Improving the Quality of Rubber Plants in Bagged Nurseries by the Use of Compost-Based Culture Substrates

The decrease of fertility soils, the rarity of quality potting soil used to fill nursery bags, the high cost of chemical fertilisers and the problems associated with their use are leading planters and rubber nurserymen in developing and/or ex-panding areas to look for alternative and sustainable fertilization. In this per-spective, a trial was carried out at Research Station of CNRA-Bimbresso and in a farmer’s environment in order to evaluate the agronomic quality of compost made from chicken droppings and dry Panicum maximum straw to improve the growth of rubber plants in bagged nurseries. Mixtures based on potting soil and/or compost in different proportions were prepared. The fol-low-up of the trial focused on determination of the physico-chemical characteristics of the soils, measurement of the parameters of vegetative growth and the grafting success rate evaluation of rubber plants in nursery. The results obtained show that compost-based crop substrates plication of urea (at Kimoukro, in the centre), which under the conditions of the present study may be the recommended doses on rubber plants in bagged nurseries at Côte d’Ivoire.


Introduction
In the context of crop diversification, rubber cultivation in Côte d'Ivoire is of considerable economic interest and has become increasingly popular among many populations in recent years. This plant has been introduced by Agro-industrial Rubber Companies in the years fifty [1]. Côte d'Ivoire, formerly 6 th in the world, has jumped 3 points. It thus becomes the only non-Asian country in the international rubber top 5 behind Thailand (1 st ), Indonesia (2 nd ) and Vietnam (3 rd ). This position was acquired thanks to its 850,000 tonnes of rubber produced in 2019, an increase of 13% compared to the previous year [2].
The prodigious development was boosted, on the one hand, by the improvement in the purchase price of rubber at the edge of the field, and on the other hand, by the subsidy for the production of rubber plants by Fonds de Développement pour l'Hévéa (FDH), which is Hevea Development funds. Also, many studies carried out in the field of genetic improvement and latex harvesting technologies [3] [4] [5] [6] have contributed to this. As a result, Côte d'Ivoire has one of the highest yields in the world, at 1650 kg•ha −1 •year −1 [7]. It is such a performance that the country intends to consolidate in a context of strong demand for natural rubber.
This performance could however, be hampered by a number of concerns, in particular land and parasite pressure in traditional rubber tree areas, the aging of the verger and the low level of adoption of the technical itineraries. Also, note that the development plan for the rubber industries envisages production of about 2 million tonnes of natural rubber over the next five years, to consolidate its leadership in Africa.
In order to achieve this objective and to meet the ever-increasing demand for natural rubber, two strategies can be adopted. The first is to improve the productivity of rubber trees in traditional growing areas by optimising latex harvesting system of rubber trees, appropriate replanting techniques, selecting clones that are more efficient and better adapted to local environmental conditions and by good plantation management, etc. The second strategy consists of extending the rubber plantations areas to non-traditional areas, described as marginal. Moreover, any replanting or extension of rubber tree plantation areas necessarily starts from nurseries, which occupying a pivotal position in the production cycle of vegetal material, is at origin of any plantation creation project.
The rubber plants in bagged nurseries remain the only effective means of ob-taining quality plants material that guarantees the profitable and sustainable exploitation of rubber tree plantations. The high demand for planting material, linked to interest in rubber tree cultivation over the last decade, has shown the need for rubber plants to shorten the production time of planting material in the nursery.
As a result, the lack of vigour of the plant material produced is one of the causes of high mortality after transplanting. The considerable loss of planting material in the nursery results in heterogeneity in tree growth, induced by rubber trees replacements and consequently a significant reduction in the number of exploitable trees. Also, Mougo [8] has revealed that losses of rubber plants in nurseries could reach more than 75% before grafting, 53% of which is due to the type of vegetal material and crop substrates.
For materials that are accessible and meet the requirements of plant growth, several studies have looked into the recycling of organic waste [9]- [15]. In fact, the use of composting organic waste is a potential choice to be developed for interesting agronomic and environmental reasons. In the context of organic waste recovery using the pit composting technique, the aim of this study is to evaluate on the one hand the effect of composts on plant growth and on the other hand its effect on the soil quality improvement.

