Potassium distribution in soil profiles under no-tillage system

ABSTRACT

Potassium (K) vertical mobility in soils has often been overestimated and used as a rationale for recommending the broadcast application of this nutrient in fertility management programs, especially in soils with low cation exchange capacity (CEC). This study aimed to evaluate the vertical distribution of K in two land uses: areas with natural vegetation and crop fields managed under no-tillage (NT) fertilized with K. For this purpose, 49 soil profiles from the Brazilian subtropical state of Rio Grande do Sul were sampled, comprising 45 profiles from areas under NT management and four profiles from sites with natural vegetation. Soil samples were collected in 19 very thin layers: 1 cm layer in the first 10 cm, 2.5 cm layer from 10 to 25 cm, and 5 cm layer from 25 to 40 cm. Sampling sites were then grouped according to their CEC, categorized as < 7.5, 7.6-15.0, and 15.1-30.0 cmol_c dm^-3. Both crop fields and natural fields exhibit a similar vertical gradient model, characterized by a strong accumulation of K in the soil within the uppermost centimeters. This gradient is notably enhanced by the addition of K fertilizers, leading to a substantial portion of K becoming inaccessible to the root system. The optimal level of available K for the topsoil soils was found within an average range of 4 to 12.5 cm of soil depth. Consequently, K fertilization resulted in two main outcomes: (i) an excess of K in the upper soil layers, which increases the potential for K loss through surface erosion and runoff, and (ii) a limited migration of K towards the deeper soil layers until reaching the root growth zone. There is an urgent need to: (a) reaffirm the official recommendations of public agencies that the replacement of K exported by crops should be carried out in the furrow, along the sowing line, and as deep as possible; and (b) reconsider the diagnostic soil layer for assessing the status of K availability in soils under NT management.

Keywords
available potassium; vertical potassium distribution; potassium movement

INTRODUCTION

Potassium (K) fertilization plays a pivotal role in ensuring robust crop growth in strongly weathered soil. The increasing demand for food, fiber, and energy is driving up the global consumption of K fertilizers (^{USGS, 2021}). In 2020, the world consumption of K fertilizers was estimated at 34.4 million Mg of K (^{USGS, 2021}). Brazil ranks as the second-largest consumer of this fertilizer, accounting for 15.3 % of the global demand (^{MME, 2019}). However, the Brazilian reserves of K are estimated at only 1.9 million Mg, while global reserves are estimated at 210 billion Mg of K (^{USGS, 2021}).

Conventional agricultural practices, such as annual soil disturbance through tillage, typically homogenize the distribution of available nutrients up to a depth of 0.20-0.30 m (^{Tshuma et al., 2021}), which can be advantageous for plant root systems. However, this practice also leads to several detrimental effects, including soil erosion, loss of nutrient and organic matter, and selective loss of clay mineral (^{Cogo et al., 2003}; ^{Panachuki et al., 2011}; ^{Tiecher et al., 2017}, ²⁰²⁰). In contrast, the no-tillage system (NT) involves fertilizers being either banded near the seed during planting or, more commonly, broadcasted on the soil surface (^{Lopes and Guilherme, 2000}; ^{Derpsch et al., 2010}). Over time, NT results in accumulation and enrichment of P and K in the soil surface (^{Rheinheimer and Anghinoni, 2001}; ^{Moreno et al., 2006}; ^{Ernani et al., 2007}; ^{Ferreira et al., 2009}; ^{Kaminski et al., 2010}; ^{Rheinheimer et al., 2019}; ^{Oliveira et al., 2022}). This nutrient accumulation primarily occurs within the first few centimeters of soil, posing a high risk for transfer to water bodies via surface runoff (^{Bertol et al., 2007}; ^{Santos et al., 2020}), leading to economic losses for farmers (on-site effects) (^{Bertol et al., 2011}) and water pollution (of-site effects) (^{Bortoluzzi et al., 2013}; ^{Tiecher et al., 2022}).

