Influence of Bentonite and MB4 on the Chemical Characteristics of an Oxisol

The growing concern with the quality of life and the environment, due to the degradation of natural resources and their contamination, mainly with agrochemicals, led to the emergence of a sustainable or alternative agriculture. The objective of this study was to evaluate the effect of the addition of increasing doses of bentonite and MB4 on the availability of nutrients to the soil. The experiment consisted of a 4 × 4 factorial, bentonite doses (0, 30, 60 and 90 t∙ha) and MB4 doses (0, 3, 6 and 9 t∙ha) with three replications. The soil mixtures with the treatments were conditioned in the plastic pots, incubated for 90 days in a greenhouse, and chemically analyzed. Data were submitted to analysis of variance and comparison of means by the Tukey test. Increasing doses of bentonite and MB4 promoted an increase in the calcium (Ca), magnesium (Mg) in the soil sample either alone or the mixture of two factors, except for the mixture of the Ca Mg doses. Increasing doses of bentonite increased the cation exchange capacity of the soil, favoring the availability of nutrients in the soil. The increasing doses of MB4 favored the increase of the pH values and, consequently, decreased the soil potential acidity values for the applied bentonite doses. On the other hand, these treatments decreased the cation exchange capacity of the soil.


Introduction
The soil can lose its nutrients through several processes: plant uptake, erosion, volatilization and leaching. Leaching is probably the most important process of nutrient loss in moist soils, but according to [1] is very difficult to avoid it.
The low productivity of crops cultivated in sandy soils is due to constant losses of the nutrients through the ease with which they allow the movement of nutrients from the superficial layers to deeper layers of the soil, far from the roots of the plants [2]. Therefore, the replacement of these nutrients, through elements and chemical compounds, is indispensable to the crop yields.
Considering the model of agricultural development adopted in Brazil, much of the demand for fertilizers for crops is supplied through the use of readily soluble fertilizers by NPK formulations, associated or not with the use of macro and micronutrients, to ensure satisfactory yields. Many farmers also use organic fertilizers as a source of nutrients, sometimes associated with mineral fertilizers.
In organic agriculture, ground natural rocks have been used as a source of nutrients. The use of natural rocks in agriculture, as natural fertilizers, has grown greatly in recent years [3].
The minerals contained in the rocks are nutrient sources of slow release of to the soil and, depending on their mineralogical composition, can contribute with a varied and expressive amount of essential elements to the plants. The use of grounded rock silicate fertilizers is attractive as these types of fertilizers have the potential to supply soils with a large array of macro and micronutrients in comparison to commercially available soluble fertilizers, which commonly only supply the main macronutrients N, P and K, but not nutrients such as Ca, Mg and micronutrients [4]. Grounded rocks are increasingly being used due to the need to recover impoverished, unbalanced soils that have lost much of the nutrient reserve of their mineral constituents [5].
The effectiveness of rock powder as a source of nutrients to the soil is questioned because of the low solubility and the need to be applied in large amounts to the soil to obtain positive responses [6]. This depends on factors such as the chemical and mineralogical composition of the rock, the granulometry of the material, the reaction time, and soil factors such as pH and biological activity [7].
In Brazil, there are still few references to the use of grounded rocks in agriculture on a commercial scale. Mixtures of several grounded rocks have been commercialized, for example, by the company MIBASA of Arapiraca, State of Alagoas, whose main product is the MB4 rock meal. MB4 is a mixture of two rocks: biotitaxisto and serpentinite, in the ratio of 1:1 [8]. This product comes from the grinding of silicate rocks and has about 48% silica in its composition.
According to [9], MB4 has been tested in various soils and has proven to be an efficient recovering and improver of soils, because it has a wide variety of chemical elements, providing essential nutrients for plants.
Bentonite has also been investigated as a possibility for agronomic use, as soil conditioning or fertilizer material [2] [10]. Bentonites are predominantly composed of mineral clay from the smectite group, usually known as montmorillonite, and quartz impurities. In some varieties are also kaolinite and illite [11]. Chemical and mineralogical analyses showed that there are differences between clays of different colors. Green and chocolate clays, for example, generally contain high smectite content and few impurities, such as ilite and kaolinite.
These clays are also chemically different because they contain low silicon content and high aluminum content relative to other colors [11].
Both bentonite and MB4 were donated materials to be used in scientific research, so there was no cost to purchase them.
In this sense, the objective of this work was to evaluate the effect of the addition of increasing doses of bentonite and MB4 on the availability of nutrients to the soil.

Materials and Methods
This experiment was carried out in a greenhouse at the Agricultural Engineering Department, Federal University of Campina Grande, Paraiba, Brazil, using soil samples collected in the superficial layer (0 -20 cm) of Eutrophic Red Latosol [12]. These samples were air-dried, crushed, sieved through a 2 mm sieve and chemically characterized according to [13], presenting the following attributes: The bentonite clay samples were collected in the Primavera mine, Paraiba State, Brazil. These samples were air dried and sieved with 0.074 mm mesh in order to precede X-ray diffraction and X-ray florescence analysis. According to these analysis, bentonite samples presents picks of smectite clays, tridymite (a silicate mineral and polymorph of high temperature of quartz), and quartz (low quantity), also presenting the following composition: SiO 2 = 76.784%; Al 2 O 3 = 13.339%; Fe 2 O 3 = 6.035%; MgO = 2.225%; CaO = 0.759% and other oxides = 0.545%. The cation exchange capacity was also determined by the methylene blue method [14], resulting in 48 meq/100g of dry clay and specific area: 375  The soil mixtures with the treatments were conditioned in the plastic buckets and placed in field capacity with water supply, remaining incubated for 90 days.
The moisture of these mixtures was maintained close to the field capacity.
After incubation, soil samples from each experimental unit were collected, air dried, sieved in a 2 mm mesh and chemically characterized, according to the methods adopted by [13].
The results were submitted to analysis of variance, using the SISVAR program [16].

