CHARACTERIZATION OF SOIL FERTILITY AT DIFFERENT RELIEF POINTS IN A HUMID RESIDUAL MASSIF IN THE STATE OF CEARÁ

Introduction: The relief acts as a soil formation factor and interferes, even indirectly, in the physical-chemical composition of soils, so that important variations in fertility can occur along a toposequence. In agricultural crops, variations in fertility have a major impact on productivity and production costs, which makes mapping soil fertility important. The aim of this study was to evaluate the fertility of soils under different agricultural crops and different points of relief in a mountainous area located in the municipality of Aratuba-CE. Theoretical reference: Various studies that relate soil types to the landscape show that deeper and more developed soils generally occupy flatter, or top, relief positions, where conditions for water percolation are favored. (ARTUR et al., 2014; SOUZA JUNIOR and DEMATTÊ, 2008). Braga et al. (2015) verified the existence of a correlation between soil fertility and distribution of tree species. Methodology: The design was in a randomized block in a split-plot scheme, with the plots formed by four cultivation areas, namely: banana trees (A1), banana trees and cashew trees (A2); grass (A3); pigeon pea (A4); and a control area (A5), maintained with natural vegetation. The subplots were formed by three relief positions: top, middle and slope. The physical-chemical variables of the soil were analyzed: pH, potassium, magnesium, calcium,


INTRODUCTION
The formation of relief in a toposequence aids in the understanding of soil genesis, as it has the potential to bring about changes in soil structure, thereby influencing water movement within it.This also provides conditions for the occurrence of different pedogenetic processes, consequently leading to the differentiation of horizons that give rise to various soil classes across different segments of a slope.This can ultimately impact soil fertility (DAI et al., 2022;LAYZELL et al., 2012;KACHANOSKI, 2011).
Various studies linking soil types to the landscape demonstrate that deeper and more developed soils tend to occupy positions on flatter terrain or hilltops, where conditions for water percolation are favorable.On slopes, relatively young and transitional soils typically develop due to the rugged terrain hindering water percolation within the profile and promoting erosive processes.In valley areas, soils are generally hydromorphic due to the greater accumulation of water (ARTUR et al., 2014;SOUZA JUNIOR and DEMATTÊ, 2008) The variations in soil classes along a toposequence can indicate significant differences in their fertility, subsequently affecting the vegetative development of cultivated species in these areas.This underscores the importance of fertility mapping across different topographies to ensure proper management of natural resources and maximize plant production.
Supporting this assertion, Braga et al. (2015) observed a correlation between soil fertility and the distribution of tree species, as well as between soil fertility and floristic similarity among populations of these plant species within a forest fragment grown on various topographies.Importantly, they noted that the physical and chemical attributes of the soil varied according to changes in the terrain's topography.Ruckowski et al. (2023) similarly discovered that even slight variations in terrain within a slope area can influence soil attribute variability and the growth of eucalyptus (Eucalyptus grandis W. Hill ex Maiden).
Given that agricultural exploitation of toposequences and rugged terrain areas is increasingly common in regions where traditional small-scale agriculture plays a significant role in the economy, such as the case of Aratuba municipality in Ceará, Brazil, soil fertility mapping becomes essential as a tool for agricultural and environmental development planning.
In this context, the current study aimed to assess soil fertility at different points along a toposequence under various agricultural crops in the mountainous region of Aratuba, within the Baturité Massif, in the state of Ceará.