From an agronomic perspective, broadcast fertilization is typically assumed to facilitate nutrient migration to the crop root system. However, certain ions, such as H₂PO₄^-/HPO₄^-2, K⁺, Zn²⁺, and Cu²⁺, tend to form high-affinity complexes with the solid surface of the soil, thereby limiting their availability and downward movement. Moreover, ion migration relies on water percolation, but the amount of these nutrients migrating in the soil profile is often minimal and insufficient to meet crop demands. In natural biomes, there are instances of negative rates of nutrient migration within the soil profile, as plant uptake from deeper soil layers surpasses downward nutrient movement, resulting in K accumulation in the upper few centimeters of the soil profile (^{Bortoluzzi et al., 2005}). Beyond that, K distribution within the soil profile can be influenced by several soil parameters, including mineralogy (^{Hinsinger et al., 1992}; ^{Bortoluzzi et al., 2005}; ^{Barré et al., 2008}; ^{Firmano et al., 2020}), cation exchange capacity (CEC), liming, clay content, cropping system (^{Ambrosini et al., 2022}), and the applied K rate (^{Hinsinger et al., 1992}; ^{Raheb and Heidar., 2012}). ^{Bortoluzzi et al. (2006)} proposed a mathematical model, utilizing multiple linear regression, to estimate the CEC and the contribution of permanent charges and pH-dependent charges (pH, clay, and organic matter content) in subtropical soils. They found soil organic matter contributed approximately 54, 45, and 39 % of CEC at pH 7 in the Ap1a, Ap1b, and Ap2 horizons, respectively, decreasing to only 14.1 % in the B horizon. Consequently, within soil sub-layers of the A horizon, where there is little variation in clay content and type, the CEC decreases along the soil profile, leading to a reduction in the capacity to store bioavailable K.

Potassium is present in the soil in two main compartments that are in equilibrium: the soil solution and adsorbed to the solid phase of the soil. Plants absorb K in its ionic form (K⁺) from the soil solution, with diffusion being the main transfer process responsible for the movement of K⁺ from the soil solution to the cell membranes of the roots (^{Barber, 1995}; ^{Simonsson et al., 2007}). Potassium application must be close to the root system for optimal uptake. Excessive K rate application increases soil K levels, in chemical forms that are less or more available to plants, such as labile K and non-exchangeable K. Non-exchangeable K form promotes occlusion within the interlayers of 2:1 clay mineral (^{Moterle et al., 2016}, ²⁰¹⁹). Additionally, the labile fraction increases, resulting in luxury absorption by plants and potentially enhancing K migration into the soil profile (^{Bell et al., 2021}). Potassium balance in the NT system consists of the total amount of K inputs through fertilization minus system losses (K exported by crops and K adsorbed by 2:1 clay minerals). This balance significantly influences K distribution in the soil profile, and K migration is primarily determined by the soil’s chemical K status rather than pedogenic factors like clay migration into the profile (^{Bortoluzzi et al., 2008}). In subtropical soils containing 2:1 clay mineral, K accumulation in the topsoil surface at the expense of migration into deeper layers is even more typical (^{Bell et al., 2021}). However, the prevalent practice of broadcast K fertilization in NT raises concerns, as the superficial accumulation of K may not align with crop root system distribution, potentially decreasing crop yields and causing economic and environmental damage.

We hypothesized when K is applied to the topsoil in the NT, it causes the accumulation of K in the superficial layers, while the subsequent K migration in the soil profile is an overestimated process. Broadcast fertilization of K in the NT lacks scientific justification and may result in economic damage to the country’s agribusiness due to reduced crop productivity. This study aims to evaluate the distribution of K in contrasted soil profiles from NT that received K broadcast fertilizer.

MATERIALS AND METHODS

This study was carried out using soil samples collected in the southern region of Brazil, in the state of Rio Grande do Sul. Sampling sites primarily consisted of cropfields (n = 45) cultivated with annual crops under NT system (Figure 1), with a few additional sites sampled in areas under natural vegetation (NV) serving as reference backgrounds (n = 4). Sampling covered two traditional soybean-growing regions under NT (Middle plateau region and Missões region) and two regions where grain crops have recently expanded, also under NT (Cental depression region and Serra Gaúcha region). Cropfields sites were selected randomly on commercial farms, with permission obtained from farmers whenever possible. Four NV sites were chosen in areas adjacent to the crop field sites, one in each region, as indicated in figure 1. Environmental characteristics of the sampling sites are detailed in table 1.

