4.2. Physico-Hydrical Properties in PLANOSOLS
Despite these variations, Bd remained below the critical threshold across all treatments and soil layers. According to [
17], the critical Bd values range from 1.30 to 1.40 g cm
−3 for clay soils, 1.40 to 1.50 g cm
−3 for clay loam soils, and 1.70 to 1.80 g cm
−3 for sandy loam soils. Nonetheless, the SG, NV, and CC treatments in the 0.00–0.10 m layer had relatively low densities of 1.38, 1.31, and 1.23 g cm
−3, respectively.
Regarding Bdmax, according to [
30], when Bdmax values approach the critical range of 2.3–2.9 g cm
−3, the soil’s physical and structural quality may be impaired, particularly concerning dynamic processes such as aeration, hydraulic conductivity, and root growth.
Worth highlighting is the maximum bulk density (Bdmax), representing the upper limit of soil compaction, showed a statistically significant variation among the evaluated treatments.
As for the soil’s relative density, according to [
15], it normally ranges from 0 to 1, and when values exceed 0.75, root development may be impaired, which may have contributed to the poor tree development in the I+SG treatments. In this context, it was observed that in all treatments except for the CC treatment in the 0.00–0.10 m layer, which had a statistically significant value of 0.66, relative density values were close to the critical threshold of 0.75, and in some cases, well above it. When analyzing the treatments, all values in the 0.10–0.20 and 0.20–0.30 m layers exceeded the established threshold, except for the SB+GC treatment, which recorded a value of 0.740. The highest mean value was observed in the I+SG treatment in the 0.10–0.20 m depth, with a relative density of 0.83.
DC is calculated based on relative bulk density, using 75% as the critical upper threshold for soil compaction. In nearly all treatments, DC values exceeded this threshold, particularly in the 0.10–0.20 m and 0.20–0.30 m layers. In the 0.00–0.10 m layer, however, DC remained below the critical threshold in the CC treatment (66.3%) and in the SG and NV treatments, with values of 72.5% and 72.3%, respectively. This confirms the compaction tendency of these horizons in Planosols, leading to slow permeability and the formation of a periodically perched water table throughout the year [
11].
Saturated hydraulic conductivity (Kθ) did not show significant differences among treatments within each soil layer, mainly due to the high variability observed for this variable. The highest mean values in the 0.00–0.10 m layer were found in the C+SG, GL+SG, CC, and VN treatments (moderate to slow). In the 0.10–0.20 m layer, the highest mean values were observed in the CC, SG, and NV treatments. Similarly, in the 0.20–0.30 m layer, the highest values were found in the NV, C+SG, and CC treatments. The lowest mean value was observed in the 0.00–0.10 m layer in the I+SG treatment (slow). The reduction in saturated hydraulic conductivity was proportional to the increase in Bd and DC.
A very important physico-hydrical relationship was observed through the Pearson analysis where in the VN and L+ BD treatments there were positive correlations between the saturated hydraulic conductivity (Kθ) and soil bulk density (Bd), with values r = 0.61 and r = 0.51, respectively (
Table 7). This behavior is associated with lower bulk density (Bd) (1.31 g cm
−3) and degree of soil compaction (DC) (72.3%), mainly in native vegetation (VN) (
Table 4). The hydraulic conductivity of saturated soil describes the functionality of its porous system, encompassing properties related to its porosity, such as quantity, size, morphology, continuity, and orientation of the pores [
31].
Soil structure, in turn, is modified under different land use and management systems. In this context, determining K
θ is essential for evaluating how these systems affect soil water dynamics [
32]. Certain land use and management practices can negatively impact soil structure by exposing it to abiotic factors, leading to a decline in physical quality, erosion, nutrient loss, compaction at deeper layers, and K
θ [
33]. Some studies have shown that conventional tillage systems can improve soil K
θ in the short term due to soil disturbance, which may explain the statistically significant (
p < 0.05) results observed in the treatment with the conventional cropping system. In this case, the number of macropores increases, but the micropores tend to be compromised.
It was observed that the estimated total porosity (
ϕTP) ranged from 0.42 to 0.75 m
3 m
−3, calculated total porosity (TP) was below 0.50 m
3 m
−3, microporosity was below 0.40 m
3 m
−3, and macroporosity was lower than 0.10 m
3 m
−3 except for the CC treatment in the 0.00–0.10 m layer. Similar results were reported in a study related to aggregate stability and the degree of flocculation in soils under an Integrated Crop–Livestock system [
30].