Study Environment
The study was carried out at the experimental site of the CNRA-Bimbresso Research Station, in the southeast (N05˚18'45.2" and W004˚9'18.9") and the Kimoukro site, in a farming area (N06˚33'0" and W005˚15'0"), in the centre of Côte d'Ivoire. Climates, in the southeast of humid subtropical type and in the centre of transitional equatorial type, are characterized by four seasons clearly differentiated by their bimodal rainfall regime [16] [17]. Average annual rainfall is estimated at 1.800 mm in the southeast and 1.200 mm in centre of Côte d'Ivoire [17] [18].
The soil at Bimbresso site, belonging to the Ferralsols class [19] is deep and tertiary sandy. It characterized by a clay-sand texture in the surface horizons.
The pedological study carried out at Kimoukro site showed that the profile soil belongs to Cambisol class [19]. Table 1 shows the physico-chemical properties of these soils before the trial was implemented.

Plant Material
As the choice of rootstock is an important element in the success of grafting [20], the plant material used consists of about one month old GT1 clone rubber tree seedling. The clone GT1 from Indonesia is considered the best rootstock and it is the reference one in Côte d'Ivoire.

Fertilizer Material (Compost and Mineral Fertilizer)
The compost used was obtained by recycling chicken droppings and dry straw Open Journal of Soil Science from Panicum maximum composted in a pit for 18 weeks. The contents of the pit were turned over once every 4 weeks to ensure good aeration and to reduce anaerobic fermentation. The physico-chemical characteristics of this compost are mentioned in Table 2. Urea [CO(NH 2 ) 2 ] at 46% N was used for fertilization of rubber plants in bagged nurseries.

Soils, Gauges and Crop Substrates Preparation
The study plots, with an area of 300 m 2 (20 m × 15 m), were manually cleared. Then, they were materialised by placing gauges on the ground with the help of stakes.
Eight (8) gauges were opened to obtain 0.2 m width; 4 m length and 0.2 m depth. The distance between two gauges was 1 m. The soil, placed on the same side when the gauges were dug, was used as a growing medium, mixed or not with compost used, to fill the bags. These bags, measuring 40 cm long and 10 cm wide, were placed 2/3 of the height and laid out in the gauges in discontinuous tetrads, with one gauge forming a micro-plot.

Establishment of Germoir and Planting of Rubber Tree Seedlings
The establishment of rubber tree nursery necessarily requires a germoir. It is made up of beds of one (1) square metre that can contain up to 1000 seeds. This strip consisted of a light medium about five to 10 cm thick of sand and was covered by a shady area made up of oil palm stalks. The seeds used were those of the GT1 clone. 30 days after sowing, the seedlings obtained were transplanted in the bags.

Experimental Design and Trial Management
The trial was conducted with a randomised complete block design. Fertilization was main factor and the dose of the compost, the second one with 6 modalities (Table 3). Each treatment, repeated 3 times, consists of 40 plants. The number of effective plants was 720 out of 1152 plants. Urea (46% of nitrogen) was applied in three applications. The first treatment was applied as a background fertilizer. The second one occured at the stage of the second mature leaf and the last one was performed one month after the second one.
The urea was dissolved in water (50 mL of water per plant) and brought to plants. However, the compost applications were done in one go (when filling the bags). The estimated water requirements of rubber plants in the nursery are 120

Measurement of Vegetative Growth Parameters
In order to evaluate the performance of crop substrates elaborated, measurements of aerial vegetative growth parameters of the rubber plants in bagged nurseries were collected monthly during the nursery cycle. The total height H (mm) of the plant was measured from collar (at ground level) to apical end of the plant using a graduated ruler. The collar diameter (mm) was determined at 5 cm from the ground using a Stainless Hardened calliper. Average monthly increases in height (ΔH) and collar diameter (ΔC) were obtained according to following formula: where Mα and Mτ, expressed in mm•month −1 , are two measurements carried out at successive observations stages.
The average monthly growth was used to determine the growth gain expressed as a percentage of reference or control substrate (S1).