Figure 1
Sampling sites in the state of Rio Grande do Sul, Brazil, and the typical landscapes of the physiographic regions sampled.

Table 1
Environmental and pedogenic characterization of soil sampling sites in Southern Brazil, state of Rio Grande do Sul

Feature(1,2,3)	Missões region	Middle plateau region	Central depression region	Serra Gaúcha region
Climate	Cfa	Cfa	Cfa	Cfb
Average annual temperature (°C)	21	21	21	19
Average annual precipitation (mm)	1250	1450	1450	1650
Geology	Basalt - Group São Bento	Basalt - Group São Bento	Sedimentary sandstones - Rosário do Sul	Dacite/Basalt - Group São Bento
Soil classification	Ferralsols (Latossolos)	Ferralsols (Latossolos)	Ultisols (Argissolos)	Ferralsols (Latossolos)

• ⁽¹⁾
  ^{Pessoa (2017)}.
• ⁽²⁾
  ^{Scherer et al. (2000)}.
• ⁽³⁾
  ^{Streck et al. (2018)}.

According to farmer records, the 45 sites sampled in cropfields have been managed under NT system for 4 to 31 years. These fields receive K fertilizer either in the sowing line or broadcast on the soil surface. These sites were classified based on the average CEC in the 0.00-0.40 m soil layer. Annual amount of K added varied between 50 and 120 kg ha^-1, in accordance with regional fertilization guidelines (CQFS-RS/SC, 2016). Of the 45 cropfield sites sampled, only two were planted with corn in the summer season; while the remaining were all planted with soybeans. Additionally, in 12 of these sites, farmers practiced a soybean/corn succession. During the fall/winter season, most areas were left fallow or reseeded with ryegrass + oats; 18 were cultivated with wheat; two were sporadically cultivated with canola; and two with barley. In 33 fields, fertilizers were routinely applied to the sowing line below the seeds. Twelve fields received annual surface applications of K, and in 11 of these, farmers broadcasted applications of NPK. Farmers preferred to remain anonymous regarding the fertilizer application method used in two sites.

Due to potential inaccuracies in the information provided by farmers and the limited sample size to represent each factor (K application method – broadcast or banding, time using NT, crop rotation, annual K rate), we have opted not to address these aspects in our study. Additionally, other factors such as different soil types and mineralogy could confound these comparisons. While discussing these differences may be appealing, we lack an experimental design or sufficient sampling representation to make such comparisons reliably. However, these aspects could certainly be explored in future studies. Therefore, we have focused our study on conveying a single clear and robust message that we can motivate future research and provide insights into the dynamics of potassium (K) in Brazilian soils: there is a significant stratification of K in depth in soils cultivated under no-tillage, regardless of soil cation exchange capacity (CEC).

Soil samples were taken from 19 layers within the soil profile (horizon A): 0-1, 1-2, 2-3, 3-4, 4-5, 5-6, 6-7, 7-8, 8-9, 9-10, 10-12.5, 12.5-15, 15-17.5, 17.5-20, 20-22.5, 22.5-25, 25-30, 30-35, and 35-40 cm. A trench of 0.40 m in width and 0.60 m in depth was opened across the seeding line to obtain these samples. Soil samples were dried in a forced circulation oven at 55 °C until a constant mass was achieved. After drying, soil samples were ground in a mechanical mill. Subsequently, the soil was sieved through a 2 mm mesh sieve and placed in a hermetically sealed flask for analysis.

