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Article

Castor Meal and Ground Hydrothermalized Phonolite Optimize Sweet Potato Nutrition, Yield, and Quality

by
Renan J. Parecido
1,2,
Rogério P. Soratto
1,2,*,
Adalton M. Fernandes
1,2,*,
Mayara C. Blanes
1,
Luis G. Fidelis
1,
Harun I. Gitari
3 and
Sérgio G. Dutra
4
1
College of Agricultural Sciences, São Paulo State University (UNESP), Av. Universitária, 3780, Botucatu 18610-034, SP, Brazil
2
Center of Tropical Roots and Starches, São Paulo State University (UNESP), Botucatu 18610-034, SP, Brazil
3
Department of Agricultural Science and Technology, School of Agriculture and Environmental Sciences, Kenyatta University, Nairobi 43844-00100, Kenya
4
Mato Grosso Cotton Institute, Cuiabá 78050-970, MT, Brazil
*
Authors to whom correspondence should be addressed.
Horticulturae 2024, 10(8), 775; https://doi.org/10.3390/horticulturae10080775
Submission received: 26 June 2024 / Revised: 18 July 2024 / Accepted: 22 July 2024 / Published: 23 July 2024
(This article belongs to the Special Issue Organic Fertilizers in Horticulture)
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Abstract

:
To assess the effect of pure castor meal and a mixture of castor meal with ground hydrothermalized phonolite rock (CM+HP mixture) in providing nutrients, particularly N and K, and optimizing yield and quality of sweet potato, a field experiment was conducted using a randomized block design. Treatments were the absence and presence of synthetic N and K fertilizers (ammonium nitrate and KCl) combined with rates of organic fertilizers (1.2 and 2.4 Mg ha−1 of castor meal, 2.25 and 4.5 Mg ha−1 of CM+HP mixture, plus a treatment without organic fertilizers). The CM+HP mixture maintained adequate N and K status in plant leaves. Organic fertilizers increased the number of storage roots per plant and the sweetness of the storage roots, while synthetic fertilizers increased the storage root mean weight. Castor meal combined with synthetic fertilizers improved soil health (increased organic matter and enzyme activity in the soil). The combined application of synthetic fertilizers with 2.4 Mg ha−1 of castor meal or 4.5 Mg ha−1 of CM+HP mixture had the greatest benefit on storage root yield, with an average increase of 128% (10.9 Mg ha−1) on marketable storage root yield, and the nutrient removal compared with the sole application of organic fertilizers.

1. Introduction

Sweet potatoes (Ipomoea batatas L. [Lam.]) are a crucial food source in many nations [1,2], helping to overcome nutrient deficiencies and reduce child malnutrition in some of the world’s most deprived regions [1,3,4]. Beyond human consumption, sweet potatoes are also used as animal feed and industrial feed for starch extraction and alcohol production [5,6]. This versatility highlights sweet potatoes as vital raw materials in a global context of constant population growth [7,8].
In South America, Brazil is the largest producer of sweet potatoes, with a cultivated area of 55,200 ha and an annual production of approximately 850,000 Mg [9]. However, sweet potatoes are also cultivated in many other countries. Globally, the leading producers are China and Nigeria, with cultivated areas of 2.2 and 1.5 million ha, respectively.
Sweet potatoes are considered a high-yield crop with good yield even in nutrient-poor tropical soils. However, they still respond to fertilization [1,10,11,12,13,14]. K and N are essential elements for sweet potatoes, being absorbed and utilized in large quantities [7,10,11,15,16,17]. Deficiencies in these nutrients can substantially reduce productivity by decreasing the size of marketable storage roots and limiting starch accumulation in reserve tissues, leading to changes in important market characteristics, such as the texture and firmness of the storage roots [13,18,19,20]. A balanced supply of N and K often enhances the response to both nutrients [21], with some studies indicating an elevated response to N in the presence of K [12].
Organic residues release nutrients to plants more gradually and consistently than synthetic fertilizers, providing chemical, physical, and biological benefits to the soil, such as improved structure, aeration, drainage, water retention, and microbial activity [22,23,24,25,26]. In this context, castor meal, a by-product obtained from the extraction of castor seeds (Ricinus communis L.), is considered a high-quality organic fertilizer [24,27,28,29,30]. Castor meal contains 4.2–7.5% N, 0.7–1.0% K, along with other nutrients, and has a C/N ratio of approximately 12:1 [31,32]. When applied to the soil, castor meal is rapidly mineralized, releasing N for plant uptake [27]. However, due to its relatively low K content, it does not meet the high K demand of crops, such as sweet potatoes, which require large amounts of K [11,17].
To balance the K/N ratio in castor meal, an alternative is to mix it with a gradual K-release natural source, such as finely ground hydrothermalized phonolite rock. Finely ground hydrothermalized phonolite rocks have been demonstrated as viable alternatives for supplying K to grain crops [33,34,35] and for the Arabica coffee crops (Coffea arabica L.) [36]. In addition to K, ground phonolite rocks contain other essential or beneficial elements, such as Si, Mg, and Ca [37,38,39,40]. This amendment, free of Cl, can be a valuable option for organic agriculture, where the use of KCl is prohibited [41], improving crop performance and enriching soil quality.
It has been demonstrated that sweet potatoes respond well to organic fertilization with green manure [13,17,42] or cattle manure [43]. Therefore, the balance of nutrients derived from using castor meal and hydrothermalized phonolite rock produced in Brazil could be a viable alternative source of essential nutrients for sweet potato cultivation. Additionally, the constant increase in Brazilian agricultural production and its reliance on imported fertilizers justify the use of ground rocks as sources of K in agriculture. Therefore, these alternative sources (castor meal and ground potassium rocks) could also enhance the economic and environmental sustainability of Brazilian agriculture and serve as a viable option for organic production systems where synthetic and highly soluble sources are not allowed.
In the present study, we aimed to evaluate the effect of alternative non-synthetic fertilizers, specifically pure castor meal and a mixture of castor meal and finely ground hydrothermalized phonolite rock (CM+HP mixture), in providing nutrients, particularly K and N, and optimizing the storage root yield and quality of sweet potato crops.