CHARACTERIZATION OF THE STUDY AREA
A área de estudo is the municipality of Aratuba, which is located within the Baturité residual wet massif in the state of Ceará, Brazil.This region is also classified as a highland marsh landscape (PORTO et al., 2004).It consists of isolated mountains with medium to low altitudes, featuring flat surfaces preserved between watersheds and steep slopes (BÉTARD, PEULVAST, & SALES, 2007).The geographical scope extends to other municipalities in the Ceará state, including Baturité, Pacoti, Palmácia, Guaramiranga, Mulungu, Capistrano, Itapiúna, Aracoiaba, Acarape, Redenção, Barreira, and Ocara.The Baturité massif is situated in the northeast-southwest direction, specifically between the coordinates Latitude S 4º 4' 30"; Longitude W 38º 52' 39.15, covering a total area of approximately 3,822 square kilometers (LIMA, 1983).
From a geological standpoint, the Baturité range is characterized by igneous and metamorphic rocks of the crystalline basement.The area exhibits significant signs of structural disruption, marked by the appearance of various steep terrains and indications of intense tectonism (SEMACE, 1992).A variety of lithologies are present, including granite, migmatite, gneiss, pegmatite, quartzite, limestone, diabase, amphibolite, and lepinites, with quartzite and granite being predominant at the summits and migmatite and gneiss on the slopes.Over time, this mosaic of rocks has proven more resistant to erosive processes when compared to the surrounding arid areas (PINHEIRO & SILVA, 2017).
Geomorphologically, the Baturité range can be described as a residual plateau, featuring a distinctive isolated massif with significant altitudes and steep topography, setting it apart from the surrounding area characterized by extensive flat surfaces (FERNANDES, SILVA, & PEREIRA, 2011).
Local altimetric levels range from 600 to 900 meters, with the highest point of the massif recognized as the second-highest altitude in the Ceará state, reaching an elevation of 1,115 meters (PINHEIRO & SILVA, 2017).The interplay between geological base, topography, climate, vegetation, and human activities has resulted in four predominant soil classes in the Baturité range: Dystrophic Red-Yellow Argisols, Eutrophic Red-Yellow Argisols, Eutrophic Lithic Neosols, Eutrophic Fluvic Neosols, and Chromic Luvosols (PEREIRA, SILVA, & RABELO, 2011).In the specific case of Aratuba, Argisols are prevalent, while Planosols also play a significant role.The vegetation in the region includes subdeciduous tropical rainforest, mainly found in higher altitudes, as well as open shrubland caatinga, which dominates lower areas (IPECE, 2017).
Aratuba lies within the watersheds of the Curu and Metropolitana Rivers, experiencing a high average annual rainfall of over 1,700 mm.Its groundwater can be categorized into two distinct hydrogeological domains: crystalline rocks and alluvial deposits.Crystalline rocks are predominant, representing what is commonly known as a fissure aquifer.The occurrence of groundwater in these rocks relies on secondary porosity, primarily fractures and fissures, ___________________________________________________________________________ Rev. Gest.Soc.Ambient.| Miami | v.17.n.9 | p.1-12 | e04155 | 2023.4 leading to irregular, discontinuous, and limited reservoirs, often resulting in low-yield wells and occasionally saline waters (CUNHA et al., 2023).
Alluvial deposits consist of recent sandy-clay sediments along the banks of major rivers and streams, providing a significant water source and substantial water flow.
The local economy of Aratuba is primarily centered around agriculture, with a focus on subsistence crops such as beans, corn, and cassava.Secondary agricultural activities include monocultures of cotton, banana, avocado, sugarcane, cashew, and various fruits.The region is also engaged in extensive livestock farming, including cattle, sheep, goats, pigs, and poultry.Extractive activities involve the production of charcoal, logging for firewood and fence construction, as well as activities involving the oiticica, babassu, and carnauba plants (VIEIRA et al., 2023).

MATERIALS AND METHODS
The study area was a private rural property known as "Sítio Dom Bosco," situated within the larger area of "Sítio Camará," located in the rural zone about 3.0 km away from the center of Aratuba-CE.
According to IPECE data ( 2017), the study area features a climate classified as Subhumid Subtropical, with an average temperature ranging from 24°C to 26°C.The rainy season occurs from January to May, with an annual average precipitation of 1,753.1 mm.
The property under study covers an approximate area of 100 hectares.The terrain is characterized by rugged relief, with flat areas (summits), followed by gently sloping sections known as "meio" (mid-slopes), and steeper slopes referred to as "encostas" (hillsides).These areas are occupied by both perennial and annual agricultural crops, including beans, corn, cassava, cashew trees, mango trees, passion fruit vines, banana plants, and native vegetation.
To evaluate soil fertility at different points within this relief, five areas were initially selected, each ranging in size from 0.1 to 0.19 hectares.These areas were located at distinct points on the terrain but were matched based on similar topography, encompassing summit, hillside, and valley zones.The five designated areas featured different crops with varying cultivation durations: banana plants (5 years of cultivation), pigeon pea (2 years of cultivation), cashew trees (10 years of cultivation), Napier grass (5 years of cultivation), and a naturally regenerated native vegetation area (50 years of fallow).The figure below shows the field location of these areas, highlighting the crops planted in each of them.
[Note: As a text-based AI, I cannot directly process or display images, so I can't interpret or analyze figures.If you need assistance with further information or analysis, please provide a description of the content.]The field areas were defined, and an experimental design was established using a randomized complete block design in a split-plot scheme.The main plots were formed by the five crops: A1 (Banana plants), A2 (Banana plants and cashew trees), A3 (Napier grass), A4 (Pigeon pea), and A5 (Native forest); and the subplots were divided based on the positions in the relief: summit, mid-slope, and hillside.This resulted in a total of sixty experimental units.
For each treatment, soil samples were collected in triplicate.Each sample was composed of four simple samples collected from a depth of 0 to 20 cm, using a straight shovel and a ruler to assist in depth delimitation.After collection, the samples were placed in plastic bags and taken to the Soil and Water Laboratory for Irrigation at the Federal Institute of Education-IFCE/Campus Sobral.They were dried in a forced-air oven, ground, and prepared for subsequent fertility analysis.
The following soil fertility determinations were conducted: soil pH in water (1:2.5 ratio), exchangeable cations calcium (Ca2+) and magnesium (Mg2+), extracted using potassium chloride (KCl mol L-1) with a complexometric method; exchangeable aluminum (Al3+), extracted with potassium chloride using volumetric analysis; available potassium (K+) and phosphorus (P), extracted with Mehlich 1 solution, and potassium and sodium determined using flame spectrophotometry and phosphorus using spectrophotometry.Additionally, potential acidity was determined, extracted with calcium acetate buffered at pH 7.0, and determined using volumetric analysis.Protocols from Embrapa (2017) were followed for all the aforementioned determinations.
The obtained results were subjected to analysis of variance using the F-test and, when significant, underwent Tukey's test (P<0.05).The analysis was conducted using R, version 4.

