Soil Characterization for Production of an Industrial Tomato Variety in South Portugal—A Case Study †

: Appropriate soil conditions are important to the success of tomatoes culture. In fact, there are mineral elements that are essential for the good and healthy development of tomatoes, namely, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, and zinc. Additionally, organic matter and pH play an important part in the process. In this context, this study aimed to characterize a soil destined to produce an industrial tomato variety in the south of Portugal. As such, mineral elements content, pH, electrical conductivity, humidity, organic matter, and color (without humidity and without humidity and organic matter) were analyzed in 16 soil samples before any type of soil preparation was carried out. Through principal components analysis (PCA) was possible to observe that electrical conductivity and humidity are more correlated with each other than pH and organic matter. Yet, pH of soil varied between 6.9 (minimum) and 7.3 (maximum) being in accordance with the ideal range values for tomato production. Also, regarding quantification of mineral elements Fe showed a higher content, followed by K, Ca, P, Mg, S, Zn, and As. However, regarding the color of the soil without humidity and without humidity and organic matter, there were significant differences between CieLab parameters (L, Chroma, and Hue). Nevertheless, soil conditions of the field presented good requirements for tomato production, despite the higher levels of Fe in the soil and the presence of As.


Introduction
Conditions of soil are a very important factor in the success of tomatoes culture. This culture grows well on most soils but prefers deep and well-drained, sandy loams soils, being moderately tolerant regarding pH [1]. Soil chemical (namely, pH) and physical properties can influence water and mineral uptake by plants and therefore the nutritional content of tomatoes [2]. For plant growth, there are twelve mineral elements essentials and most of them come from soil (namely, nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, and zinc) [3,4]. Nevertheless, without these essential mineral elements, tomatoes can't grow properly [3]. Yet, each mineral element varies according to its mobility within the plant and every crop has different needs [3]. For instance, N, P, K, Ca, Mg, and S are needed in large quantities for good crop production, and namely, Fe and Zn are needed in fewer quantities. Additionally, soil cannot provide adequate amounts of N, P, and K there's a need for soil fertilization to lead to good crop production [4]. Also, organic matter of soil is related to crop nutrient composition [5] and pH can affect plant growth and influence different soil properties (namely, nutrient absorption) [6]. In addition to organic matter content and pH, also electrical conductivity of soil is related to crop productivity [5][6][7][8]. Regarding soil moisture (or humidity of soil) a study carried out by [9], showed that soil moisture deficiency can affect tomato yield. Nevertheless, the color of soil is one of the most significant characteristics and can also indicate erosion or excess of salinity [10] 2005.

Experimental Fields
This study focused on the characterization of soil from one experimental field, located in São João de Negrilhos (Aljustrel)-South of Portugal (GPS coordinates: 37.948157, −8.173834)-intended to produce an industrial tomato variety (H1534). The location of the field is shown in Figure 1. Geographic location of the field (images obtained through Google Earth). Indication (in blue) of the limit of soil sample collection in the field. (Note: when the soil samples were taken, there was no type of plantation in the field. The image shown is for geographic location only-they do not correspond to the date of collection of soil samples).

Soil Analysis
In the experimental field (about 750 m 2 ) (intended for tomato production), 16 soil samples (100 g, picked up at 30 cm depth) were collected in a hexagonal grid, for physical and chemical analysis, before any type of soil preparation was carried out. Soil samples were sieved (using a 2.0 mm nylon sieve) to remove stones and other debris before analysis. Mineral content in soils were determined, following [11], using a XRF analyzer (model XL3t 950 He GOLDD+) under helium atmosphere. Additionally, pH and electrical conductivity were carried out following [12], being these parameters determined in the decanted supernatant of a mixture (ratio 1:2.5 g soil mL −1 water milli-q) for 1h with tiring (25 °C for 30 min) in a thermal bath. Humidity (also known as soil moisture) and organic matter were carried out following [13]. Colorimetric parameters of soil were carried out after the remotion of humidity and after the remotion of humidity and organic matter, following [14].

Statistical Analysis
Data normality and homogeneity of variance was carried out. The principal component analysis was done on the correlation matrix and the first two components were retained and rotated using Varimax rotation.
Additionally, data were statistically analyzed using a One-Way ANOVA to assess differences among the different type of soil (without humidity and without humidity and organic matter), followed by a Tukey's for mean comparison. A 95% confidence level was adopted for all tests.