2. Materials and Methods

2.1. Site Characteristics and Climate

A field experiment was conducted from January to June 2023 in São Manuel, São Paulo State, southeastern Brazil (22°46′22″ S; 48°34′11″ W; 762 m asl). According to the Köppen classification system, this tropical region experiences a Cwa climate with hot and rainy summers and dry winters. During the experimental period, temperatures and rainfall were recorded daily, as shown in Figure 1.
The soil is classified as a sand-textured Ferralsol [44]. Prior to the experiment, the area was previously cultivated with okra [Abelmoschus esculentus (L.) Moench] for two years, followed by a 12-month fallow period. The chemical properties of the soil were determined following the procedures described by van Raij et al. [45], while the soil texture was determined using the pipette method [46]. The topsoil layer (0–0.2 m) had a pH(CaCl2) of 4.8, organic matter of 16 g kg−1, SO4-S of 4 mg kg−1 and Presin-extractable of 8 mg kg−1. In addition, it had 1.1, 3.4, 9.5, 16.4, and 30.2 mmolc kg−1 of exchangeable K, Mg, Ca, H+Al, and cation exchange capacity, respectively. The base saturation was 46%, and the silt, sand, and clay contents were 35, 824, and 141 g kg−1, respectively.

2.2. Experimental Design, Treatments, and Crop Management

The experimental design was a randomized complete block in a 2 × 5 factorial scheme with four replicates. The test crop was the sweet potato cultivar Canadense, the most widely grown cultivar in the São Paulo State. Treatments with synthetic fertilizers included the absence or presence of application of recommended N and K fertilizations, such as ammonium nitrate (32% N) and KCl (60% K2O) fertilizers, respectively. Organic fertilizer treatments consisted of a control (without organic fertilizer) and applications of 1.2 and 2.4 Mg ha−1 of pure castor meal, as well as 2.25 and 4.5 Mg ha−1 of CM+HP mixture. The castor meal contained 5.0% N, 1.2% K2O, 0.7% Ca, 1.5% P2O5, 0.4% S, 0.5% Mg, and traces of all micronutrients. The hydrothermalized phonolite contained 12.0% total K2O, 3.0% soluble K2O in 5% tartaric acid + 0.5% sodium fluoride solution, 25% Si, 0.06% Ca, 0.10% Mg, and traces of Mn, Zn, and Co. Pure castor meal rates were calculated to provide the recommended and twice the recommended N rate (i.e., 60 and 120 kg N ha−1) [47]. The CM+HP mixture rates were calculated to provide 60 or 120 kg N ha−1 and 140 or 280 kg K2O ha−1. Thus, the CM+HP mixture was calibrated to form an N–K2O formulation of 2.7–6.2 (i.e., balancing the K concentration). Each plot measured 5.2 by 4 m (20.8 m2), and the sweet potato spacing was 1.30 × 0.35 m. The dimensions of the usable area were 2.6 by 2 m (5.2 m2) because the outermost rows and 1.0 m at each border were excluded from the evaluation.
Eighteen days before planting, 700 kg ha−1 of dolomitic limestone (40% CaO, 28% MgO, and 70% effective CaCO3 equivalence) was applied to raise the base saturation to 60%. The soil was tilled uniformly to a depth of 0–0.20 m with disk harrows. On 26 January 2023, ridges of approximately 0.30 m high were mechanically raised using a tractor-mounted ridger for sweet potatoes. The pure castor meal and CM+HP mixture were applied immediately before raising the ridges. On the same day, all plots received phosphate fertilizer at a rate of 160 kg P2O5 ha−1 (triple superphosphate, 41% P2O5 and 10% Ca) and micronutrients (1 kg B ha−1, 5 kg Zn ha−1, 0.4 kg Cu ha−1, 1.1 kg Mn ha−1, 1.7 kg Fe ha−1, and 0.06 kg Mo ha−1, as fritted trace elements), according to the recommendations of Feltran et al. [47]. In treatments with synthetic N and K fertilizers, 20 kg N ha−1 and 70 kg K2O ha−1 were also applied. Sweet potato cuttings, 40 cm long, were planted on 30 January 2023. Topdressing fertilization with synthetic fertilization at 40 kg N ha−1 and 70 kg K2O ha−1 was carried out on 10 March 2023 [39 days after planting (DAP)], according to the treatments.

2.3. Sampling and Analyses

2.3.1. Leaf Nutrient Concentrations

At 65 DAP, 15 of the most recently mature leaves were collected from the sampling area of each plot [47]. The leaves were rinsed with deionized water and dried in an oven under forced-air circulation at 65 °C for 72 h. The samples were then ground and sieved using a 40-mesh stainless steel screen. The nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentrations were determined for each sample [48].

2.3.2. Soil Health Indices

Before sweet potato harvesting (27 June 2023), soil samples were collected from the ridges. Three simple samples were taken from the usable area of each plot at a depth of 0–0.20 m using a tubular soil probe to form a composite sample. These samples were analyzed for soil organic matter content [45], and the activities of soil arylsulfatase and β-glucosidase enzymes [49] were determined.

2.3.3. Storage Root Yield and Sorting

Harvesting was performed on 27 June 2023 (148 DAP). At harvest, the storage roots from two 2.0 m-long ridges were collected from the usable area of each plot. The harvested storage roots were washed and counted to obtain yield data. Smooth storage roots with an elongated, uniform shape and weighing between 80 and 800 g were considered marketable [50].

2.3.4. Storage Root Dry Matter Content

A representative subsample of marketable storage roots (proportional to all sizes) was randomly collected from each plot, weighed (fresh weight), sliced, dried in an oven under forced-air circulation at 65 °C for 96 h, and reweighed to obtain the dry weight for computing dry matter (DM) content.

2.3.5. Nutrient Concentration and Removal in the Storage Roots

The samples of dried storage roots were subsequently ground and sieved using a 40-mesh stainless steel screen. Thereafter, the nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentrations were determined [48]. Nutrient removal was calculated by multiplying the amount of DM accumulated in the storage roots per treatment with the concentration of each nutrient. The values were converted accordingly into kg ha−1 or g ha−1.

