DiscussionTop

The RPs have different yield depending on the cultivation system, due to the different conditions encountered by the cacao tree. The yield of the shaded cacao tree was lower than in full sun, due to the smaller number of plants per area and the light restriction (Almeida & Gattward, 2018). In addition, the trees that make up the shading can compete for resources (Isaac et al., 2007), such as water and nutrients. Plants grown in full sun show greater gas exchange, transpiration, and photosynthesis, and consequently they produce more carbohydrates and need to extract more nutrients from the soil (Almeida & Gattward, 2018). In turn, plants grown in full sun and fertigated have the additional advantage of not only being limited in terms of light availability, but also having a better and better distributed supply of water and nutrients throughout the year. Shading affected the nutrition of plants on N, P, S, and K, which corroborates with what is described by Costa et al. (1998), Isaac et al. (2007), Bai et al. (2017), and Van Vliet & Giller (2017).

High yield cacao trees grown in full sun had a lower average leaf content (KW norm) than those under AGFs (Table 1), possibly because K is the most exported nutrient for the fruit (Souza Júnior et al., 2018), and since in the areas under full sun the yield was higher, this nutrient is more translocated from the leaf to the fruit. In turn, ca-cao trees grown in full sun had higher average levels of N, P, and S (Table 1), probably as a result of needing a greater protective apparatus against excess radiation and its consequences, such as photoinhibition, which reduces photosynthesis. The excess of radiation makes it neces-sary to synthesize regulatory, protective, and/or energy storage molecules such as proteins, alkaloids, carotenoids, glutathione, ATP, NADP etc., increasing the demand for structural function nutrients, such as N, P, and S (Taiz et al., 2017).

Despite the differences pointed out between cacao trees nutrition with and without shading, for K, N, P, and S, there was a high agreement of the KW norms (mean and variance) for most nutrients between the two cultiva-tion systems (Table 1) and of nutritional diagnoses (Table 2), when general norms are compared with specific ones according to the cultivation system. This corroborates with Dias et al. (2010), who recommend the use of gene-ral norms for cupuaçu. This is reinforced by the diagnosis not being attributed to chance, since according to Wadt et al. (1998), when the frequencies of nutrients diagnosed in a class are not attributed to chance, these norms can be used for nutritional diagnosis.

On the other hand, the feasibility of using general nor-ms is pointed out by Reis Júnior (2002) when reporting that the norms can be extrapolated when the contents of the high-yield populations in the locality to be extrapolated are similar to the contents of the norms. This can be verified with the fact that the average levels of nutrients in the RP (KW norms) are within the general sufficiency ranges proposed by Souza Júnior et al. (2018), with the exception of K, which was slightly below. In addition, general norms have better results when extrapolated (Wadt & Dias, 2012).

Regardless of the norm, general or by cultivation system, the lowest CVs were observed for N, P, K, Mg, and B (Table 1), indicating greater sensitivity in the diagnosis for these nutrients (Hermida et al., 2013). In turn, the highest CVs were observed for metallic micronutrients: Mn, Zn, Fe, and Cu (Table 1).

Rocha (2008) considers that variability is the main limitation to the use of the KW method, due to the sensitivity of the method to diagnose nutritional deficiencies or excesses decreasing with increasing variability. Kurihara (2004) suggests the CV limit of the KW norm of 40%, for a good sensitivity of the diagnosis. According to this limit, the norms for Zn and Mn should cause less precision in the diagnosis, except for the Zn norm in shaded cacao trees (Table 1).

The high variability of leaf nutrient content in cacao confirms that observed by Marrocos et al. (2012), which may be a consequence of the diversity of edaphoclimatic conditions of its cultivation in the south of Bahia, with soils with wide variability of mineralogical, chemical, and physical attributes (Santana et al., 2002; Araujo et al., 2018; Arévalo-Hernández et al., 2019).

