Introduction

Several crops, such as rice (Oriza sativa), maize (Zea mays), coffee (Coffea arabica), oil palm (Elaeis guineensis), and cacao (Theobroma cacao), characterize the Peruvian Amazon. The latter has become a crop of national interest due to its main product, the chocolate, and the high quality of Peruvian cacao about other competitors, such as African cacao (^{International Cocoa Organization, 2021}). In Peru, there are nearly 180,000 ha of cacao crops from which the San Martin and Loreto region represent 36 % of the nation’s production (^{Dirección de Estadística e Información Agraria (DEIA), 2021}). Cacao in Peru is mainly produced by small farmers (<5 ha), generally from former coca producers. Furthermore, this represents a challenge in terms of cost of production since the power of negotiation of these producers is low.

Cacao production directly interacts with natural factors and agronomic practices. One of the essential parts of the crop’s success is the seedlings’ quality (^{Sodré Andrade & Gomes Sena, 2019}). Therefore, adequate nutrition and substrate are needed to maintain adequate growth and tolerance for abiotic and biotic stress (^{Cruz Neto et al., 2015}; ^{Marrocos et al., 2020}). The main characteristics of an adequate substrate are good physical conditions and nutrients to sustain the plants during the nursery stage, being the first three months important for good root growth and nutrient acquisition (^{Reyes Moreno et al., 2021}).

Rivers dominate Loreto region, therefore, logistics costs are higher than in other regions of Peru, making commercial substrates inviable, for that reason, alternatives are always promoted. This region also has a prevalence of soils of the type fluvents characterized for good physical conditions and relatively high fertility due to the influence of the rivers. This type of material is generally used for the production of seedlings in many Latin-American countries since it can promote good physical conditions (^{Sodré Andrade & Gomes Sena, 2019}).

This region is also one of the country’s leading producers of oil palm (^{DEIA, 2021}). However, this crop has many residues, mainly the empty fruit bunch after the harvest. Nevertheless, many producers have decided to make compost out of this residue, a product nowadays of suitable chemical and physical characteristics at a low price. Consequently, its use as an amendment has increased in the last years in many crops (^{Supriatna et al., 2022}). The compost has many advantages since it can improve the physical and chemical properties of the soil, such as macroporosity, nitrogen and potassium nutrition (^{Da Costa Leite et al., 2023}), being the latter, also the most demanded nutrient in cacao (^{Marrocos et al., 2020}).

Taking into consideration the demands for alternative substrates in cacao due to higher logistics costs for traditional supplies in this crop, the objective of this work was to evaluate the effect of the combination of sandy soil and oil palm compost on the substrate, growth, and nutrition of cacao seedlings under nursery conditions.

Materials and methods

Plant analysis and nutrient uptake

Dried samples were milled and passed through 20 mesh sieves for plant analysis. Afterward, the procedures specified in ^{Arévalo-Hernández et al. (2022)} were used. For nitrogen, the Kjeldahl method was used. For the other elements (P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn), digestion with HNO3 65 % was performed in a hot block at 120 °C until the samples were fully digested. Phosphorus, sulfur and boron were determined with a UV-Vis spectrophotometer, and the cations were determined using an AAS.

For the determination of nutrient uptake (macro in grams per plant and micronutrients in milligrams per plant), the equation 1 was used (^{Arévalo-Gardini et al., 2021}; ^{Arévalo-Hernández et al., 2022}).

Element concentration= g kg^-1 for macronutrients and mg kg^-1 for micronutrients

Results

The results of means for biometric measurements are presented in Table 2. The analysis of variance showed that only the variables of root volume (cm²), shoot dry weight (SDw) (g), and root dry weight (RDw) (g) presented significant differences (p ≤ 0.05). In the case of the other variables studied (height, diameter, leaf area, and root length), not significant differences were observed (p > 0.05).

