diseases.

Treatments

The treatments were defined according to ^{Crespo del Campo & Crespo (1997)} and the soil analysis.

T1= Application of N (2.4g), P (0.6g); K (2.4g), Ca (2.3g), Mg (1.1g), (^{Crespo del Campo & Crespo, 1997}), without discounting the minerals in the soil.

T2= This calculation was made based on the recommended dose and the soil analysis, for this treatment only 7 g de Ca and 3.2 g of Mg were applied.

T3= 50% of the recommended dose without discounting the content of nutrients in the soil. Dose: N (1.2g), P (0.3g), K (1.2g), Ca (1.15g), Mg (0.55g).

T4= Application of 150% of the recommended dose without discounting the content of nutrients in the soil. Dose: N (3.6g), P (0.9g); K (3.6g), Ca (3.4g), Mg (1.65g).

T5= Control.

Experimental design

The study was carried out using a completely random design (CRD), consisting of five treatments and three repetitions. The experimental area amounted to 24 square meters. The treatments were arranged at random.

Variables

Plant height (PH). With a graduated ruler, every 15 days, seedlings were measured from the base of the stem to the apex during three-months.

Total leaf area (TLA). The evaluation was carried out at 30, 60, and 90 days; three plants were selected, and the leaves were measured from the middle third using the graph paper method.

Individual leaf area (ILA). At the end of the investigation, one plant was taken at random and the area of each leaf was measured using the graph paper method.

Diameter at transition zone (DTZ). Using a caliper or Vernier, the diameter of the plants was measured at the base of the stem (Transition zone).

Number of leaves (NL). The leaves of each plant were counted every 15 days,

Total fresh weight (TFW). Seedling by each treatment were harvested after 90 days and weighed on an analytical balance.

Stem fresh weight (SFW). Seedlings were cut at the transition zone. Then seedling stems by each treatment were weighed on an analytical scale.

Root-fresh weight (RFW). The roots were carefully cleaned to remove all traces of soil and washed under running water, then dried at room temperature and weighed on an analytical scale.

Root length (RL). These roots then were measured.

Stem dry weight. Stems were dried at 70°C to a constant weight and weighed on an analytical scale.

Root dry weight (RDW). Seedling roots were dried in an oven at the same temperature as the stem and then weighed on an analytical scale.

Total dry weight (TDW). Dry stems and roots were weighed on an analytical scale.

Statistical analysis

An analysis of variance for a completely randomized design was performed to identify treatment differences. Tukey's HSD (honestly significant difference) test, was performed to compare treatment differences. Data processing and analysis was performed using the statistical software Infostat, V 9.0., with 95% of probability.

RESULTS AND DISCUSSION

In the analysis of variance, there were contrasting statistical differences among treatments. Treatment T2 showed statistical differences (P>0.05) in in plant height, total and individual leaf area, diameter at transition zone, total fresh and dry weight, root fresh and dry weight, and total dry weight. Variables plant height, amount of leaves, root length, and root dry had not statistical differences (Table 1).

Stem diameter (mm). In the analysis of variance, there were significant statistical differences (P<0.05) among treatments; the highest value was found in treatment 2 with 8.47 mm, the lowest values were recorded in treatments 1, 5 and 3. The T1 treatment, with the application of the recommended dose, presented the lowest average value with 6.57mm compared to T2 where the minerals in the soil was discounted, with an average of 8.47mm. According to ^{Martinez et al. (2015)}, it is important to highlight the diameter of the stem of cocoa plants because the greater the diameter, the better the performance of the seedlings on the field.

Leaf area (cm²). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 2 with 82.93 cm2, the lowest values were found in treatments 1, 3 and 4. Treatment T1 had the lowest mean with 39.03 cm2. Nutrients associated with metabolism are found in greatest concentration in the early stages of leaf or other organ formation. A decrease of concentration trigger low seedlings performance, due to concentration dilution effects from increases in cell wall material during leaf expansion and from nutrient reabsorption during senescence. (^{Epstein & Bloom, 2004}).

Leaf area (cm²). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 2 with 7.53 cm2, the lowest in treatments 1, 5 and 4. From these T1 presented the lowest value with 5.73 cm2. According to ^{Barceló et al. (2001)}, the greater growth of the leaf area of the plants is due to the higher content of nutrients, such as mineral ions; Seedling relative growth rate (RGR) obtained under optimal growing conditions might be considered a valuable bioassay of a species' potential capacity to exploit its advantageous development prospects, namely its growth strategy (^{Wright & Westoby, 2001}).

The leaf area of a tree defines its capacity to intercept incident solar radiation, a primary source of energy used by plants for the manufacture of tissues and food compounds (^{Almeida & Valle, 2007}, ^{Tang et al., 2014}). Leaf area is synonymous with photosynthetic potential and influences growth, development, biological and agronomic yield potential, solar radiation interception, efficient water use and mineral nutrition (^{Rodríguez et al., 2016}); in addition, it is related to agronomic, biological, environmental and physiological processes.

Stem fresh weight (g). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 2 with 14.13 g, the lowest in treatments 3, 5 and 1. Among these T3, 50% of the recommended dose, had the lowest with 8.63g, Plants absorb a number of nutrients from the soil in specific proportions, and it is important that they are kept in balance to facilitate their absorption. Furthermore, ^{Amores et al. (2014)} state that prior to the development of a fertilization program, it is necessary to have a diagnosis of the level of natural fertility of the soil and of the substrates, if it is at the nursery level, through soil and foliar analysis.