Production of Rubber Plants in Bagged Nurseries
The most common method of reproduction for the establishment of plantations is vegetative propagation. This multiplication is performed using several techniques, but it is the dormant eye patch grafting which is the most common in modern rubber plantations, as it allows much more homogeneous planted areas to be obtained, from which future rubber production can be accurately estimated [21]. Open Journal of Soil Science After the grafts are taken from the woodlot, the best graft protectors are also selected. These are healthy plants with a diameter of more than 10 mm at 6

Effects of Growing Substrates on the Physico-Chemical Properties of the Soil
The mean values of pH, organic carbon and total nitrogen and C/N in soil, at the end of experiment for different crop substrates, are presented in Table 4. With the exception of N content and C/N ratio, the ANOVA of these parameters showed significant effects (p < 0.05) between crop substrates, regardless of the study site. These parameters (pH and organic matter) are significantly improved by the compost-based crop substrates compared to initial values at the time of trial and substrates S1 (no fertilizer or control) and S2 (fractional application of urea).
The improvement in chemical parameter values was dependent on the compost dose applied to production of crop substrates. This improved effect was Open Journal of Soil Science

Effect of Substrates on Vegetative Growth Parameters
In response to the different crop substrates used, the mean monthly increment   On the site of Bimbresso (Table 5 and Table 6 (Table 7 and Table 8), the average monthly increases ranged Table 6. Average monthly increase in collar diameter (C) and growth gain compared to the control substrate (S1) at Bimbresso site.

Grafting Success Rate
Forty-two (42) days after grafting, the grafting success rates ranged from 92.61% to 100% respectively for substrates S2 and S4 at Bimbresso and from 81.86% (substrate S4) to 96.59% (substrate S5) at the site of Kimoukro. The lowest grafting success rates were observed with crop substrates S2 (92.61%) and S4 (81.86%), respectively at the site of Bimbresso and Kimoukro (Figure 2). The grafting success rate of the crop substrates S1, S3, S4, S5 and S6 at the site of Bimbresso, and S1, S2, S3 and S5 at Kimoukro site were statistically equivalent (p ≥ 0.05).

Effects of Crop Substrates on the Chemical and Physico-Chemical Properties of Soils
The significant effects of compost-based substrates on soil chemical parameters Open Journal of Soil Science The compost used, through its good physical quality (more than 90% fine elements, Ø ≤ 10 mm) and its fertilizing value of an acceptable level, ensures a high availability of nutrients, as well as after cultivation. In fact, organic matter, which is essential for soil fertility, produces humus. When it decomposes, it not only releases the nitrogenous matter essential for plant development, but also plays a major role in maintaining a favourable soil structure. Therefore, its use in the manufacture of crop substrates has created better growing and nutrition conditions for rubber plants in bagged nurseries. and S6 to neutralise the H + and Al 3+ ions responsible for acidity [11] [27]. According to Boyer [28], the application of organic matter to the soil can complex the Al 3+ ions responsible for soil acidity and making them inactive. These ions, which are strongly retained, pass with some difficulty into the soil solution. This also indicates the decrease in soil acidity observed in the compost-based sub-Open Journal of Soil Science strates, particularly the high-dose compost substrates (S4 and S6).
An increase in soil acidity was observed in the substrate S2 as a result of fractional application of urea (46% N). This increase in soil acidity of this substrate corroborates the results of earlier work by Kaho [29], which revealed the acidifying effect of chemical fertilizers on Altisols and Ultisols in Ghana and Nigeria.
Brady [30] reported that most complete fertilizers unless specially treated tend to form an acid residue in soils. This is mainly due to the influence of some of nitrogen carriers, especially those that supply ammonia and produce ammonium ions by hydrolysis.
However, ammonium ions, presented as potential sources of soil acidity, also behave as an antagonistic cation of K + , Ca 2+ and Mg 2+ ; excesses of NH 4+ can cause a deficiency of these elements (K, Ca and Mg) and conversely, an addition of Ca reduces the toxicity of NH 4+ [31]. Chemical amendments also gradually increase soil acidity, which consequently has a negative impact on the development of soil microorganisms [32].
Results also showed a significant improvement in soil organic matter content and cation exchange capacity (CEC) with the use of compost. According to Abobi [11], the increase in CEC is due to organic amendments, which would have affected the phenomena of fixation of exchangeable cations on the adsorbent complex of soils. The increase in CEC is more dependent on the increase in organic matter than on the low clay fraction found in the soils, and indirectly on the pH through its reaction behaviour with these two colloids (clays and organic matter).
Cation exchange capacity is related to the clay-humus complexes formed by humus and mineral particles, via the cations Ca 2+ , Fe 2+ and Al 3+ , which are responsible for improving the soil properties that the compost helps to form [33].
In addition, Huber and Schaub [34] report that soil organic matter content would be a determining factor in the CEC value. Other studies have also shown strong correlations between CEC, organic carbon content and pH (H 2 O) of the soil [35] [36]. Several authors have also demonstrated the increase in CEC following the application of composts [36] [37] [38]. High levels of organic matter result in higher CEC and the soil retains nutrients to make them available to plants for growth [39].
Regarding soil nitrogen content, no substrate effects have been observed at any site. However, compared to initial soil content, the N values recorded at the end of trial show a slight increase in compost-based substrates, especially substrates with a high dose of compost (S4 and S6). N'Dienor [40] indicated that composts contain relatively little nitrogen (0.5% to 0.6%) according to the basic components), their quantitative application can provide the necessary additional nitrogen and start its dynamics in the soil. According to Nyembo [41], organic manures release nitrogen in a form that is assimilable by the plant and their actions are slow and progressive. The fertilizing value of the nitrogen contained in the compost is therefore generally low, but should not be neglected in the case of Open Journal of Soil Science high inputs.