2.3.6. Texture Properties and Soluble Solids of Storage Roots

Another sample of three marketable storage roots was collected from each plot to evaluate quality parameters. Firmness was determined using the TA.XTPlus texturometer (Stable Micro Systems, Surrey, UK) by penetrating 18 mm deep into the pulp of the storage root at a speed of 2.0 mm s−1 using a texture probe equipped with a TA 9/1000 tip. Soluble solids extracted from crushed slices of storage root pulp were measured by placing drops of the solute in the prism of a portable refractometer RTD-95 (Instrutherm Instrumentos de Medição Ltd., São Paulo, SP, Brazil), and the results were expressed in °Brix.

2.3.7. Starch, Reducing Sugar, Total Sugar, and Crude Fiber Contents in the Storage Roots

Starch, reducing sugar, and total sugar contents were determined in storage root samples (the same ones used to determine DM content and nutrient concentration) according to Somogyi’s procedures [51]. The absorbance of the samples was recorded at 535 nm using a spectrophotometer. Starch, reducing sugar, and total sugar content were then calculated based on a standard curve and expressed as a percentage of fresh weight. In addition, the crude fiber content was assessed following the methodology described by the Association of Official Analytical Chemists [52] and expressed as a percentage of fresh weight.

2.4. Statistical Analyses

Data were processed using a two-way analysis of variance (ANOVA) with the SISVAR statistical software package. Synthetic and organic fertilizers were considered as fixed main factors. The blocks and all of the block interactions were considered random effects. If ANOVA identified a significant effect (F ≤ 0.05) of main factors or their interaction, the treatment means were separated at the 0.05 probability level using Fisher’s protected least significant difference test. When the interaction was significant, a two-way mean separation was performed.

3. Results

3.1. Leaf Nutrient Concentrations

The combined application of synthetic NK and organic fertilizers significantly influenced only the concentrations of Cu and Zn in sweet potato leaves; however, both types of fertilizers affected the concentrations of the other nutrients as well (Table 1). The application of CM+HP mixture at a rate of 4.5 Mg ha−1 resulted in increased concentrations of N, Mg, and B in the sweet potato leaves compared to those in other treatments (Table 1). By contrast, the application of synthetic NK fertilizer reduced the leaf concentrations of P, Ca, B, Mg, and Mn. Specifically, in treatments where synthetic NK fertilizers were applied along with a CM+HP mixture at the rate of 4.5 Mg ha−1, there was an increase in leaf Cu concentration compared to that in the other treatments (Figure 2a). Without synthetic NK fertilizers, the application of 1.2 Mg ha−1 of castor meal resulted in increased Cu concentration in the sweet potato leaves. The highest leaf Zn concentration occurred in the treatment with 4.5 Mg ha−1 of CM+HP mixture combined with synthetic NK fertilizers (Figure 2b).

3.2. Storage Root Yield and Chemical Composition

The treatments that received organic fertilizers (pure castor meal or the CM+HP mixture) had a greater total number of storage roots per plant than those in the treatment without organic fertilizers (Table 2). In the presence of synthetic NK fertilizers, the application of castor meal resulted in the highest number of marketable roots per plant; however, the application of 4.5 Mg ha−1 of CM+HP mixture provided the highest value, differing from the application of the same rate of CM+HP mixture alone (Figure 3a). The application of synthetic NK fertilizers increased the mean storage root weight compared to that in the treatments without synthetic NK fertilizers, regardless of organic fertilizer application (Table 2).
The application of synthetic NK fertilizers, as well as the isolated application of castor meal or the CM+HP mixture, increased the yield of total and marketable storage roots (Figure 3b,c). However, in the presence of synthetic NK fertilizers, treatments with 2.4 Mg ha−1 of castor meal or 4.5 Mg ha−1 of CM+HP mixture yielded increased total and marketable storage roots compared to those in the other treatments.
Minimal effects of the treatments were observed on most of the quality traits of sweet potato storage roots, such as firmness and DM, sugar, starch, and fiber contents (Table 2). Nevertheless, the application of 2.4 Mg ha−1 of castor meal or 2.25 Mg ha−1 of CM+HP mixture increased the soluble solids content in storage roots compared to that in treatments without organic fertilizer, regardless of synthetic NK fertilizer application.