Soil samples underwent to several chemical determinations, such as: active acidity (pH in water - ratio 1:1 (v/v)); potential acidity - estimated using the equation proposed by ^{Kaminski et al. (2001)} [H⁰ + Al³⁺ = e^{10.665–1.1483∗TSM}/10] and adopted by CQFS-RS/SC (2016); TSM index was measured by ^{Toledo et al. (2012)} method; and exchangeable Ca²⁺, Mg²⁺, and Al³⁺ extracted by KCl 1.0 mol L^-1 (soil: extractant ratio 1:25; ^{Tedesco et al., 1995}). Exchangeable Al was determined by titration with 0.0125 mol L^-1 NaOH solution, while Ca²⁺ and Mg²⁺ were determined using atomic absorption spectrophotometry. Cation exchange capacity at pH 7.0 (CEC_pH7.0) was calculated as the sum of H⁰ + Al³⁺ + Ca²⁺ + Mg²⁺ + K⁺.

Soil available K content was extracted using Mehlich-1 solution [(HCl 0.05 mol L^-1 and H₂SO₄ 0.0125 mol L^-1 – ^{Mehlich (1953)}]. Briefly, 3.00 dm³ of soil was transferred to a snap-cap with a screw cap, and 30 mL of Mehlich-1 extractant solution was added. Containers were closed and stirred for 5 min. Afterward, the containers were uncapped and left to stand for 16 h. Then 10 mL of the supernatant was pipetted off, and the K content was measured using emission spectrophotometry.

Descriptive statistics were conducted to determine the mean values and the confidence intervals (95 %) for the K contents extracted by Mehlich-1. The data was segregated into three CEC classes: <7.5, 7.6-15.0, and 15.1-30.0 cmol_c dm^-3. Soil critical level of K for these classes are 60, 90, and 120 mg dm^-3, respectively, for annual crops, based on the regional guidelines provided by the ^{CQFS-RS/SC (2016)}. Subsequently, the available K data was classified into availability classes.

RESULTS AND DISCUSSION

Vertical distribution of available K

All sampled soil profiles, both in cropfields under NT and under natural vegetation, regardless of soil CEC, exhibited a pronounced gradient of available K with depth (Figures 3a, 3c, and 3e). These findings contrast with some reports suggesting potential K percolation in subtropical soils (^{Neves et al., 2009}). If a chemical element exhibits high vertical mobility in the soil, potentially leading to loss through percolation, two scenarios for its vertical distribution can be expected: (i) homogeneous distribution, characterized by a uniform nutrient distribution throughout the soil profile, typically resulting in concentrations close to zero as observed in nitrate concentration assessments (^{Ernani et al., 2002}) or chlorine studies (^{Liu et al., 2021}); or (ii) accumulation with depth, in which the chemical element is distributed throughout the soil profile with increasing accumulation as the sampled depth increases, as observed for sulfate (^{Eckert et al., 2023}; ^{Costa et al., 2022}). However, neither of the two possibilities was found in the present study; on the contrary, the vertical distribution of available K in the studied soil profiles is inverse to the intense percolation process, even with highly contrasting physical and chemical characteristics, as indicated by the CEC of the studied soils varying from 4.6 to 30.0 cmol_c dm^-3 (Figure 2). Moreover, the substantial K accumulation in the uppermost centimeters of soil (Figure 3) suggests a high risk of nutrient loss due to erosion and surface runoff. Indeed, the few existing experiments that concretely evaluate K losses from the system clearly demonstrate that K losses through percolation are negligible compared to losses through surface runoff, even in sandy soils with low CEC. For instance, the five-year monitoring of K losses in a Ultisol with 170 g kg^-1 clay, 9.6 cmol_c dm^-3 of CEC, and a very gentle slope (4 %) showed only 0.3-0.7 % of the total applied K was lost through leaching (below 0.60 m depth) (^{Girotto et al., 2013}), while losses through surface runoff ranged between 9 and 17 % of the total K applied (^{Ceretta et al., 2010}).

Figure 2
Distribution of no-till cropfield soil profiles according to the cation exchange capacity (CEC) range for the 0.00-0.20 m soil layer.

Figure 3
Vertical distribution of soil-available K content extracted with Mehlich-1 in soils with CEC <7.5 cmol_c dm^-3 (a, b), soils with CEC between 7.6-15.0 cmol_c dm^-3 (c, d), soils with CEC between 15.1-30.0 cmol_c dm^-3 (e, f), and mean distribution of soil-available K and 95 % confidence interval in no-till cropfields sites with different CEC classes (g). For the CEC class <7.5 cmol_c dm^-3, it could not sample soil under natural vegetation.