The highest calculated total porosity (TP) in the 0.00–0.10 m soil layer (0.49 m3 m−3) is attributed to the sandy texture of the surface layer of the Planosols, along with the total organic carbon content derived from organic matter accumulation and root system decomposition, particularly from Brachiaria grass. The I+SG and CC treatments in the 0.00–0.10 m layer showed statistically significant mean values (p < 0.05), with 0.40 and 0.49 m3 m−3, respectively. Treatments involving Brachiaria grass intercropping contributed to soil structure improvement by increasing soil porosity through organic matter input.
Macroporosity (Ma) in the 0.00–0.10 m layer showed a statistically significant difference (
p < 0.05), with the highest mean value observed in the CC treatment (0.11 m
3 m
−3), exceeding the critical threshold of 0.10 m
3 m
−3 as defined by [
33]. The lowest Ma value (0.03 m
3 m
−3) was found in the GL+SG treatment in the 0.00–0.10 m layer and in the BD, I+SG, and C+SG treatments in the 0.20–0.30 m subsurface layer. This result may be associated with increased Bd in the subsurface layers, which restricts root penetration and impairs plant development. Additionally, reduced porosity limits water infiltration, potentially leading to the formation of gullies and overall soil degradation [
34].
Bulk density is widely used as an indicator in most soil studies due to its strong relationship with other soil attributes. It is well established that as Bd increases, there is a reduction in total porosity, macroporosity, hydraulic conductivity, and ionic absorption, along with a corresponding increase in microporosity and mechanical resistance to root penetration [
35]. In pasture areas, soil compaction is primarily caused by animal load, with the bulk density increasing due to surface applied pressure.
According to [
36], the TP is an important indicator of soil quality, as higher porosity reflects conditions in which the soil has not been altered by pressure from traffic or tillage. Inadequate soil use, such as excessive tillage and the absence of conservation practices, can lead to increased Bd and reduced TP and macroporosity [
37]. TP tends to be higher in native forest areas than in other systems, regardless of soil depth. Reference [
38] further notes that the reduction in total pore volume in pasture areas may be primarily due to a decrease in macroporosity, since microporosity does not appear to be directly influenced by soil management.
Microporosity is associated with soil texture, a pedological characteristic that can be altered by land use; however, the influence of management on this attribute is generally limited [
39]. According to [
40], in clayey soils, the proportion of micropores tends to be greater than that of macropores at greater depths, a pattern also observed in the present study.
Based on the threshold proposed by [
18], the I+SG, NV, and CC treatments exhibited values equal to or above the recommended limit in the 0.00–0.10 m layer. It is also worth noting that the GL+SG and C+SG treatments in the 0.20–0.30 m layer showed adequate SAC values (0.14 and 0.16 m
3 m
−3, respectively). The remaining treatments had values below the minimum required for adequate gas diffusion in the rhizosphere zone of plants.
In the 0.00–0.10 m layer, statistically significant values (p < 0.05) were observed in the NV and CC treatments, with SAC values of 0.20 and 0.16 m3 m−3, respectively. In the 0.10–0.20 m layer, the treatments that showed significant values were C+SG, I+SG, CC, and NV. In the 0.20–0.30 m layer, significant values were observed in the C+SG, GL+SG, I+SG, CC, and NV treatments.
This observation aligns with [
41], who stated that when aeration porosity is below 0.10 m
3 m
−3, the oxygen flux rate to the plant root system is severely restricted, negatively impacting the physiological and metabolic processes responsible for root growth and development. Furthermore, a marked increase in SAC above the critical threshold of 0.34 m
3 m
−3 may indicate low water retention and availability in the soil for plants [
2]. However, no values exceeding 0.34 m
3 m
−3 were observed in this study. Regarding θ
RAC, statistically significant differences (
p < 0.05) were observed only in the surface layer (0.00–0.10 m). The treatments that showed significant values were C+SG, SG, NV, and CC.
In addition to its many essential functions in agricultural systems, soil also acts as a reservoir for water storage for crops. The available water capacity (AWC) in the soil, which plants can absorb, is defined as the amount of water held between field capacity (θ
FC) and the permanent wilting point (θ
PWP). Studies [
21,
42] have shown that, in most soils and under typical conditions, soil reaches field capacity when the water matric potential (Ψm) ranges from −10 kPa (in sandy soils and Oxisols, even those with a high clay content) to −30 kPa (in clay soils). It has also been established that the matric potential corresponding to the θ
PWP is −1.500 kPa [
21]. Moreover, according to [
43], soil moisture content knowledge is crucial for making informed decisions regarding when and how much to irrigate cultivated crops.