3.3. Concentration and Removal of Nutrients in the Storage Roots

Nutrient concentrations and removal by sweet potato storage roots were influenced by the application of organic and synthetic fertilizers, as well as their interaction (Table 3). In the presence of synthetic NK fertilizers, applying the highest rate of castor meal (2.4 Mg ha−1) increased the concentrations of N, P, Fe, K, and Mn in sweet potato storage roots compared to those in the other treatments (Figure 4a–e). When synthetic NK fertilizer was not applied, the application of 2.4 Mg ha−1 of castor meal and 2.25 Mg ha−1 of CM+HP mixture yielded the highest N concentrations in storage roots (Figure 4a). Conversely, without the application of synthetic NK fertilizer, applying 2.4 Mg ha−1 of castor meal resulted in reduced Fe concentrations. A similar case was noted when the CM+HP mixture was applied at an equivalent rate, resulting in reduced Mn concentrations in storage roots (Figure 4d,e). Organic fertilizers had no significant effect on storage root P and K concentrations in the absence of synthetic NK fertilizer (Figure 4b,c). The application of synthetic NK fertilizer increased root Ca concentrations compared to that in the control treatment (Table 3). However, the application of organic fertilizers (pure castor meal or the CM+HP mixture) reduced B concentrations in storage roots, irrespective of the application of synthetic NK fertilizers. Storage roots from treatments with 2.4 Mg ha−1 of castor meal or 2.25 Mg ha−1 of CM+HP mixture exhibited high Cu concentrations, irrespective of the application of synthetic NK fertilizers. Furthermore, the application of synthetic NK fertilizer increased the Cu and Zn concentrations in storage roots compared to those in the treatments without synthetic NK fertilizer.
In the absence of synthetic NK fertilizer, the application rates of 2.4 Mg ha−1 of castor meal and 2.25 Mg ha−1 of CM+HP mixture resulted in higher N removal by the roots than with treatments without fertilization (Figure 5a). However, the highest N removal by the sweet potato storage roots was achieved upon the application of 2.4 Mg ha−1 of castor meal combined with synthetic NK fertilizers.
Similarly, the highest removal of P, K, and Mg was observed with the integrated use of synthetic NK fertilizers along with the highest rates of castor meal or the CM+HP mixture (Figure 5b–d). In the absence of synthetic NK fertilizer, all treatments with organic fertilizers increased P removal by storage roots. The application of 2.25 Mg ha−1 of CM+HP mixture resulted in the highest K and Mg removals. Conversely, the lowest removals of N, P, K, and Mg were observed in the absolute control (Figure 5a–d). Ca removal was enhanced in the treatment with synthetic NK fertilizer compared to that in the treatment without synthetic fertilizer, regardless of organic fertilizer usage (Table 3). The application of the highest rate of the CM+HP mixture increased Ca removal, while the lowest removal was observed in the treatment without organic fertilizers, regardless of synthetic NK fertilizers. In the absence of synthetic NK fertilizer, treatments with the highest rate of castor meal and both rates of the CM+HP mixture provided greater S removal than treatments without organic fertilizer (Figure 5e). However, S removal was higher in the treatment with 4.5 Mg ha−1 of CM+HP mixture combined with synthetic NK fertilizers than in the other treatments.
The application of the highest rates of castor meal and the CM+HP mixture combined with synthetic NK fertilizers resulted in increased removal of B, Cu, and Mn by sweet potato storage roots compared to that in the other treatments (Figure 6a,b,d). In treatments where synthetic NK fertilizers were applied, 2.4 Mg ha−1 of castor meal increased Fe removal, whereas the application of 4.5 Mg ha−1 of CM+HP mixture resulted in the highest Zn removal (Figure 6c,e). When no synthetic NK fertilizer was used, the application of 2.4 Mg ha−1 of castor meal and 2.25 Mg ha−1 of CM+HP mixture increased B removal. Additionally, with the application of the highest rate of castor meal and both rates of CM+HP mixture, an increase in Cu removal was observed compared to that in the absolute control. Treatment with high rates of CM+HP mixture demonstrated increased Fe removal, whereas low rates of CM+HP mixture resulted in enhanced Zn removal compared to that in the absolute control when synthetic NK fertilizer was not applied.

3.4. Soil Health Indices

An interaction between the studied factors and soil health indices was observed (Figure 7). The application of 2.4 Mg ha−1 of castor meal without synthetic NK fertilizer increased soil organic matter content compared to the absolute control (Figure 7a). When synthetic NK fertilizers were used, the highest soil organic matter content was observed with the application of 1.2 Mg ha−1 of castor meal. In the absence of synthetic NK fertilizer, the activity of the arylsulfatase enzyme in the soil was elevated upon application of the highest rate of castor meal (Figure 7b). Conversely, in the presence of synthetic NK fertilizer, application of the lowest rate of castor meal resulted in higher arylsulfatase enzyme activity in the soil than in the other treatments. The activity of the β-glucosidase enzyme in the soil was higher in the treatment with synthetic NK fertilizers combined with 1.2 Mg ha−1 of castor meal than in the other treatments with organic fertilizers (Figure 7c). When synthetic NK fertilizers were not applied, the activity of the β-glucosidase enzyme in the soil of the absolute control was higher than that in the treatments with organic fertilizers.