These results indicate that the mixture of compost with sandy soil promoted better growth in cacao seedlings, which is a positive outcome, as sandy soils are readily available for cacao producers in these regions; likewise, the presence of fruit bunch residues of oil palm for nursery in greenhouse conditions. Since similar results between independent variables were obtained, but only SDw indicated an interaction between the factors studied, this variable will be discussed in this research. For this purpose, a response surface was constructed and presented in Figure 1.

Figure 1 Effect of combinations of oil Palm Compost (Elaeis guineensis) and sandy Soil levels on the dry weight of cocoa seedlings (Theobroma cacao) under nursery conditions. Yurimaguas, Peru, 2022. 

In general, the higher accumulation of biomass was observed with the higher dose of oil palm compost (Figure 1), with no indication of toxicity even when the seedlings were directly grown in 100 % compost. However, the lowest growth was recorded at 60 % of sandy soil and 40 % of oil palm compost; this may be related to low input of nutrients. Also, higher macroporosity and high drainage in the soil may have restricted water nutrition in plants, especially in high temperature days.

In the case of soil chemical analysis, all the attributes studied presented significant differences (p ≤ 0.05) between treatments, both for the single factor and the interaction between the two variables. The results of the interactions of each soil factor are presented in Figure 2. In the case of pH, it is possible to observe a decrease as the dose of oil palm stalk increases, from the treatment without application to the treatment with 100 % of the compost, indicating that this substrate tends to decrease soil pH with higher doses. In the case of the EC, OM, Ca, Mg, K, and Al, the opposite happens; with increasing doses of compost, higher values of these variables are observed. In the case of phosphorus, it was the only element that presented a quadratic curve in the soil; however, the optimum value achieved for this element was with nearly 80 % of compost and 20 % of the sandy soil, a relation of 1:4.

Figure 2 Effect of the interaction between doses of oil Palm Compost (Elaeis guineensis) and sandy Soil as substrate on cocoa seedlings (Theobroma cacao) under nursery conditions, on the chemical characteristics of the substrate: pH (A), electrical conductivity - EC µs cm^-1 (B), organic matter - OM % (C), phosphorus - P mg kg^-1 (D), cation exchange capacity - CEC cmolc kg^-1 (E), and aluminum - Al cmolc kg^-1 (F). Yurimaguas, Peru, 2022. 

The results of chemical analyses of nutrient concentrations (N, P, K, S, Ca, Mg, B, Cu, Fe, Mn, and Zn) in cocoa leaves are presented in Table 3. All the nutrients had significant differences (p ≤ 0.05) between treatments, except sulfur and boron, which did not show significant differences (p > 0.05). The results of the effect of the doses for nitrogen, which presented significance in the main factors, need to be presented since it followed the same trend that the elements that presented interaction. In the case of the elements that showed interaction (K, Ca, Mg, B, Mn, Zn), the results are observed in Figure 3. All the elements followed the same trend, a direct positive correlation between the dose of applied oil palm compost and the nutrient amount. This is related to the highest nutrient composition of the compost (Table 1) in the sandy soil.

Figure 3 Effect of the interaction of doses of oil Palm Compost and sandy Soil on nutrient contents: potassium - K (A), calcium - Ca (B) and magnesium - Mg (C) in grams per plant, and boron - B (D), manganese - Mn (E) and zinc - Zn (F) in milligrams per plant of cacao seedlings in nursery conditions. Yurimaguas, Peru, 2022. 

Discussion

Oil pam compost increased growth in cacao seedlings, and the highest dose indicated that cacao seedlings could be grown in a pure oil palm compost substrate. In general, compost of any origin promotes growth by increasing plant nutrition, and reduces soil pathogens populations and bacteria (^{Milinković et al., 2019}). The use of compost of this residue represents an opportunity for large-scale waste in the oil palm industry, which can benefit small producers and nurseries dedicated to cacao crops. Also, compost can enhance biomass accumulation and reduce heavy metals such as cadmium in cacao (^{Argüello et al., 2023}; ^{Chavez et al., 2016}), which is essential regarding seedling quality. Additionally, higher doses of compost improve the carbon stocks in the soil and the plant, which is generally related to higher plant growth (^{Zhang et al., 2020}), as seen in this study.