RESULTS AND DISCUSSION
The analysis of variance data for the soil physicochemical parameters are presented in Table 1.For the "Area" (main plots), significant effects were found in all analyzed variables, except for potassium (K).Regarding the relief positions (subplots), the non-significant variable was hydrogen + aluminum (H + Al).In the interaction between "Area" and relief positions, all variables exhibited significant effects (Table 1).
Soil pH values ranged between 4.67 and 5.51, falling within the acidic range on the pH scale (EMBRAPA, 2017) (Table 2).The lowest pH values were observed in the Napier grass area (A3), situated on the hillside, with a pH of 4.67.This pH value was significantly lower compared to the other areas and also in comparison to other relief positions.Similar results were found for the "Mid-slope" region of the same treatment, with a pH of 4.96, which was significantly lower compared to other crops.
[Note: If you have more specific questions about the results or discussion, or if you would like me to elaborate on any particular aspect, please let me know.
From the perspective of nutrient availability, the values found may indicate a reduction in the availability of certain soil nutrients in these areas, as acidic pH can affect nutrient availability.pH ranges between 6.0 and 7.0 are considered most suitable for the cultivation of most crops, and pH can be easily corrected using soil amendments such as lime (AGEGNEHU; YIRGA; ERKOSSA, 2019).
However, it's important to note that different species have varying degrees of tolerance to soil acidity and alkalinity.Uchida and Hue (2000) demonstrated in their work that forage legumes, like the pigeon pea in area A4, can have some strains of symbiotic nitrogen-fixing bacteria affected by pH values below 6.0.On the other hand, plants like potatoes tend to favor growth at pH levels between 5.0 and 5.5.
Soil calcium values ranged from 0.75 to 3.01 cmolc kg-1, generally being higher in the summit region (Table 2).Similar to the case of pH, the Napier grass area (A3) on the hillside exhibited lower absolute calcium values, although these values were statistically equal to those observed in treatments involving banana plants, pigeon pea, and native forest (A5) areas, respectively.A similar situation occurred in relation to the calcium content of treatment A3 in the mid-slope area.This fact might be related to the lower pH values in these treatments.
[Note: If you have more questions or if you'd like further elaboration on specific aspects, please feel free to ask.____________________________________________________________________________________________________________________ Rev. Gest. Soc. Ambient. | Miami | v.17.n.9 | p.1-12 | e04155 | 2023.7 Table 1 -Summary of Analysis of Variance (ANOVA) for soil physicochemical variables.* Significant F-value at the 5% probability level (P < 0.05) ** Significant F-value at the 1% probability level (P < 0.01) ns -Non-significant F value (P > 0.05) Source: the authors Lange et al. (2021), in their study on the relationships between soil Ca:Mg and the development of soybeans and corn, also mention the direct correlation between soil pH and calcium and magnesium content.They found that soil with an initial pH of 4.37 had its pH increased to 5.4 after 90 days of incubation with a lime application rate of 8 tons per hectare, and to 5.7 after 200 days.
Magnesium levels ranged from 0.887 to 2.363 cmolc kg-1, with significantly higher values in the summit region and also in the Napier grass area (A3).The same parameter was significantly lower in the mid-slope and hillside positions for the same treatment, as well as compared to other areas (Table 2).In terms of agronomic aspects, the Ca:Mg ratio is considered more important than the absolute quantities of these nutrients, with a 3:1 ratio being deemed the most appropriate for the proper growth of most crops.
However, Lange et al. (2021) found in their experiments involving different Ca:Mg ratios throughout the soybean and corn growth cycles that for corn, the 06:01 ratio was unfavorable as it reduced the mass of one thousand grains and affected the availability of potassium due to competitive inhibition.For soybeans, Ca:Mg ratios of 02:01, 01:01, and 05:01 all resulted in higher grain yields compared to other evaluated Ca:Mg ratios.
A similar trend was observed for potassium.Values found in the summit positions were generally significantly higher compared to the mid-slope and hillside regions.There were also significant variations among the areas.For example, the Napier grass area (A3) showed higher potassium levels in the summit region but lower values in the hillside area.
It's important to note that potassium, being a monovalent nutrient present in both primary and secondary minerals, can be found in soils with varying degrees of weathering, depending on the parent material.Another factor influencing the quantity of this nutrient in the soil is its extraction by crops.Literature has shown that banana plants have high potassium extraction rates, with potassium being the most required nutrient for this species compared to nitrogen, which is the most demanded macronutrient for most plants (ANDRADE et al., 2020).This could partially explain the lower potassium values in areas A1 and A2, where banana plants and banana plants combined with cashew trees are cultivated, respectively, especially in the summit and mid-slope regions (Table 2).