Some recent studies also demonstrate this vertical gradient of K availability in the soil, but typically, they utilize much thicker soil layers, such as 0.05 or 0.10 m (^{Calegari et al., 2013}; ^{Tiecher et al., 2017}; ^{Almeida et al., 2021}). Our study took it a step further and showed there is stratification of the soil available K, even in the top 0.05 or 0.10 m layers. Available K content is at its highest in the first centimeter, demonstrating it is an element with exceptionally poor mobility in subtropical soils (Figure 3). Only two soils demonstrated different behavior, where the maximum available K content was found in the layer between 2 and 4 cm deep (Figures 3c and 3e), and these are precisely soil profiles where fertilization is done in the furrow of the sowing line. Nevertheless, most soils that had fertilization in the furrow displayed their highest levels of available K in the top 0–1 cm of soil, highlighting the significant role that plants play in K cycling. Plants uptake the nutrient from the entire soil profile explored by the root system, store it in their aboveground biomass, and later, as there is no soil disturbance and incorporation of plant residues, K accumulates in the top few centimeters of soil (^{Costa et al., 2009}; Figure 3). Because of the high capacity of plants to cycle K and the superficial addition of K fertilizers during crop fertilization, the natural gradient of K distribution in the profile inexorably worsens, keeping most of this nutrient out of reach for the roots.

Vertical distribution pattern of soil-available K in the reference areas (under native vegetation) already demonstrates negligible K migration in the profile, with a negative balance (i.e., migration is lower than plant uptake in depth and accumulation on the surface), a behavior that is accentuated in NT cropfields (Figures 3d and 3f). However, due to K fertilization, the cultivated areas consistently exhibited a higher available K content than the native areas in the first 0.10 m of soil (Figures 3d and 3f). Even so, the available K content in the soil in the 0.00-0.10 m layer in native vegetation areas was above the critical level established for grain crops under NT.

In areas cultivated under NT, the influence of the CEC value on the vertical distribution of K (Figure 3g) is noticeable, especially in the first centimeters. Soils with CEC <7.5 cmol_c dm^-3 exhibit lower K contents and a more drastic decrease in K content than soils with CEC >7.6 cmol_c dm^-3 up to approximately 0.20 m deep. Although some soil profiles with CEC between 15.1-30.0 cmol_c dm^-3 display more discrepant K contents, the 95 % confidence interval for K content throughout the soil profile is similar to those soils with CEC between 7.6- 15.0 cmol_c dm^-3 (Figure 3g). This behavior may be attributed to the similar K buffering capacity of soils with CEC higher than 7.6 cmol_c dm^-3. Despite the official fertilization guidelines in South Brazil (^{CQFS-RS/SC, 2016}) separating these soils into different CEC classes, recent research by ^{Souza Junior et al. (2022)}, using 23 soils from the states of Santa Catarina and Rio Grande do Sul, demonstrates the K buffer capacity increases linearly with increasing CEC up to 8.5 cmol_c dm^-3, but remains constant for soils with CEC ranging from 8.5-15.5 cmol_c dm^-3. These findings combined with our results, underscore the need for future studies evaluating the K buffering capacity in soils with CEC greater than 15.0 cmol_c dm^-3.

Decrease in available K content at depth cannot be attributed to a lower soil CEC at depth (Figure 4). While it is true that deeper soil layers generally contain less organic matter, which contributes to CEC, other factors also play significant roles. In some soils, there might be an increase in clay content with depth, along with less weathered minerals with higher CEC. This led to similar or slightly smaller CEC values over the first 0.40 m, and, in some cases, even an increase in CEC with increasing depth (Figure 4). Therefore, as the saturation of K in the CEC tends to decrease with depth, this indicates the decrease in K content at depth is not directly proportional to changes in soil CEC.

