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Least limiting water range in Oxisols under different levels of machine traffic

This work aimed, to evaluate the structural behavior of Oxisols based on the least limiting water range (LLWR) and establish relations with corn crop. The experiment was carried out in a randomized block design with five treatments and four replications. Soil samples collected at the layer of 0-0.20 m depth in a Haplustox (LVd) and an Eutrustox (LVef) were used. The compaction treatments consisted of T0= no additional compaction; T1 and T2= two and four passes with a 4 t tractor, respectively; T3 and T4 = two and four passes with a 10 t tractor, respectively. The range of LLWR variation in the LVd was the lowest one and varied from 0.01 to 0.04 m3 m-3, whereas in the LVef varied from 0.03 to 0.13 m3 m-3 for the critical soil penetration resistance (PRc) of 2 MPa. It was observed that critical bulk density (BDc) values were 1.76 and 1.40 Mg m-3 in the LVd and the LVef, respectively. The highest frequency of bulk density (BC ≥ BDc) occurred from the T1 to T4 (LVd) and from the T2 to T4, however, a reduction in corn yield was not observed. The determination of the LLWR was efficient and complementary for identifying more favorable or more impeditive conditions to compaction in the LVef for the corn crop development in relation to the LVd.


Introduction
As natural ecosystems are being replaced by intense and continuous agricultural activities, the soil physical properties may undergo temporary or permanent changes and consequently may lead to unfavorable conditions for the plant development. Therefore, the soil physical degradation is established and generally related to the losses of organic matter and structural integrity of the soils (Guimarães et al., 2013).
The soil bulk density, porous system, soil penetration resistance, soil aggregation, S-index, soil water retention, the degree of compaction, water infiltration, least limiting water range (LLWR) etc., are the most commonly attributes to evaluate soil degradation. Therefore, laboratory analyses are still performed, although they are expensive and time-consuming, mainly, when there is a high number of samples and attributes evaluated. This is because, the visual quantification of the soil structural behavior in a given agricultural management is a hard task (Omuto, 2008).
The LLWR is a soil quality index that integrates into a single variable (soil bulk density) the factors that directly affect the plant growth.
The decreasing of LLWR reduces the range in which soil moisture can vary without restricting plant growth, related to water availability, aeration, and soil penetration resistance.

Plant Production and Crop Protection
Com. Sci., Bom Jesus, v.8, n.2, p.337-346, Abr./Jun. 2017 Therefore, it can be considered a well-established index to evaluate the quality of cultivated soils (Cavalieri et al., 2011). However, for Gubiani et al. (2013) the use of LLWR is imprecise to estimate the risk of losses in grain production related to soil compaction.
Another way to evaluate the use of LLWR is through of the critical soil bulk density (BDc), which occurs when the LLWR is zero, that is, the one in which the lower limit crosses the upper limit of the interval. From this condition, severe restrictions to planting growth occur. In a medium-textured red Oxisol, Freddi et al. (2009) found that the BDc was 1.46 kg dm -3 at the layer clay) and 0.20-0.40 m (365 g kg -1 clay) depth, respectively. These differences exist because the LLWR estimator is the soil bulk density, which varies according to the soil management.
The aim of this study was to evaluate the compaction in a Haplustox (LVd) and an Eutrustox (LVef) by means of the least limiting water range (LLWR) for corn crop.