Accuracy in diagnosing the nutritional status of plants is a key element for decision making in the competitive market (Rozane et al., 2016). There was an improvement in the efficiency of the nutritional diagnosis of cacao by the BIK interpretation ranges obtained by the upper boundary-line method (Table 4). They add biological significance and model the response in relative yield according to the BIK, being specific for each nutrient (Figs. 1 and 2), unlike the original range by Kenworthy (1961), which is based only on statistical criteria, considering a fixed CV of 20%, where high variability limits the use of this method (Mendonça, 2016). In addition, Kenworthy considered the existence of symmetry in the diagnostic ranges for the different nutrients, in addition to the similarity in the response of the different nutrients to the variation in the degree of nutritional balance and especially the deficiency and excess zone. Fact not observed for the curves (Figs. 1 and 2) and diagnostic ranges (Table 4) for cacao, which corroborates with Fernández et al. (2016), who observed different responses for the relationship between the biomass production of cacao seedlings and the leaf contents of N, P, and K.

In addition, the border population is made up of plots of higher yield within each cultivation system, but also plots of lower yield due to deficiencies or excess of nutrients; resulting in specific models of the relationship between RY as a function of the BIK for each nutrient (Figs. 1 and 2), which reinforces the improvement in the quality of the diagnosis, as it allows the establishment of specific BIK ranges for the cacao tree (Table 4).

For macronutrients, the best fit model was the quadratic model (Fig. 1), indicating symmetry between the regions of deficiency and toxicity. In turn, for micronutrients, the predominant models express an abrupt increase in eRY between the deficiency and sufficiency ranges and, after this, the decrease in eRY due to the increase in BIK is more gradual, with no symmetry, except for Mn which presented a behavior similar to that observed for macronutrients. The fact that Mn behaves differently from other micronutrients possibly occurs because the cacao tree tolerates high concentrations of this nutrient, including leaf contents in order of magnitude from those observed for some macronutrients (Souza Júnior et al., 2012).

In the productive phase of cacao, and independent of the cultivation system, it is also possible to establish diagnostic ranges for leaf nutrient levels based on two distinct criteria: original KW method (Table 3) and specific boundary-line method for the cacao tree (Table 6). It should be noted that for cacao there are few studies that establish suf f iciency ranges. Oliveira et al. (2019) obtained sufficiency ranges for the leaf contents of cacao clones CCN51 and PS1319, using 20 commercial plots; these authors indicate these ranges only for the south of Bahia and the north of Espirito Santo and for these clones. Souza Júnior et al. (2018) in a broad review of this theme in the international literature, identified 14 sufficiency ranges proposed by different authors, for cacao, in different countries. Souza Júnior et al. (2018) also point out that there is a great divergence between these ranges of sufficiency for various nutrients and that there is often a lack of information on the methodology used to obtain them, being commonly based on the authors' experience with the crop and / or a small number of crops; indicating that they need to be improved and redefined. In the present work, the ranges were obtained from a database with 309 plots and statistical methods used for other crops.

Analyzing the normal range, its lower limit, which generally equals the critical level (Souza Júnior et al., 2018), was higher by the original KW methodology (Table 3) than by the boundary-line methodology (Table 6), except for K and Mg, which had the opposite behavior, and for B, which presented similar values. The original KW method was more rigid in establishing the limits of the normal ranges (narrower ranges) than the frontier method, which was more flexible (wider range), except for N (Tables 3 and 6), a fact also reported for various nutrients for cof fee (Mendonça, 2016) and eucalyptus (Lima Neto et al., 2020). The absence of symmetry in the response between the deficiency and toxicity ranges for Cu, Fe, Zn, and B (Fig. 2) helps to understand the enlargement of the upper limit of the normal range by the boundary-line method (Table 6).

Most of the low yield plots (plots that did not make up the RP), regardless of the diagnostic ranges used for the interpretation of the KW method, had adequate nutrition (Table 5), indicating that other non-nutritional factors, together, influenced yield, among which: water relations (deficit or excess of water), excess of shading, low genetic potential and attack of diseases and pests.