Figure 4
Vertical distribution of cation exchange capacity (CEC) at soil pH 7.0 and K saturation in the CEC for soils with <7.5 cmol_c dm^-3 (a, b), soils with CEC between 7.6-15.0 cmol_c dm^-3 (c, d), and soils with CEC between 15.1-30.0 cmol_c dm^-3 (e, f).

Availability of K in depth

Soil K availability can be evaluated based on critical levels established in calibration studies for predefined diagnostic soil layers. Current guidelines for the states of Santa Catarina and Rio Grande do Sul in southern Brazil establish the following critical levels of K in soil extracted with Mehlich-1 for grain production: 60, 90, and 120 mg dm^-3, for soils with CEC <7.5, 7.6-15.0, and 15.1-30.0 cmol_c dm^-3, respectively (^{CQFS-RS/SC, 2016}). These critical levels are established for the 0.00-0.10 m layer for NT, and for the 0.00-0.20 m layer for conventional cultivation systems or for the implementation of NT system. In areas under NT, it is possible to use these critical levels also to monitor whether the 0.10-0.20 m soil layer remains above these reference values. Therefore, the soil-available K contents will be compared for each layer individually and considering the possible diagnostic layers according to ^{CQFS-RS/SC (2016)}.

In soils with CEC <7.5 cmol_c dm^-3, only in the 0.00-0.04 m soil layer (or in the 0.00-0.03 m layer when considering the 95 % confidence interval) exhibited an available K content above the threshold level when comparing soil layers individually (Figure 3b). For soils with CEC between 7.6-15.0 cmol_c dm^-3, the available K content was above the critical level up to 12.5 cm deep (or up to 9 cm deep when considering the 95 % confidence interval) (Figure 3d). For soils with CEC between 15.1-30.0 cmol_c dm^-3, the available K content was above the critical level up to 17.5 cm deep (or up to 8 cm deep when considering the 95 % confidence interval) (Figure 3f).

When considering the diagnostic layers, it is noted that the soil in the 0.00-0.10 m layer, 87 % of the sampling sites in NT cropfields have a K content above the critical level (Figure 5d). For soils with CEC between 7.6-15.0 cmol_c dm^-3, this percentage reaches 96 % (Figure 5b). A similar trend occurs when evaluating the 0.00-0.20 m layer. However, as the gradient of availability in depth is very strong, there is dilution due to lower K contents in the subsurface soil (i.e., 0.10-0.20 m). Therefore, only 66 % of sampling sites in NT cropfields had K content above the critical level (Figure 5d). On the other hand, considering only the 0.10-0.20 m layer, only 27 % of sampling sites in NT cropfields showed K content above the critical level. This percentage is even lower for soils with CEC <7.5 cmol_c dm^-3, which was only 11 % (Figure 5a).

Figure 5
Interpretation of soil-available K content in the 0.00-0.10, 0.10-0.20, 0.20-0.40, and 0.00-0.20 m layer of no-till cropfields in Southern Brazil, for soils with CEC <7.5 cmol_c dm^-3 (a), 7.6-15.0 cmol_c dm^-3 (b), 15.1-30.0 cmol_c dm^-3 (c), and for the entire set of soils sampled (d). Interpretation according to the ^{CQFS-RS/SC (2016)} guidelines.

Depletion of available K reserves in the deeper soil layers (0.10-0.40 m) and the enrichment in the superficial layers (up to 0.05-0.10 m) indicate a mismatch between the site of the fertilizer application in NT and the distribution pattern of the crop root system. This highlights the need to implement strategies for redistributing K within the soil profile. During crop seasons, the first centimeters of soil are precisely the first to lose moisture, resulting in decreased diffusion of K present in the soil solution to the cell membranes of the roots. Consequently, even if the soil has high or very high availability of K in the superficial diagnostic layers, it may not adequately meet the crop’s needs. Furthermore, acidity in deeper layers under NT further hinders root growth and access to K in depth (^{Tiecher et al., 2023}). Studies have indicated crop yields are associated with a higher concentration of nutrients in deeper soil layers or a better distribution of nutrients in the soil profile (^{Nora et al., 2014}; ^{Corassa et al., 2018}). This behavior has been observed for phosphorus as well (^{Bellinaso et al., 2021}). These findings underscore the urgent need to develop recommendations that consider the accelerated stratification of K in the soil profile of NT cropfields, including diagnostic subsurface layers, to better infer the real accessibility of K by plants. Furthermore, it is crucial to reduce K use in surface broadcasting and promote fertilizer use in the furrow and in-depth to increase K use efficiency and mitigate potential losses due to erosion and surface runoff.