Materials and Methods
The experiment was carried out in Jaboticabal county, São Paulo state, Brazil, located at the geographic coordinates of 21° 14'05 "S, 48 ° 17'09" W, and altitude of 615 m asl.
According to the classification of Köppen, the climate of the region belongs to Cwa type, with hot summers and dry winters, an average annual rainfall of 1428 mm and an average temperature of 21ºC ( Figure 1). The soils were characterized by Embrapa (2013) as a medium texture Haplustox ("Latossolo Vermelho distrófico típico" -LVd), moderate A horizon, kaolinite-hypoferric and clayey texture Eutrustox ("Latossolo Vermelho eutroférrico típico" -LVef), moderate A horizon, kaolinite-oxidic . The soils, at the layer of 0-0.20 m depth, showed 348 and 560 g kg -1 clay content, 598 and 240 g kg -1 sand content, 54 and 200 g kg -1 silt content, 2.89 and 2. 74 Mg m -3 soil particle density, 14 and 24 g kg -1 soil organic matter content, in the LVd and the LVef, respectively. Before the installation of the experiments, the conventional soil preparation at 0,30 m depth was carried out by a disc chisel with 18 discs of 660 mm (26 "); and by a Marchesan Tatu off-set disk harrow, model: ATCR, with seven discs in each section (two), with discs of 0.61 m and distance between disks of 0.23 m.
The experiment was arranged in a randomized block design with five treatments and four replication for each soil class. The plots were constituted by five maize rows with 6 m length, the three central rows were considered the useful plot, and 1.5 m at each end of rows were discarded. The soil compaction induction was performed on November 26, 2013, two days after rainfall, when the soil water content was close to the soil field capacity at the layer of 0-0.20 m depth.
The treatments were: T0 = soil prepared in the conventional tillage system and without additional tractor traffic; T1 and T2 = two and four passes with a 4 t tractor, respectively; T3 and T4 = two and four passes with a 10 t tractor, respectively. Soil compaction was then performed by passing the wheels of these Com. Sci., Bom Jesus, v.8, n.2, p.337-346, Abr./Jun. 2017 tractors, covering the whole surface of the plots, so that the tires compressed areas parallel to each other. Therefore, the number of passes by the tractors varied according to the treatment; the traffic was overlapped on the previous one, so that whole area of each plot was passed with an equal number of times.
In order to establish T1 and T2 treatments, a Ford 5610 tractor with 56 kW (70 hp), traction (4 × 2) with 4 t mass, whereby the distribution of 30% of the total mass in the front diagonal wheels and 70% in the rear radial wheels, was used. The inflation pressure in the R1 front tires was 83 kPa whereas in the rear tires (R1 18.4-30) was 96 kPa.
Corn sowing (Maximus simple hybrid) was carried out on December 03, 2013, using a no-tillage seeder-fertilizer machine with five rows.
In this case and for other cultivation practices, a Massey Fergusson 620 tractor with 77 kW (105 hp), traction (4 x 2 front wheel drive assist), 7 t mass whereby the distribution of 40% of the total mass in the front radial wheels and 60% in the rear radial wheels, was used. The inflation pressure in the front tires (14.9-24R1) was 95 kPa whereas in the rear tires (19.4-34R1) was 110 kPa.
The seed density ranged from five to six seeds per  (Grossman & Reinsch, 2002).
Microporosity was obtained in the water retained at -100 hPa matric potential (Flint & Flint, 2002), and macroporosity by the difference between total porosity and microporosity.
For determination of the LLWR, the soil water retention curve was adjusted by the nonlinear model used by Silva et al. (1994) and the soil penetration resistance (PR) curve was adjusted using the Busscher (1990)  (4), respectively, obtained from equation (1): The water content from which RP is limiting was estimated by equation (5), obtained from equation (2): where PRc is the critical PR of 2 MPa. The water content in which the aeration porosity is 10% was calculated by the equation (6): Eq. (6) where Pd is soil particle density (g dm -3 ).
The θ AP , θ FC , θ PWP and θ PR were plotted as a function of the BD, forming the LLWR, representative of the three layers (0-0.20 m), since it was equivalent in all layers, thus, the average value was used.
On April 29, 2014, after the stage of physiological maturity of the grains, the grain mass was evaluated in 20 plants per plot. When grain moisture was greater than 25%, corn cobs of two useful row with 2.0 m length were harvested (0.5 m was discarded at each end of the 3.0 m row), for the threshing and the grain mass determination. Corn grain yield was obtained by extrapolating grain yield from the useful plot area to 1 ha, adjusting grain moisture to 13%. The analysis of variance and Tukey's test (p < 0.05) were performed. Also, correlations between soil bulk density, and plant height and corn yield were performed.