Results and Discussion
The analysis of variance showed that the interaction between the bentonite and MB4 factors was significant at p < 0.01 for all elements, except for potassium and H + Al, which did not present significant effects. Bentonite doses influenced all analyzed parameters (p < 0.01), while MB4 doses had a significant effect for all parameters except for potassium (Table 1).
In the treatments without MB4, represented by the curve M0 (Figure 1 The Ca contained in MB4 interacted with the bentonite providing a reduction of the available Ca (Figure 1(a)). This occurred, possibly, to the adsorption of Ca by bentonite. According to [17], this is due to the higher specific surface area of the bentonites and, consequently, higher cation adsorption capacity. However, from the minimum point, the levels of Ca available in the soil increased in the M3, M6 and M9 curves, probably because of the saturation of the clay exchange sites, i.e., it exceeded the maximum adsorption capacity of the bentonite. Magnesium (Figure 1   probably by exceeding its cation adsorption capacity. On the other hand, with the incorporation of the doses 6 and 9 t•ha −1 of MB4, there was an increase of Na; however, the two curves were overlapped, showing that there was no difference between these two treatments. Potassium (K) was significant only with the effect of increasing doses of bentonite (Table 1), showing a better fit in the quadratic form (Figure 1(d)). Although bentonite retains exchangeable cations, it is generally found that increasing doses of bentonite favored cation exchange capacity (CEC), especially without the presence of MB4 (M0) (Figure 2(a)), corroborating [19]. Clays have high micro porosity and greater specific surface, increasing the number of available sites for the bonds, thus favoring a greater CEC. According to [17], fine soil aggregates and clay minerals have a greater capacity to retain heavy metals due to their larger surface area, corroborating [20]. It can be inferred that the bento- there is an increasing quadratic behavior for the MB4 doses, ranging from 1.08 (0 t•ha −1 MB4) to 1.39 (9 t•ha −1 of MB4), corresponding to a 28.7% increase in the highest dose comparing to the control (Figure 3(a)). It can be verified that in spite of the curve B0 within the increasing doses of MB4 present higher values of Ca, the increment between the highest dose and the control was relatively small, agreeing [21]. These authors evaluated the effect of increasing doses of grounded basalt on an acid Yellow Latosol for a period of 180 days, which showed that the increases in the concentration of calcium and magnesium were relatively low, indicating that these elements must be present in minerals of low solubility. Similar results were found by [22].  Toscani & Campos [23] working with grounded rock during one year, verified that Ca was the element that increased more sharply, varying from 1 to 2.6 cmol c /dm 3 . Von Wilpert and Lukes [24] observed positive effects of the use of silicate rock powder on forest soils in Germany, since increased Ca, K and pH levels were observed as a function of the application of 6 t•ha −1 of rock dust. In the present study, when mixing MB4 with bentonite (B30, B60 and B90), Ca values were lower than those shown in curve B0. Probably, Ca was retained in bentonite due to competition for the exchange sites, which is in accordance with [25] who claim that the levels and types of clay influence the reactions of adsorption/desorption.
Increasing doses of MB4 without bentonite (B0), increased Mg content in the soil only 2.3% when comparing to the highest MB4 dose (9 t•ha −1 ) (Figure 3(b)), probably due to the MB4 low releasing of nutrients, corroborating [21]. Howev- This shows that MB4 did not release Al 3+ exchangeable in the reaction of the rock powder with the soil solution. Besides, MB4 present the liming potential with increasing application rates according to [28]. Melo et al. [21], in an incubation experiment with different doses of grounded basalt, observed that the addition of these doses presented greater efficiency for the neutralization of the potential acidity.
Increasing doses of MB4 reduced CEC, especially in the absence of bentonite (B0). Possibly, this rock powder reacted in some way with the available cations in the soil solution were unavailable (Figure 4(b)). The opposite result was verified by [29]; the incubation of 36 months in weathered soils treated with equivalent doses up to 300 t•ha −1 of basalt powder indicated an increase in CEC. Gillman et al. [30] also observed an increase in CEC of seven soils of Queensland, Australia, incubated with increasing doses of basalt powder (0, 1, 5, 25 and 50 t•ha −1 ). It is interesting to note that CEC, like soil fertility, depends directly on the chemical quality of the parent rock which, when milled, can release mineral nutrients to the soil.
According to [31], the application of rock powders to the soil does not replace chemical fertilizers, but rather leads to a change of conception on the management of agroecosystem fertility.  Leonardos [3] verified that the application of volcanic rock powder in a sandy soil significantly increased the pH, which can be explained by the lower buffer power of the studied soil, corroborating [30]. On the other hand, [18] did not observe significant pH differences between treatments in a similar soil.