Effects of Crop Substrates on Vegetative Growth Parameters
Regarding the growth of rubber plants in bagged nursery, the compost-based substrates performed better than the control substrate (S1, no fertilizer), with statistically significant differences for vegetative growth parameters and grafting success rate.
At Bimbresso, the average values of the monthly increase in collar diameter (ΔC), most important parameter in the management of rubber plants in nursery, were significantly higher for plants of compost-based substrates compared with plants of control substrate (S1). Moreover, these values are even higher than that observed by Compagnon [21], which are of order of 1.5 mm•month −1 on average. These results would indicate that the nutrients, specifically nitrogen, phosphorus and potassium, were more available in the compost-based crop substrates than other substrates (S1 and S2). It was noted that independently of the site, a delay in plant growth was observed in substrates containing high doses of compost.
Compared to the control substrate S1, these growth retardation rates were as much as −9.77% in height and −6.48% in collar diameter of plants at Kimoukro site. This observed retardation could be the consequence of the alkalinizing action of compost on the soil, which considerably reduces soil acidity, as mentioned above. Compared to initial values, soil pH increased from 1.4 to 1.8 unit at Bimbresso and from 0.4 to 0.6 unit at Kimoukro, respectively, for S4 and S6 substrates.
This increase in pH to neutrality level was to a certain extent prejudicial to the growth of rubber plants in nursery. In fact, the results of pot cultivation, Compagnon [21], indicate that rubber tree is a very hardy acidophilic plant, capable of adapting to variations in soil pH (H 2 O). However, rubber tree growth is inhibited on soils with a pH above 6.0. This phenomenon has been well demonstrated in experiments with potted crops where chlorosis and growth failure were already evident at pH 6.5 [43]. Open Journal of Soil Science Furthermore, Ferrand [44], made the same observation using the practice of burning in the valorisation of forest soils intended for the rubber trees in nursery. According to this author, the pile of ash left after burning the wood stumps locally alkalinises the soil and makes stains on which the young rubber trees grow with difficulty. On the same topic, Compagnon [45] mentioned that pH values most frequently recorded for satisfactory rubber tree growth are between 4 and 6.5. He stated that young rubber trees are very sensitive to excess alkalinity. The response curves of height growth as a function of soil pH obtained in this experiment seem to explain this phenomenon. Indeed, they show good vegetative growth of rubber plants in nursery at acidic pH levels (between 4.5 and 6.0).
The grafting success rate does not differ significantly according to treatment and study site. These results suggest that parameter graft success in Hevea brasiliensis is not influenced by application of fertilizers (compost or fractional application of urea). However, Ondo [46] concluded that in tropical climates, the number of successful grafts and the grafting success rate are influenced differently, depending on the type of rubber tree clones, method of rootstock transplanting (in bags or in ground), month of grafting and origin of the graft wood.

Conclusions
At the end of this study, we can retain that compost increases, in proportion to doses applied, the soil's levels of soil organic matter, nitrogen, the sum of exchangeable bases (SBE) and cation exchange capacity (CEC).
For soil pH, compost input resulted in a decrease in soil acidity. Compost-based substrates performed better than S1 (no fertiliser) and S2 (fractional application of urea) substrates with statistically significant differences for vegetative growth parameters. However, high doses of compost (54 t•ha −1 ) tend to raise the soil pH However, our results also showed that water availability for rubber plants in bagged nurseries was a limiting factor, especially in Kimoukro, an area described as marginal for rubber cultivation. Therefore, a factorial experimentation that would make it possible to define the best combination of fertilization and irrigation would be conceivable. In addition, the economic profitability of this organic fertilizer linked to the costs of compost production (purchase of inputs, cost of technical equipment and labour) remains to be evaluated.