Results
Regarding pH, organic matter content, electrical conductivity (named conductivity in Figure 1A), and humidity of soil samples ( Figure 1A,B), through principal component analysis (PCA) it was possible to identify that the interrelations among the parameters are explained in the projections of components 1 and 2. Considering the F1/F2 factorial plane (component 1/component 2), there is a greater correlation between electrical conductivity and humidity ( Figure 1A) with a correlation matrix of 0.709 ( Figure 1B). Additionally, through Pearson's correlation, we can also observe that both parameters have the highest correlation value (0.834) ( Figure 1C). Considering the pH and organic matter, they are close to the origin according to F1 (but not close to the origin in F2), as such, the variability of both parameters is better explained by F2 than by F1. In the organic matter parameter, there is a greater correlation between humidity and conductivity than with pH ( Figure  1B,C). The pH is the parameter with the lowest correlation between the remaining parameters analyzed ( Figure 1A-C). Additionally, the pH of soil varied between 6.9 (minimum) and 7.3 (maximum) (data not shown). Mineral content of soil was assessed ( Figure 3A-C) and Fe showed the highest content, followed by K, Ca, P, and Mg. Sulfur, Zn and As were the mineral elements presented in lower concentration in the soil samples, being As a contaminating mineral element. Colorimetric parameters were assessed in soil without humidity and without humidity and organic matter (Table 1). Regarding L and Chroma, showed significantly higher values and Hue parameter showed significantly lower content in soil samples without humidity compared to soil samples without humidity and organic matter.

Discussion
Considering the importance of soil chemical and physical properties that can affect water and mineral uptake by plants [2], soil chemical characteristics were assessed. Regarding pH, organic matter content, electrical conductivity, and humidity (or moisture) of soil samples ( Figure 2) were possible to identify that they correlated differently. In fact, electrical conductivity and humidity showed a greater correlation between each other. This correlation can be due to electrical conductivity being influenced by different properties-namely, clay content and soil water content- [15] since the range of values were very different between both parameters (humidity showed values between 10 and 19.8% and electrical conductivity varied between 134 and 244 µ S.cm −1 -data not shown). Additionally, tomato plants are moderately sensitive to soil salts [16] and the values obtained for electrical conductivity of soil showed much lower values than the threshold of tolerance of tomato crop [17], being suitable for tomatoes production. Also, the pH of the soil is in accordance with the ideal range for tomatoes production [1]. Regarding macro and micro elements of soil, Fe showed a higher content, followed by, K, Ca, P, Mg, S, Zn, and As ( Figure 3). For instance, despite Fe being needed in fewer quantities for good crop production [4], the high content obtained in the soil, mapped as Luvisol (WRSDB, 2009) with the code "Pag", is due to the pedogenesis that occurred on sands and gravels with reddish-brown clayey intercalations, containing abundant ferruginous pisoliths and ferromanganese and limonitic impregnations and crusts, of the Plio-Pleistocene (PQ) and the Miocene (M) geological units. As such, the color of the soil without humidity and without humidity and organic matter were red, being associated with Fe oxides [19] and in accordance with the abundance of Fe of soil in our study ( Figure 3A). Nevertheless, soil color reflects Fe oxide composition and content. Also, a higher Chroma parameter can be correlated to a Fe oxide content [20], being observed in our data ( Table 1) that after the removal of organic matter (only the mineral part remaining) there was an increase in Chroma, which is related to a higher Fe content (since there is a removal of the organic A B C fraction in the soil samples). In fact, the color of soil is one of the most significant characteristics, being an indicator of soil formation [10] and can be used to describe soil profiles [19]. According to [4] our data showed higher content of P, K, Mg, and Ca in soil considering the desirable levels of nutrients for tomatoes production. In fact, P, K, and Ca are absorbed by tomatoes in large amounts and a higher content of Mg can lead to an increase the tomato fruit production [4]. Regarding the contaminating mineral element (As), the content obtained was below the critical limits for a pH higher than 7.0 [21].

Conclusions
The soil samples collected in the experimental field located in São João de Negrilhos (Aljustrel), exhibit different interrelations between pH, organic matter content, electrical conductivity, and humidity (or moisture). A higher correlation was observed between electrical conductivity and humidity probably due to electrical conductivity being influenced by different properties, namely soil water content. Regarding pH was in accordance with the ideal range for tomato production. Overall, soil conditions of the field presented good requirements for tomato production, despite the higher levels of Fe (due to the geological substratum of the region) and the presence of As that were below the critical limits for the pH of the field.