Root fresh weight (g). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 5 with 7.83 g, the lowest in treatments 4, 1 and 3. Treatment T4, 150% of the recommended dose, had the lowest with 1.63 g. This makes it possible to highlight the importance of adding organic matter as a conditioner of soil characteristics and a source of nutrients for plants, as well as an activating substrate for soil-dwelling microorganisms (^{Félix et al., 2010}; ^{García et al., 2011}). For this reason, treatment T5 registered higher values in the root fresh weight variable.

Total fresh weight (g). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 2 with 18.87 g, while the lowest values were found in treatments 3, 4 and 1. These treatment T3 had the lowest value with 9.43 g. This is possibly the nutrients applied to the plants of T2, thus favoring their strengthening, which therefore allowed them to have better growth of the seedlings.

Stem dry weight (g). The analysis of variance showed significant statistical differences (P<0.05) between treatments; the highest value was found in treatment 2 with 7.5 g and the lowest in treatments 1, 3 and 5. From these, T1 had the lowest mean with 2.13 g. Root and stem malformations originating from container production of Pinus nigra plants cause mechanical instability and plant mortality when seedlings are transplanted (^{Zahreddine et al., 2004}).

Variables with non-significant statistical differences

The statistical analysis of variance revealed no statistically significant differences (P>0.05) in the variables of PH, NL, RL, and RDW. Explanations of these findings are provided below.

Plant height, ^{Gómez (1999)} mentions that cocoa seedlings normally remain in the nursery for five to six months. In many cases, they do not reach the necessary vigor for planting (40 cm height), mainly due to inadequate management of irrigation, fertilization, and substrates, and even after six months in the nursery, they do not reach the ideal height for transplanting. In our case, in only four months of research, the seedlings showed a high vigor to be taken to sowing; the lowest height was 45.07 cm, and the highest was 61.53 cm, thanks to the fertilization.

Planting seedlings that are not vigorous results in slow growth in the field, long production time and susceptibility to disease attack. For these reasons, cocoa growers should choose seedlings with good physiological performance. (^{Reyes & González, 2003}).

Regarding the number of leaves per plant, (^{NeSmith & Duval, 1998}) state that, with the decrease in root volume, less leaf area per plant is yielded. The reduction in leaf area occurs due to the reduction in the size and number of leaves per plant.

Concerning root length and root dry weight, ^{Peterson et al. (1991)} assert that confining roots in a container that limits their growth intensifies competition for vital resources. As root biomass increases and rooting space decreases, there is a corresponding competition for oxygen and a decrease in accessible pore space. The total dry weight, reported earlier, may be attributed to the application of only the necessary fertilizers to the plants at T2, as per the soil requirements.

A significant challenge faced was the insufficient research on the application of chemical and organic fertilizers to cocoa during the nursery stage. Therefore, we suggest conducting research using various fertilization methods and different cocoa hybrids or varieties to determine the best dosage for the crop. Some of the findings obtained are already over a decade old, and we need current information to facilitate a more comprehensive comparison.

Financial analysis

In the financial analysis, the treatments showed differential performance. Treatment T4 was the most expensive, while treatment T5 had the lowest cost. In the profit analysis (Table 2), with a selling price of $3000 Colombian pesos per plant, it can be seen that each treatment had a profit of more than $1000 per plant, being the highest profit for treatment T5, which was the control, due to the fact that this treatment did not apply the doses of fertilizers, which resulted in low cost; of the fertilized treatments, the highest profit was presented by treatment T3, followed by treatment T2.

Treatment T2 had exceptional performance in almost all the assessed aspects. Although it placed third in terms of profitability among all the treatments, it achieved the second position among the fertilized therapies in our analysis. The results suggest that the application of fertilizers at the nursery stage of cocoa plantation is beneficial as it allows for the generation of seedlings with outstanding development at a low cost.

Treatment T1, the dosage advised by ^{Crespo & Crespo (1997)}, had the third highest value. Among the assessed variables, this treatment displayed the poorest performance, indicating that the soils in each region are diverse and inconsistent with universal formulas for cocoa fertilization.

In their study, ^{Quiroz & Amores (2000)} assert that the fertilization of cocoa does not yield any economic advantages. They highlight that the application of fertilizers to the crop in a profitable manner is more intricate compared to most tropical crops. They further comment that the challenges mostly stem from the genetic diversity of the plant material. The aforementioned comments align with the conclusions of this study, since the conclusive determination of the beneficial impact of fertilizer application remains uncertain.

Proper fertilization of cacao crops is crucial for the physiological growth and optimal development of the crop, ensuring the attainment of predicted production levels based on the planted age and variety. The composition of essential nutrients in the soil or topsoil for plants can vary because the soil is a dynamic organism influenced by changing climatic conditions, weathering of source materials, and the organisms that coexist with it (^{Compañía Nacional de Chocolates, 2021}).

It should be emphasized that the plants generated are rootstock, which will subsequently be grafted with scions from high-yielding cacao clones. It is at this crop phase that the impact of fertilization on plant vigor and responsiveness to grafting is properly determined.

CONCLUSIONS

The different doses of fertilizers applied to the soil presented significant statistical differences; Treatment T2 showed the best performance in the variables: stem diameter, leaf area, stem fresh and dry weigh, and total fresh weight, demonstrating to be the best treatment evaluated and determining an optimal growth and development for the cocoa seedlings. Treatment T5 (control) displayed the highest root fresh weight.

In the economic analysis, treatment T4 had the highest costs, while the lowest was treatment T5. The highest profitability was obtained with treatment T5, despite the low performance in its physiological variables.

The findings apply to places or soils that share similar characteristics, such as cocoa plantations located in comparable soil and climatic circumstances. Nevertheless, it is crucial to take into account the soil analysis to administer the appropriate amount of fertilizer without causing excess strain on the seedlings.