Results and Discussion
There was no interaction between compaction treatments and the soil layers, in the LVd and LVef for the soil bulk density (BD), macroporosity, microporosity, and soil penetration resistance (PR) ( Table 1). The lowest BD average value was found in the T0 (without additional machine traffic), however, it was statistically different only compared to the T4 (four passes with a 10 t tractor) in the LVd (Table   2). Even so, macroporosity in this soil was similar for all treatments.
It was verified that the increase in the number of passes and the tractor mass in both soils, LVd and LVef (Table 3)   Macroporosity values close to or higher than 0.10 m 3 m -3 were verified after tractor traffic in the LVd and LVef. This value is considered as the minimum necessary for water flowing and gaseous exchanges between the external environment and the soil, and critical for the root growth of most crops.
In the LVd, the BD average absolute values of all treatments were higher than those found in the LVef. This result is due to the mineralogy since the sand content and the particle density (2.89 Mg m -3 ) in the LVd are higher than in the LVef. This is due to the predominance of quartz and kaolinite with massive structure, which confers lower porosity in the LVd. Despite the statistically significant increase of BD for the T4 compared to T0, at the layer of 0-0.20 m depth, in the LVd, it was not observed reducing of corn grain yield (Figure 2a). The corn yield in this soil showed a quadratic behavior as a function of BD, however, it was not significant. From the BD value of 1.73 kg dm -3 , the maximum yield of 5.30 t ha -1 decreased by 21.32% for the T4. It can be observed in Figure  1 that the development of the crop manifested agronomic changes due to the scarcity of rainfall during the 2013/14 growing season. Rainfall was only 291 mm from January to March 2014 ( Figure   1) and probably resulted in water stress.  Table 4). The least limiting water range (LLWR) values corresponding to the gray-hatched area in the graphic (Figure 3). The LLWR in the LVef was higher than in the LVd (Figure 3). It was observed a reduction of the LLWR as the BD values increase, as already stated by Silva et al. (1994) and Blainski et al. (2012). The θ and the RP ( Table   4)   degradation since it starts to restrict the water range where the roots can grow and develop within the available water range (Cavalieri et al., 2011). Using the critical PR of 2 MPa in the LVd, the LLWR lower limit was the θ PR . While in the LVef this limit showed the water content at the permanent wilting point (θ PWP ) only for BD values ≤ 1.33. Since, from values higher than these, the θ PR has become the LLWR lower limit. Thus, in the LVef there is a more restricted amplitude of BD compared to the LVd. It indicates that the higher the BDc, the less chance the plant will remain under stress conditions. Regardless of the soil class, the available water content (AWC = θ FC -θ PWP ) was always higher than the LLWR, which characterizes the soils as physically degraded. Due to such behavior, Silva et al. (1994) point out the LLWR as a tool for analysis of soil structure more sensitive than the concept of available water. In general, in managed soils, LLWR is limited in the upper limit by the θ FC and in the lower limit by the θ PR as verified by Guimarães et al. (2013) in a sandy loam LVd. These authors affirmed that it was considered the LLWR lower limit for the entire range of BD, excepted for the native forest. In this study, this behavior did not occur in the LVef.  A similar condition of the LLWR variation in the LVd found in the present study was also verified by Lima et al. (2015) in an Oxisol Table 4. Soil water retention curve and soil penetration resistance curve for an LVd and an LVef at the layer of 0-0.20 m depth for different agricultural management systems.
Therefore, the results of this study agree with the results of several authors who emphasize that the LLWR is a more sensitive parameter than the soil water retention curve to evaluate changes in soil structure (Roque et al., 2011;Silva et al., 2011e Guedes Filho et al., 2013.

Conclusions
The treatments submitted to the LVd provided physical degradation by increasing BD and PR, added to lower clay and organic matter content of this soil comparing to the LVef.