4. Discussion

Although sweet potatoes can grow and be cultivated in soils with low nutrient availability [1,14], the findings of the present study indicate that organic fertilization (using pure castor meal or castor meal mixed with a K source based on hydrothermalized phonolite rock) improves the nutrition and storage root yield of this root crop, especially when combined with synthetic NK fertilization. The absence of notable interaction between synthetic and organic fertilizers on leaf N and K concentrations indicates that, regardless of the addition of synthetic NK fertilizers, the use of CM+HP mixture proved to be an efficient source of N and K for sweet potatoes, maintaining leaf concentrations at sufficient levels (33–50 g N kg−1 and 30–60 g K kg−1) [47]. In sandy, K-deficient soils like the one in the present study (exchangeable K concentration <1.6 mmolc kg−1) [47], failing to provide K results in a deficiency in sweet potato plants, particularly under heavy rainfall during the growing season [53]. The present study demonstrates that the CM+HP mixture effectively met the K requirements of sweet potato crops.
The application of 4.5 Mg ha−1 of organic fertilizer composed of CM+HP mixture increased leaf Mg and B concentrations, even when these were already at sufficient levels (3.0–12.0 g Mg kg−1 and 45–75 mg B kg−1) [47]. This indicates that the organic fertilizer either supplied or improved the acquisition of Mg and B from the soil by the sweet potato crop. Conversely, the addition of synthetic NK fertilizers promoted plant development (as evidenced by the increased yield of storage roots), but this growth resulted in the dilution of leaf P, Ca, Mg, B, and Mn concentrations (Table 2). However, this reduction did not lead to nutritional deficiencies in plants, as the concentrations of P, Ca, Mg, B, and Mn remained within the ranges suitable for the growth of sweet potato crops (2.3–5.0 g P kg−1, 7.0–12.0 g Ca kg−1, and 40–250 mg Mn kg−1) [47]. Despite the beneficial effects of the combined application of synthetic NK fertilizers and organic fertilizers on Zn and Cu concentrations in sweet potato leaves, these nutrients did not reach levels of deficiency or toxicity, remaining within the ranges (20–50 mg Zn kg−1 and 10–20 mg Cu kg−1) considered suitable by Feltran et al. [47].
The combined application of synthetic NK fertilizers with castor meal or CM+HP mixture may have provided a more balanced nutrient supply, favoring the yield of sweet potato storage roots. For instance, in a study conducted in Brazil, top-dressing with 100 kg N ha−1 combined with 120 kg K2O ha−1 resulted in increased sweet potato yields compared to the yields obtained by applying isolated N rates [12]. Organic fertilizers (castor meal or CM+HP mixture), in addition to providing nutrients, can also act as soil conditioners, improving soil structure and enhancing the retention of nutrients and water [54,55,56,57]. Furthermore, organic fertilizers may have promoted soil microbial activity and positively influenced plant physiology and metabolism, thereby supporting growth, nutrient uptake, and storage root yield [57,58,59,60]. Our results demonstrated that sweet potato crops responded favorably to organic fertilization, with the application of 2.25 Mg ha−1 of the CM+HP mixture increasing the total storage root yield by 52%. Organic fertilization, such as using green manure, can have effects comparable to those by NPK fertilization for sweet potato crops or may even help reduce the rate of synthetic N fertilization [13,17,42].
Despite the benefits of using organic fertilizers, synthetic NK fertilizers also proved to be important for the growth of sweet potato storage roots, increasing their weight by an average of 31% irrespective of the organic fertilizer used. The positive impact of the application of synthetic NK fertilizers on the mean storage root weight of sweet potatoes has been particularly observed in K-deficient soils [53,61]. Nitrogen enhances the distribution of DM in plants and, when supplied in adequate quantities, promotes the allocation of DM to storage roots [62,63]. Potassium increases the cambial activity of storage roots by enhancing the translocation of photoassimilates to the roots, thereby increasing root size [64,65]. Therefore, the amount of N supplied via synthetic NK fertilizer was sufficient to promote storage root growth without causing excessive growth of the aboveground portion at the expense of the growth of storage roots, as reported in other studies [17,66,67]. Similarly, K supplied by synthetic fertilizer was essential for increasing the biomass of storage roots, as verified in previous studies [7,53].
The increased P and K concentrations in sweet potato storage roots, observed with the highest application rate of castor meal (2.4 Mg ha−1) combined with synthetic NK fertilizers, can be attributed to the N stimulus in promoting the growth of absorbent roots, thereby enhancing their capacity to uptake and translocate P [68]. Additionally, N uptake can facilitate K uptake due to the synergistic interaction between these two nutrients [21,69]. Baligar et al. [70] highlighted this synergistic effect in plant tissues, noting that a lower supply of N resulted in low K concentrations in plants.
The variation in storage root yields (Table 2 and Figure 3) influenced nutrient removal by sweet potato storage roots (Table 3 and Figure 5 and Figure 6), as the nutrient levels in the roots showed little variation (Figure 4). The nutrient uptake by sweet potato crops is directly correlated with increased storage root yield [71]. Plant biomass and nutrient concentrations in plant parts were also positively correlated with the amount of nutrients taken up [17]. For most tuber crops, K is the nutrient required in the greatest quantities, followed by N and Ca [10,15,16,72]. In the present study, macronutrient removal by sweet potato storage roots followed the order K > N > P > Ca > S > Mg > Fe > Mn > B > Zn > Cu.
Soil organic matter is closely related to improvements in soil quality [73,74,75,76]. In the present study, the use of organic fertilizers increased the levels of organic matter and the activity of the enzymes β-glucosidase and arylsulfatase in the soil, indicating an improvement in soil quality. The activities of these enzymes in soil are influenced by several factors. The complexity of interactions can be affected by the amount of organic matter and plant residues, microbial activity, nutrient availability, pH, temperature, and salinity [49,77,78,79,80].
In the present study, castor meal proved to be an effective organic fertilizer for sweet potato cultivation since the isolated application of 2.4 Mg ha−1 of castor meal increased the total and marketable yields of storage roots by 45% and 23%, respectively. These increases were slightly smaller than those provided by the isolated use of the recommended synthetic NK fertilizer. The use of green manure as an organic fertilizer has also yielded results comparable to those of synthetic NPK fertilizers in sweet potato cultivation [42]. In the absence of synthetic NK fertilizer, the CM+HP mixture did not outperform pure castor meal. However, the combined use of synthetic NK fertilizers with the highest rates of organic fertilizers (castor meal or CM+HP mixture) was particularly effective, resulting in average increases of 116% and 128% in the total and marketable yields of storage roots, respectively, compared to those in the absolute control (without any fertilization). Under these conditions, the highest rates of organic fertilizers also increased the total and marketable yields of storage roots by an average of 60% and 67%, respectively, compared to those in treatments with synthetic and organic fertilizers alone. In the presence of synthetic NK fertilizers, the application of castor meal or a CM+HP mixture, especially at high rates, contributed to a more balanced supply of nutrients, resulting in improvements in the storage root yield and, consequently, greater nutrient removal.
However, neither synthetic nor organic fertilizers decreased the internal quality of the storage roots. On the contrary, organic fertilizers increased the soluble solid content in the storage roots. Other studies have shown that synthetic N fertilizer only improves sweet potato quality parameters, such as starch content if the application is not combined with organic fertilization using green manures [61], which was not observed in this study. Therefore, organic fertilization with the CM+HP mixture is a promising management technique for sweet potato production systems.

5. Conclusions

Regardless of the application of synthetic NK fertilizers, the CM+HP mixture has been shown to be an excellent source of N and K for sweet potatoes. This mixture is particularly effective in maintaining adequate leaf N and K levels. The highest rate (4.5 Mg ha−1) of the CM+HP mixture also increased Mg and B concentrations in sweet potato leaves, irrespective of the use of synthetic NK fertilizers. The application of organic fertilizers increased the number of storage roots per plant and enhanced the soluble solid content in the storage roots, while synthetic NK fertilizers increased the mean weight of the storage root. The combined application of synthetic NK fertilizers with 2.4 Mg ha−1 of castor meal or 4.5 Mg ha−1 of the CM+HP mixture yielded greater benefits in terms of storage root yield (total and marketable) and nutrient removal than organic fertilizers alone. Moreover, fertilizing with castor meal at a rate of 1.2 Mg ha−1 in combination with synthetic NK fertilizers improved soil health, as indicated by the increased levels of organic matter and activity of arylsulfatase and β-glucosidase enzymes in the soil.