As shown in
Table 6, θ
FC was highest in the NV treatment in the 0.00–0.10 m layer, with a value of 0.201 m
3 m
−3, while the lowest value was observed in the C+SG treatment in the 0.10–0.20 m layer, with 0.107 m
3 m
−3. Significant differences (
p < 0.05) were found across all three layers analyzed for this variable. In the 0.00–0.10 m layer, the GL+SG and NV treatments showed statistically distinct means. The NV and CC treatments differed significantly in the second layer (0.10–0.20 m). Finally, in the 0.20–0.30 m layer, the I+BD, NV, and CC treatments differed. According to [
21], the soil behaves as a reservoir for plant-available water. Although this reservoir is open to the atmosphere and deeper soil profile horizons, it retains water by interacting with the soil matrix.
The permanent wilting point is generally considered a static property of the soil, unlike the field capacity [
21]. For this variable, a statistically significant variation (
p < 0.05) was observed across all three soil layers. In the 0.00–0.10 m and 0.10–0.20 m layers, significant differences were found in the NV and CC treatments. In the 0.20–0.30 m layer, in addition to the NV and CC treatments, the I+SG treatment also showed distinct values.
The treatments that presented the highest and lowest permanent wilting point values were NV and GL+SG. Furthermore, it was observed that the native vegetation and conventional cropping system showed statistically significant variation across all three soil layers for both the field capacity and permanent wilting point. Furthermore, for the variable plant-available water (θAW), a significant difference (p < 0.05) was observed only in the second layer (0.10–0.20 m) for the C+SG treatment. Across all treatments, mean values ranged from 0.057 to 0.097 m3 m−3, corresponding to the lowest available water content in the C+SG treatment and the highest in the NV treatment.
However, regarding the critical threshold for this parameter, ref. [
34] indicates a critical range of 0.15 to 0.25 m
3 m
−3 for plant-available water. All treatments in this study fall well below that critical limit. Although the soil is a Planosol with a higher sand content in the surface layer, a greater degree of homogenization has been observed by the eighth year of system implementation.
When analyzing available water capacity (AWC), it was observed that only the C+SG treatment in the 0.10–0.20 m layer showed a statistically different mean value (p < 0.05), with 8.35 mm of stored water, which was also the lowest AWC value among all treatments. The highest values were recorded in the I+SG treatment in the 0.20–0.30 m layer (12.69 mm) and the NV treatment in the 0.00–0.10 m layer (12.53 mm) of stored water. This behavior is associated with better pore distribution in Integrated Crop–Livestock systems and native vegetation environments. In NV environments, water retention and availability are generally related to soil organic matter content, whereas in conventional cropping systems, soil bulk density tends to be more effective for water retention compared to SOM.
Regarding the relative field capacity (RFc), a statistically significant difference (
p < 0.05) was observed across all three depths. In the 0.00–0.10 m layer, the I+SG and NV treatments showed significant mean values of 0.36 and 0.44 m
3 m
−3, respectively. In the 0.10–0.20 m layer, the NV and CC treatments had mean values of 0.45 and 0.40 m
3 m
−3, respectively. Finally, in the 0.20–0.30 m layer, only the I+SG and C+SG treatments showed significant differences, with, respectively, 0.39 and 0.29 m
3 m
−3 values. Although these differences were statistically significant (
p < 0.05), it is evident that all values remained below the range considered ideal. According to [
44], the optimal RFc should range between 0.6 ≤ RFc and ≤0.7 to ensure a proper balance between water availability and soil aeration in the root zone. Values outside this range may reduce soil microbial activity due to inefficient water or air content [
17], with reference values being: <0.6 indicating low water content and >0.7 indicating low air content.
The highest values of the S-index found in the surface layers are associated with Bd and organic matter content in the soil. These layers tend to have better structure, assuming that greater porosity is present. Among the treatments analyzed for the 0.00–0.10 m layer, the highest S-index value was observed in the NV treatment, with an average of 0.25 m3 m−3, while the lowest value was 0.08 m3 m−3, found in the CC treatment in the 0.20–0.30 m layer.
Finally, it is important to highlight that after eight years of implementing the system, some treatments already show significant differences in relation to physical-hydraulic properties.