From a nutritional point of view, the most limiting macronutrients were K, Mg, and Ca (Table 5), possibly because cacao has a high demand for these nutrients (Sou-za Júnior et al., 2018), in addition to the many plots in this study that are found in Oxisols with low availability of exchangeable cations. It is important to highlight that the K+, Ca2+, Mg2+ ions present competitive inhibition in the absorption process, that is, they compete for the same root absorption site, consequently the greater presence of one inhibits the absorption of the other (Malavolta, 2006), which justifies the fact that K, Mg, and Ca deficiencies occur in plots where it was diagnosed that one of these nutrients was in excess or luxury consumption. Focusing on Mg and Ca, in addition to this competitive inhibition, the fact that cacao requires a close Ca / Mg ratio (Souza Júnior et al., 2018). These interactions highlight the need to also study the balance of nutrients and be careful in fertilizing these interactions.

Deficiencies of all micronutrients were found, except for Mn (Table 5). B and Zn deficiencies in cocoa trees are the most reported in the literature (Souza Júnior et al., 2018). Zn deficiency occurred in plots with excess of K and or P, corroborating with Malavolta (2006) who reports a negative correlation between leaf K contents with B, Mn, and Zn, as well as that the high availability of P can induce deficiency of Cu, Fe, and Zn.

Considering both KW methods, the nutrients that had higher and lower frequencies due to excess were S and Mn, respectively (Table 6). This, in a way, corroborates the observations made by Souza Júnior et al. (2018), for the cultivation of cacao trees in the region covered by this study; these authors state that deficiencies by S have little occurrence and that very high Mn leaf levels are frequent-ly found, without these causing toxicity to the cacao tree. In addition, Mn is a micronutrient with high demand in areas of high yield, mainly in the productive phase (Ma-rrocos et al., 2020).

For making decisions about the nutritional manage-ment of crops, the use of tools that assess the nutritional balance is very important, especially for the diagnosis of deficiencies or excesses, which would indicate the need for adjustments in nutrient doses; this can be achieved using five (original Kenworthy tracks) or six ranges. However, the high range (luxury consumption) should deserve greater attention by technicians, aiming at the ra-tional use of nutrients and reduction of fertilization costs, since this can only be done with the subdivision of the original normal KW range. There was a high frequency of plots in “luxury consumption”, with only Cu and Fe the nutrients that had low LCI (Table 5). This demonstrates that there is a need to improve the process of fertilization and nutritional management of cacao, considering not only soil analysis, but also leaf analysis and diagnosis with appropriate tools.

In conclusion, full sun cacao plants have higher require-ments for N, P, and S compared to shaded plants. Norms of contents and sufficiency ranges were obtained for balanced indices of Kenworthy and for leaf nutrient contents for cacao, regardless of the cultivation system, which maximize the assessment of nutrition as a production factor. In general, the boundary-line method provides sufficiency ranges for broader leaf contents of nutrients with a lower limit than those of the original Kenworthy method.

Table 4. Specific ranges for interpreting balanced indices of Kenworthy (BIK, in %), as a function of estimated rela-tive productivity (eRY) for cacao, Southern Bahia, Brazil, 2019-: values not obtained in the data range

Table 5. Relative frequency (in %) of cacao plots in the Kenworthy diagnostic ranges and specific for cacao in the BIK indexes, and luxury consumption index (LCI), Southern Bahia, Brazil, 2019 ^[1] Limiting for lack (deficient + tendency for deficient) and limiting for excess (tendency for excess + excessive). -: values not obtained in the data range

Table 6. Interpretation ranges for leaf nutrient levels, obtained by specific ranges for balanced indices of Kenworthy, as a function of estimated relative productivity (eRY) for cacao, Southern Bahia, Brazil, 2019:values not obtained in the data range