Author Contributions

Conceptualization, R.J.P., R.P.S., A.M.F. and S.G.D.; methodology and data collection, R.J.P., M.C.B. and L.G.F.; funding acquisition and project administration, R.P.S.; data curation, R.J.P. and R.P.S.; writing—original draft preparation, R.J.P., M.C.B. and L.G.F.; writing—review and editing, R.P.S., A.M.F., H.I.G. and S.G.D.; supervision, R.P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially co-funded by A. Azevedo Indústria e Comércio de Óleos Ltda and Mineração Curimbaba Ltda. Additional funding was received from the Dean of Research of the São Paulo State University (Call PROPe 13/2022).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Acknowledgments

Thanks to the Dean of Research of the São Paulo State University (PROPe Call 13/2022) for providing a post-doctoral scholarship to the first author and the National Council for Scientific and Technological Development (CNPq) for providing an award for excellence in research to the second and third authors.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Daily rainfall, irrigation, and maximum and minimum temperatures during the sweet potato growing from January to June 2023 in São Manuel-SP.
Figure 1. Daily rainfall, irrigation, and maximum and minimum temperatures during the sweet potato growing from January to June 2023 in São Manuel-SP.
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Figure 2. Concentrations of Cu (a) and Zn (b) in the leaves of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 2. Concentrations of Cu (a) and Zn (b) in the leaves of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
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Figure 3. Number of marketable storage roots per plant (a), total storage root yield (b), and marketable storage root yield (c) of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 3. Number of marketable storage roots per plant (a), total storage root yield (b), and marketable storage root yield (c) of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
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Figure 4. Concentration of N (a), P (b), K (c), Fe (d), and Mn (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 4. Concentration of N (a), P (b), K (c), Fe (d), and Mn (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
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Figure 5. Removal of N (a), P (b), K (c), Mg (d), and S (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 5. Removal of N (a), P (b), K (c), Mg (d), and S (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
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Figure 6. Removal of B (a), Cu (b), Fe (c), Mn (d), and Zn (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 6. Removal of B (a), Cu (b), Fe (c), Mn (d), and Zn (e) in the storage roots of sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Horticulturae 10 00775 g006
Horticulturae 10 00775 g007
Figure 7. Soil organic matter content (a), activity of arylsulfatase (b), and β-glucosidase (c) enzymes in soil cultivated with sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Figure 7. Soil organic matter content (a), activity of arylsulfatase (b), and β-glucosidase (c) enzymes in soil cultivated with sweet potato crop as affected by synthetic NK fertilizers and rates of organic fertilizers composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). The red and blue bars indicate treatments without and with recommended synthetic NK fertilization, respectively. Bars indicate the standard error. Different lowercase letters indicate significant differences by synthetic fertilizer level within each organic fertilizer level, while different uppercase letters indicate significant differences by organic fertilizer level within each synthetic fertilizer level, at p ≤ 0.05, according to the LSD test.
Horticulturae 10 00775 g007
Table 1. Leaf nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentrations of sweet potato crop as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
Table 1. Leaf nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentrations of sweet potato crop as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