This study offers valuable insights into the complex dynamics of K availability in subtropical soils managed under NT and native vegetation. Vertical distribution pattern of K, characterized by significant accumulation in superficial layers and depletion in deeper layers, highlights the discrepancy between fertilization practices and the actual requirements of plants. The pronounced stratification of K, especially in the top few centimeters of soil, poses a considerable challenge in ensuring crops have adequate access to this nutrient throughout their growth cycle. The disparity between the K availability in the superficial diagnostic layers and the plant requirement highlights the urgency of revising fertilization strategies. Our findings emphasize the urgency of developing management guidelines that consider the pronounced K stratification in the soil profile, including subsurface diagnostic layers, to accurately assess K accessibility by plants in NT systems. Furthermore, it is imperative to reduce indiscriminate K application on the soil surface and promote deep fertilizer application in the seeding line. This approach aims to increase the K utilization efficiency and mitigate potential losses through erosion and surface runoff.

To support new interpretations of K availability and refine recommendations for dosages and methods of K application, conducting new K calibration studies is essential. These studies should involve corrections of soil K levels for different volumes or soil layers to ensure accuracy in K assessments. Moreover, future investigations on the vertical distribution of K should expand to other regions, such as the tropical region of Brazil, particularly focusing on the Center-West and the region encompassing the states of Maranhão, Tocantins, Piauí, and Bahia. Additionally, increasing the sampling density is crucial to facilitate comparisons among various land use histories in areas under NT. Factors such as application methods, annual doses of K, and the duration of direct planting adoption should be considered. Furthermore, exploring the diversity of soil types and mineralogy across different regions will provide valuable insights into the variability of K dynamics in Brazilian soils. These comprehensive studies will contribute significantly to advancing our understanding of K management practices and optimizing agricultural productivity and K use efficiency in Brazil.

CONCLUSION

Application of fertilizers containing K, particularly when distributed over surface residues, exacerbates the natural model of a decreasing gradient of bioavailable K in the soil profile. Soils with cation exchange capacity (CEC) lower than 7.5 cmol_c dm^-3 exhibit lower K contents and a more pronounced decrease in K content in soil profile (>0.20 m) compared to soils with CEC greater than 7.6 cmol_c dm^-3. The optimal level of available K for topsoil soils was found within an average range of 4 to 12.5 cm of soil depth. Consequently, K fertilization resulted in three main outcomes: (i) an excess of K in the upper soil layers, which increases the potential for K loss through surface erosion and runoff, (ii) limited migration of K towards the deeper soil layers until reaching the root growth zone, and (iii) a suggestion to consider changing the diagnostic soil layer for estimating the status of K availability in soils under NT.

ACKNOWLEDGMENTS

We are grateful to the University of the State of Santa Catarina, Center for Agroveterinary Sciences (UDESC-CAV); Federal Rural University of Pernambuco (UFRPE); Luiz de Queiroz College of Agriculture, University of São Paulo (ESALQ/USP) for allowing the development of part of the laboratory analyses.

• How to cite: Artuso DR, Moterle DF, Santos DR, Tiecher T. Potassium distribution in soil profiles under no-tillage system. Rev Bras Cienc Solo. 2024;48:e0230125. https://doi.org/10.36783/18069657rbcs20230125

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Edited by

Editors: José Miguel Reichert https://orcid.org/0000-0001-9943-2898 and Edicarlos Damacena Souza https://orcid.org/0000-0003-3719-8615

Publication Dates

• Publication in this collection
  04 Oct 2024
• Date of issue
  2024

History

• Received
  23 Oct 2023
• Accepted
  11 Apr 2024