The substrate chemical results indicate that the main differences in growth in plants in cacao seedlings, especially with higher oil palm compost rates, may be related to the improvement of the conditions of the substrate in these seedlings. This is related to the higher quantity of nutrients, cations, and organic matter of the compost. Also, the high aluminum concentration in excessive doses of compost may restrict plant growth, photosynthesis, and nutrition (^{Arévalo-Hernández et al., 2022}; ^{Siecińska & Nosalewicz, 2016}). The low effect on plant growth despite aluminum may be related to the higher concentration of calcium with increasing doses of compost; this element is known to reduce the stress generated by aluminum, reducing its activity in soils (^{Takala, 2019}). Additionally, the higher concentration of organic matter reduces the acidity’s activity by complexing aluminum (^{Lazicki et al., 2022}).

Also, the compost has increased nitrogen uptake in plants, which may be related to the organic forms of nitrogen that have a lower energetic cost in comparison to inorganic forms such as NO3- or NH4+, and enhance nutrient use efficiency (^{Franklin et al., 2017}), while improving overall soil conditions, since this type of amendment does not impact on soil pH as common nitrogen fertilizers such as urea or ammonium sulfate, which are commonly used in tropical soils.

Oil compost has also improved phosphorus availability in the soil. In general, organic forms tend to have a higher solubility in comparison to inorganic sources, which can explain the higher phosphorus uptake in the seedlings. Additionally, among the main nutrients in plant nutrition, phosphorus is significant for plant growth and root development in the early stages (^{Cellier et al., 2014}). Generally, tropical soils have low available phosphorus in the soil, and it may restrict plant growth in many cases (^{Viana Cunha et al., 2022}). The use of this product provides readily available phosphorus for plant growth, so it may constitute an effective amendment source to improve nutrition of this element in tropics (^{Smitha et al., 2016}).

These results indicate that the promotion of growth of cacao seedlings is not only explained by better substrate conditions but also by higher nutrition in the plants. Nitrogen and potassium are the main nutrients required by this crop (^{Carmona-Rojas et al., 2022}; ^{Marrocos et al., 2020}). However, even though phosphorus was affected by the different treatments at a soil level, in the plant, this was not observed. These contradictory results may be related to the phosphorus requirements at the seedling level that are significantly lower at the nursery stage (^{Marrocos et al., 2020}). Also, potassium is not only the element with the highest cycling ratio in comparison to other nutrients at the compost level (^{Cavalcante Santos et al., 2021}), but it’s also related to biotic and abiotic stress tolerance (^{Sardans & Peñuelas, 2021}).

In the case of micronutrients, both boron and zinc nutrition was improved by oil palm compost, even though the concentrations in the compost were low. This may be explained by the chelated forms of these elements present in the compost that have higher uptake in plants (^{Rosa et al., 2022}). Boron is very important in the early stages of growth of plants, since it promotes higher root development (^{Wang et al., 2015}). Additionally, these elements are scarce in tropical soils, so amendments that promote better nutrition should be used. Boron is essential for root development since many plants have specific boron transporters, which are crucial in plant metabolism (^{Pereira Leal et al., 2021}). At the same time, zinc acts mainly as a transporter and in hormonal control of plants affecting cacao seedlings’ overall growth and development (^{Cruz Neto et al., 2015}). However, is important to apply these elements with caution since research suggests that they have an antagonistic effect, but, under high application ratios, the use of zinc or boron may enhance balance of these nutrients (^{Long & Peng, 2023}).

Conclusions

This study demonstrated that using 100 % compost derived from oil palm empty fruit bunches significantly improved the overall growth of cacao seedlings. At this particular dosage, the compost created favorable conditions for the growth of these seedlings by enhancing substrate fertility and promoting better nutrition for the cacao plants. These findings set a precedent for utilizing oil palm bunches as compost, thereby reducing waste in this crop and improving the conditions for high-quality cacao seedling production in Peru.

Interests conflict

The authors declare that they have no conflicts of interest.