VariableSynthetic FertilizersOrganic Fertilizers (Mg ha−1)ANOVA (p > F)CV (%)
Without+NK01.2CM2.4CM2.25CM+HP4.5CM+HPSFOFSF × OF
N (g kg−1)37.1 ± 0.6 a36.3 ± 0.5 a35.3 ± 0.4 b36.4 ± 0.6 b35.8 ± 0.9 b36.9 ± 0.7 ab38.9 ± 0.9 a0.220.020.365.6
P (g kg−1)4.2 ± 0.10 a3.6 ± 0.08 b3.7 ± 0.15 a4.1 ± 0.23 a3.8 ± 0.18 a3.8 ± 0.17 a4.2 ± 0.17 a<0.010.090.9310.3
K (g kg−1)38.5 ± 0.8 a38.4 ± 0.5 a37.8 ± 0.7 a38.3 ± 0.8 a39.4 ± 1.2 a37.8 ± 1.0 a39.0 ± 1.7 a0.870.820.778.8
Ca (g kg−1)7.9 ± 0.14 a7.4 ± 0.11 b7.4 ± 0.21 a7.7 ± 0.17 a7.6 ± 0.16 a7.3 ± 0.18 a8.0 ± 0.29 a<0.010.130.326.8
Mg (g kg−1)4.1 ± 0.07 a3.7 ± 0.08 b3.8 ± 0.13 b3.9 ± 0.10 b3.8 ± 0.16 b3.8 ± 0.10 b4.3 ± 0.13 a<0.01<0.010.126.7
S (g kg−1)4.3 ± 0.05 a4.2 ± 0.07 a4.2 ± 0.09 a4.3 ± 0.08 a4.3 ± 0.14 a4.1 ± 0.06 a4.1 ± 0.08 a0.290.410.126.1
B (mg kg−1)51.2 ± 1.2 a47.8 ± 1.2 b46.7 ± 1.6 b47.7 ± 1.5 b48.5 ± 0.9 b47.0 ± 1.8 b57.9 ± 1.1 a<0.01<0.010.396.7
Cu (mg kg−1)12.4 ± 0.3 a11.7 ± 0.4 b11.7 ± 0.6 bc12.6 ± 0.5 ab11.6 ± 0.4 bc11.3 ± 0.3 c13.0 ± 0.7 a0.040.01<0.018.4
Fe (mg kg−1)147.2 ± 7.7 a154.7 ± 7.2 a165.9 ± 10.1 a151.4 ± 8.8 a128.1 ± 12.2 a165.5 ± 14.1 a143.9 ± 9.6 a0.420.080.1919.2
Mn (mg kg−1)119.0 ± 4.4 a107.9 ± 4.1 b134.0 ± 8.5 a111.8 ± 6.8 b101.6 ± 2.3 b105.3 ± 1.7 b114.5 ± 6.8 b0.04<0.010.8014.5
Zn (mg kg−1)46.1 ± 0.6 a44.7 ± 0.9 a44.9 ± 1.0 ab47.2 ± 1.0 a44.3 ± 1.3 b43.6 ± 0.9 b47.0 ± 1.4 a0.120.030.025.8
Values followed by the same letter in the row within each factor are not significantly different at p < 0.05 according to the LSD test.
Table 2. Total number of storage roots per plant, number of marketable storage roots per plant, storage root mean weight, total and marketable storage root yield, and chemical composition of storage roots of sweet potato crop as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
Table 2. Total number of storage roots per plant, number of marketable storage roots per plant, storage root mean weight, total and marketable storage root yield, and chemical composition of storage roots of sweet potato crop as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
VariableSynthetic FertilizersOrganic Fertilizers (Mg ha−1)ANOVA (p > F)CV (%)
Without+NK01.2CM2.4CM2.25CM+HP4.5CM+HPSFOFSF × OF
Roots plant−17.8 ± 0.30 a8.1 ± 0.36 a6.0 ± 0.26 b8.0 ± 0.39 a8.6 ± 0.30 a8.5 ± 0.42 a8.7 ± 0.56 a0.37<0.010.3613.6
Mark. roots plant−13.3 ± 0.15 b4.1 ± 0.17 a3.0 ± 0.19 b3.6 ± 0.30 a3.7 ± 0.24 a4.0 ± 0.19 a4.1 ± 0.37 a<0.01<0.010.0113.9
Root mean weight (g)98.9 ± 1.8 b129.6 ± 3.8 a118.4 ± 8.9 a111.0 ± 8.1 a118.0 ± 8.3 a108.1 ± 3.0 a115.7 ± 7.3 a<0.010.380.0710.7
Total yield (Mg ha−1)15.3 ± 0.60 b21.0± 0.85 a14.3 ± 1.19 d17.4 ± 1.10 c20.8 ± 1.74 a18.4 ± 0.69 bc19.9 ± 1.80 ab<0.01<0.01<0.019.5
Mark. yield (Mg ha−1)10.4 ± 0.40 b16.4 ± 0.72 a11.0 ± 1.06 d12.3 ± 0.20 cd14.8± 1.72 ab13.5 ± 0.67 bc15.4 ± 1.76 a<0.01<0.01<0.0111.9
Root dry matter (%)22.2 ± 0.4 a22.2 ± 0.8 a21.5 ± 0.6 a21.8 ± 0.8 a21.9 ± 0.8 a21.6 ± 0.7 a24.3 ± 1.4 a0.980.260.8412.7
Firmness (N)30.1 ± 0.4 a30.0 ± 0.4 a29.0 ± 0.4 a29.8 ± 0.9 a30.6 ± 0.6 a29.9 ± 0.8 a30.9 ± 0.5 a0.870.270.285.9
Total sugar (%)3.6 ± 0.16 a3.7 ± 0.26 a3.2 ± 0.15 a3.3 ± 0.22 a3.3 ± 0.12 a4.1 ± 0.43 a4.3 ± 0.44 a0.810.060.8626.1
Reducing sugars (%)2.3 ± 0.07 a2.3 ± 0.17 a2.2 ± 0.10 a2.4 ± 0.14 a2.5 ± 0.15 a2.0 ± 0.13 a2.6 ± 0.35 a0.990.240.1323.1
Starch (%)19.4 ± 0.6 a18.4 ± 0.9 a17.5 ± 0.8 a17.7 ± 1.1 a19.5 ± 1.2 a18.9 ± 1.0 a20.7 ± 1.4 a0.360.310.8117.9
Soluble solids (°Brix)5.7 ± 0.09 a5.4 ± 0.12 a5.2 ± 0.14 b5.5 ± 0.18 ab5.7 ± 0.16 a5.9 ± 0.15 a5.6 ± 0.17 ab0.070.040.147.9
Crude fiber (%)7.0 ± 0.24 a7.4 ± 0.38 a7.3 ± 0.47 a7.4 ± 0.52 a6.9 ± 0.54 a6.6 ± 0.21 a7.7 ± 0.69 a0.380.650.7821.2
Values followed by the same letter in the row within each factor are not significantly different at p < 0.05 according to the LSD test.
Table 3. Nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentration and removal in the sweet potato storage roots as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
Table 3. Nutrient (N, P, K, Ca, Mg, S, B, Cu, Fe, Mn, and Zn) concentration and removal in the sweet potato storage roots as affected by synthetic NK fertilizers (SF) and rates of organic fertilizers (OF) composed of castor meal (CM) or castor meal plus hydrothermalized phonolite mixture (CM+HP). Mean ± standard error.
VariableSynthetic FertilizersOrganic Fertilizers (Mg ha−1)ANOVA (p > F)CV (%)
Without+NK01.2CM2.4CM2.25CM+HP4.5CM+HPSFOFSF × OF
N conc. (g kg−1)10.0 ± 0.2 a10.3 ± 0.4 a10.3 ± 0.2 b10.3 ± 0.2 b11.6 ± 0.8 a9.6 ± 0.4 bc9.0 ± 0.2 c0.29<0.01<0.019.5
P conc. (g kg−1)1.8 ± 0.07 a1.7 ± 0.07 a1.7 ± 0.10 a1.8 ± 0.07 a1.9 ± 0.14 a1.6 ± 0.15 a1.7 ± 0.07 a0.130.24<0.0112.1
K conc. (g kg−1)14.8 ± 0.3 b15.9 ± 0.3 a15.4 ± 0.3 a15.3 ± 0.5 a16.0 ± 0.9 a15.6 ± 0.6 a14.4 ± 0.3 a<0.010.170.048.4
Ca conc. (g kg−1)1.5 ± 0.08 b1.8 ± 0.06 a1.6 ± 0.12 a1.6 ± 0.07 a1.5 ± 0.16 a1.6 ± 0.13 a1.8 ± 0.12 a<0.010.240.5617.5
Mg conc. (g kg−1)0.6 ± 0.015 a0.7 ± 0.017 a0.6 ± 0.018 a0.6 ± 0.024 a0.7 ± 0.034 a0.7 ± 0.027 a0.7 ± 0.027 a0.110.760.159.5
S conc. (g kg−1)0.7 ± 0.011 a0.7 ± 0.010 a0.7 ± 0.015 a0.7 ± 0.011 a0.7 ± 0.023 a0.7 ± 0.021 a0.7 ± 0.014 a0.940.670.086.2
B conc. (mg kg−1)14.4 ± 0.25 a14.2 ± 0.27 a15.4 ± 0.17 a14.2 ± 0.40 b14.1 ± 0.33 b14.3 ± 0.38 b13.4 ± 0.42 b0.46<0.010.406.2
Cu conc. (mg kg−1)4.6 ± 0.10 b5.0 ± 0.13 a4.4 ± 0.10 c4.6 ± 0.15 bc5.1 ± 0.31 a5.1 ± 0.16 a5.0 ± 0.10 ab<0.010.020.509.7
Fe conc. (mg kg−1)72.9 ± 2.6 a69.5 ± 3.1 a74.3 ± 4.0 a68.2 ± 3.4 a75.3 ± 6.5 a71.3 ± 3.7 a66.9 ± 4.9 a0.250.30<0.0112.9
Mn conc. (mg kg−1)23.0 ± 0.8 a21.6 ± 0.9 a21.9 ± 1.9 b22.5 ± 1.1 ab25.3 ± 1.2 a19.7 ± 0.7 b22.1 ± 0.9 b0.130.020.0213.4
Zn conc. (mg kg−1)6.2 ± 0.16 b7.2 ± 0.15 a6.7 ± 0.33 a7.1 ± 0.28 a6.7 ± 0.33 a6.7 ± 0.24 a6.4 ± 0.31 a<0.010.440.5710.8
N rem. (kg ha−1)34.2 ± 1.6 b48.7 ± 3.8 a31.9 ± 3.3 c38.9 ± 2.8 bc53.8 ± 7.1 a38.2 ± 2.3 bc44.5 ± 6.4 ab<0.01<0.01<0.0121.9
P rem. (kg ha−1)6.1 ± 0.34 b7.9 ± 0.51 a5.1 ± 0.51 c6.7 ± 0.30 b8.4 ± 0.0.97 a6.4 ± 0.54 bc8.2 ± 1.01 a<0.01<0.01<0.0119.6
K rem. (kg ha−1)50.3 ± 2.5 b74.0 ± 4.3 a47.6 ± 4.7 d57.7 ± 4.4 cd73.9 ± 9.0 a61.6 ± 3.2 bc70.0 ± 8.4 ab<0.01<0.01<0.0116.7
Ca rem. (kg ha−1)5.1 ± 0.42 b8.4 ± 0.65 a5.1 ± 0.66 c5.9 ± 0.49 bc7.3 ± 1.22 ab6.3 ± 0.60 bc9.2 ± 1.41 a<0.01<0.010.1629.1
Mg rem. (kg ha−1)2.2 ± 0.11 b3.1 ± 0.20 a2.0 ± 0.22 d2.3 ± 0.14 cd3.0 ± 0.36 ab2.6 ± 0.16 bc3.2 ± 0.40 a<0.01<0.010.0119.2
S rem. (kg ha−1)2.4 ± 0.12 b3.3 ± 0.21 a2.2 ± 0.23 d2.7 ± 0.18 cd3.3 ± 0.23 ab2.8 ± 0.16 bc3.5 ± 0.46 a<0.01<0.010.0218.7
B rem. (g ha−1)48.8 ± 2.0 b65.4 ± 3.2 a47.3 ± 4.3 b53.1 ± 2.7 b63.6 ± 5.2 a56.4 ± 2.4 ab64.9 ± 7.6 a<0.01<0.010.0116.0
Cu rem. (g ha−1)15.9 ± 0.9 b23.7 ± 1.6 a13.7 ± 1.5 d17.4 ± 1.5 c23.6 ± 2.9 ab19.9 ± 0.7 bc24.3 ± 3.1 a<0.01<0.01<0.0118.5
Fe rem. (g ha−1)247.8 ± 13.4 b324.7 ± 24.5 a227.4 ± 22.8 c253.3 ± 14.0 c354.3 ± 55.4 a282.2 ± 19.2 bc314.1 ± 27.8 ab<0.01<0.01<0.0120.7
Mn rem. (g ha−1)78.1 ± 3.9 b103.5 ± 9.7 a64.8 ± 4.2 b84.2 ± 6.0 b116.8 ± 13.4 a77.5 ± 2.6 b110.5 ± 18.5 a<0.01<0.010.0126.9
Zn rem. (g ha−1)21.4 ± 1.2 b33.6 ± 2.3 a20.7 ± 2.4 b26.9 ± 2.1 ab31.0 ± 3.9 a26.4 ± 1.2 ab32.7 ± 5.9 a<0.010.020.0425.5
Values followed by the same letter in the row within each factor are not significantly different at p < 0.05 according to the LSD test.
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MDPI and ACS Style

Parecido, R.J.; Soratto, R.P.; Fernandes, A.M.; Blanes, M.C.; Fidelis, L.G.; Gitari, H.I.; Dutra, S.G. Castor Meal and Ground Hydrothermalized Phonolite Optimize Sweet Potato Nutrition, Yield, and Quality. Horticulturae 2024, 10, 775. https://doi.org/10.3390/horticulturae10080775

AMA Style

Parecido RJ, Soratto RP, Fernandes AM, Blanes MC, Fidelis LG, Gitari HI, Dutra SG. Castor Meal and Ground Hydrothermalized Phonolite Optimize Sweet Potato Nutrition, Yield, and Quality. Horticulturae. 2024; 10(8):775. https://doi.org/10.3390/horticulturae10080775

Chicago/Turabian Style

Parecido, Renan J., Rogério P. Soratto, Adalton M. Fernandes, Mayara C. Blanes, Luis G. Fidelis, Harun I. Gitari, and Sérgio G. Dutra. 2024. "Castor Meal and Ground Hydrothermalized Phonolite Optimize Sweet Potato Nutrition, Yield, and Quality" Horticulturae 10, no. 8: 775. https://doi.org/10.3390/horticulturae10080775

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